2021/10/25 更新

写真a

オクニシ コウイチ
奥西 巧一
OKUNISHI Kouichi
所属
教育研究院 自然科学系 数理物質科学系列 准教授
自然科学研究科 数理物質科学専攻 准教授
理学部 理学科 准教授
職名
准教授
外部リンク

学位

  • 博士(理学) ( 1999年3月   大阪大学 )

研究キーワード

  • 磁性

  • Magnetism

  • Statistical Physics

  • 統計物理学

研究分野

  • 自然科学一般 / 磁性、超伝導、強相関系

  • 自然科学一般 / 数理物理、物性基礎

経歴(researchmap)

  • 新潟大学   自然科学系   准教授

    2011年2月 - 現在

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  • - 新潟大学理学部助手

    2000年11月 - 2011年1月

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  • - Niigata University, Research assistant

    2000年

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  • 日本学術振興会特別研究員(PD)

    1999年4月 - 2000年10月

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  • 日本学術振興会特別研究員(DC1)

    1996年4月 - 1999年3月

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経歴

  • 新潟大学   理学部 理学科   准教授

    2017年4月 - 現在

  • 新潟大学   自然科学研究科 数理物質科学専攻   准教授

    2011年2月 - 現在

  • 新潟大学   自然科学研究科 数理物質科学専攻   准教授

    2011年2月 - 現在

  • 新潟大学   物理学科   准教授

    2011年2月 - 2017年3月

  • 新潟大学   物理学科   助教

    2004年4月 - 2011年1月

学歴

  • 大阪大学   理学研究科   物理学

    - 1999年

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    国名: 日本国

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  • 大阪大学

    - 1999年

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所属学協会

 

論文

  • Unconventional magnetic excitations and spin dynamics of exotic quantum spin systems BaCo$$_2$$V$$_2$$O$$_8$$ and Ba$$_3$$CuSb$$_2$$O$$_9$$

    Yibo Han, Shojiro Kimura, Kouichi Okunishi, Masayuki Hagiwara

    Applied Magnetic Resonance   52 ( 4 )   349 - 362   2021年4月

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    掲載種別:研究論文(学術雑誌)   出版者・発行元:Springer Science and Business Media LLC  

    DOI: 10.1007/s00723-020-01296-w

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    その他リンク: https://link.springer.com/article/10.1007/s00723-020-01296-w/fulltext.html

  • Tensor-network strong-disorder renormalization groups for random quantum spin systems in two dimensions 査読

    Kouichi Seki, Toshiya Hikihara, Kouichi Okunishi

    Physical Review B   102 ( 14 )   2020年10月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:American Physical Society ({APS})  

    DOI: 10.1103/physrevb.102.144439

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  • Corner transfer matrix renormalization group analysis of the two-dimensional dodecahedron model 査読

    Hiroshi Ueda, Kouichi Okunishi, Seiji Yunoki, Tomotoshi Nishino

    Physical Review E   102 ( 3 )   2020年9月

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    掲載種別:研究論文(学術雑誌)   出版者・発行元:American Physical Society (APS)  

    DOI: 10.1103/physreve.102.032130

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    その他リンク: http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevE.102.032130/fulltext

  • Finite-m scaling analysis of Berezinskii-Kosterlitz-Thouless phase transitions and entanglement spectrum for the six-state clock model 査読

    Hiroshi Ueda, Kouichi Okunishi, Kenji Harada, Roman Krčmár, Andrej Gendiar, Seiji Yunoki, Tomotoshi Nishino

    Physical Review E   101 ( 6 )   2020年6月

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    掲載種別:研究論文(学術雑誌)   出版者・発行元:American Physical Society (APS)  

    DOI: 10.1103/physreve.101.062111

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    その他リンク: http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevE.101.062111/fulltext

  • Lattice Unruh Effect and World-line Entanglement for the XXZ Chain 査読

    Kouichi Okunishi

    Journal of the Physical Society of Japan   88 ( 11 )   114002   2019年11月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    DOI: 10.7566/jpsj.88.114002

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  • Magnetic Properties of One- and Two-Dimensional Functional Materials: Oxygen Molecules Encapsulated in Single-Walled Carbon Nanotubes and Copper Ions Embedded into Phthalocyanine Sheets 査読

    Masayuki Hagiwara, Takanori Kida, Kazuyuki Matsuda, Haruka Kyakuno, Yutaka Maniwa, Zentaro Honda, Yuya Sakaguchi, Makoto Tashiro, Masamichi Sakai, Takeshi Fukuda, Norihiko Kamata, Kouichi Okunishi

    Open Chemistry Journal   2019年2月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    DOI: 10.2174/1874842201906010027

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  • A low-energy effective model for the S=1/2 Kagome-triangular-strip Heisenberg model 査読

    Okunishi, Kouichi, Onozato, Yoshitaka, Seki, Kouichi

    AIP Advances   8 ( 10 )   2018年10月

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    掲載種別:研究論文(学術雑誌)  

    We study low-energy properties of the S=1/2 kagome-triangular-strip Heisenberg model. Using exact diagonalization, we calculate magnetization curves of the model in the ferromagnetic kagome coupling regime, where the singlet ground state and 1/3 plateau emerge. In order to analyze formation of the singlet ground state, we further derive a low-energy effective ladder model, focusing on the parity degrees of freedom for the two degenerating doublets of the unit triangle. We then discuss the entanglement between the spin and parity degrees of freedom. <P />...

    DOI: 10.1063/1.5042702

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  • Magnetism and High-magnetic Field Magnetization in Alkali Superoxide CsO<SUB>2</SUB> 査読

    Miyajima, Mizuki, Astuti, Fahmi, Kakuto, Takeshi, Matsuo, Akira, Puspita Sari, Dita, Asih, Retno, Okunishi, Kouichi, Nakano, Takehito, Nozue, Yasuo, Kindo, Koichi, Watanabe, Isao, Kambe, Takashi

    Journal of the Physical Society of Japan   87 ( 6 )   2018年6月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:Physical Society of Japan  

    Alkali superoxide CsO<SUB>2</SUB> is one of the candidates for the spin-1/2 one-dimensional (1D) antiferromagnet, which may be sequentially formed by an ordering of the π-orbital of O<SUB>2</SUB><SUP> - </SUP> molecule below T<SUB>S</SUB> ̃ 70 K. Here, we report the magnetism and the high-magnetic field magnetization in pulsed-magnetic fields up to 60 T in powder CsO<SUB>2</SUB>. We obtained the low temperature phase diagram around the antiferromagnetic ordering temperature T<SUB>N</SUB> = 9.6 K under the magnetic field. At T = 1.3 K, we observed a remarkable up-turn curvature in the magnetization around a saturation field of ̃60 T, which indicates the low-dimensional nature of the spin system. The saturated magnetization is also estimated to be ̃1μ<SUB>B</SUB>, which corresponds to spin-1/2. In this study we compare it with the theoretical calculation. <P />...

    DOI: 10.7566/JPSJ.87.063704

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  • Critical behavior of the two-dimensional icosahedron model 査読

    Ueda, Hiroshi, Okunishi, Kouichi, Krčmár, Roman, Gendiar, Andrej, Yunoki, Seiji, Nishino, Tomotoshi

    Physical Review E   96 ( 6 )   2017年12月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER PHYSICAL SOC  

    In the context of a discrete analog of the classical Heisenberg model, we investigate the critical behavior of the icosahedron model, where the interaction energy is defined as the inner product of neighboring vector spins of unit length pointing to the vertices of the icosahedron. The effective correlation length and magnetization of the model are calculated by means of the corner-transfer-matrix renormalization group (CTMRG) method. A scaling analysis with respect to the cutoff dimension m in CTMRG reveals a second-order phase transition characterized by the exponents ν =1.62 ±0.02 and β =0.12 ±0.01 . We also extract the central charge from the classical analog of entanglement entropy as c =1.90 ±0.02 , which cannot be explained by the minimal series of conformal field theory. <P />...

    DOI: 10.1103/PhysRevE.96.062112

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  • Spin-Wave Analysis for Kagome-Triangular Spin System and Coupled Spin Tubes: Low-Energy Excitation for the Cuboc Order 査読

    Ochiai, Masahiro, Seki, Kouichi, Okunishi, Kouichi

    Journal of the Physical Society of Japan   86 ( 11 )   2017年11月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PHYSICAL SOC JAPAN  

    The coupled spin tube system, which is equivalent to a stacked Kagome-triangular spin system, exhibits the cuboc order — a non-coplanar spin order with a twelve-sublattice structure accompanying spontaneous breaking of the translational symmetry — in the Kagome-triangular plane. On the basis of the spin-wave theory, we analyze spin-wave excitations of the planar Kagome-triangular spin system, where the geometric phase characteristic to the cuboc spin structure emerges. We further investigate spin-wave excitations and dynamical spin structure factors for the coupled spin tubes, assuming the staggered cuboc order. <P />...

    DOI: 10.7566/JPSJ.86.114701

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  • Magnetic structure and dispersion relation of the S =1/2 quasi-one-dimensional Ising-like antiferromagnet BaCo<SUB>2</SUB>V<SUB>2</SUB>O<SUB>8</SUB> in a transverse magnetic field 査読

    Matsuda, M., Onishi, H., Okutani, A., Ma, J., Agrawal, H., Hong, T., Pajerowski, D. M., Copley, J. R. D., Okunishi, K., Mori, M., Kimura, S., Hagiwara, M.

    Physical Review B   96 ( 2 )   2017年7月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER PHYSICAL SOC  

    BaCo<SUB>2</SUB>V<SUB>2</SUB>O<SUB>8</SUB> consists of Co chains in which a Co<SUP>2 +</SUP> ion carries a fictitious spin 1/2 with Ising anisotropy. We performed elastic and inelastic neutron scattering experiments in BaCo<SUB>2</SUB>V<SUB>2</SUB>O<SUB>8</SUB> in a magnetic field perpendicular to the c axis which is the chain direction. With applying magnetic field along the a axis at 3.5 K, the antiferromagnetic order with the easy axis along the c axis, observed in zero magnetic field, is completely suppressed at 8 T, while the magnetic field gradually induces an antiferromagnetic order with the spin component along the b axis. We also studied magnetic excitations as a function of transverse magnetic field. The lower boundary of the spinon excitations splits gradually with increasing magnetic field. The overall feature of the magnetic excitation spectra in the magnetic field is reproduced by the theoretical calculation based on the spin 1/2 X X Z antiferromagnetic chain model, which predicts that the dynamic magnetic structure factor of the spin component along the chain direction is enhanced and that along the field direction has clear incommensurate correlations. <P />...

    DOI: 10.1103/PhysRevB.96.024439

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  • Snapshot spectra in the world-line quantum Monte Carlo for one-dimensional quantum spin systems 査読

    Seki, Kouichi, Okunishi, Kouichi

    Journal of the Physical Society of Japan   88 ( 2 )   2017年6月

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    掲載種別:研究論文(学術雑誌)   出版者・発行元:arXiv e-prints  

    We introduce a snapshot density matrix and snapshot spectrum for world-line (WL) quantum Monte Carlo simulations, by integrating out the continuous-imaginary-time index of WL snapshots. For the transverse-field Ising chain, we reveal fundamental properties of the snapshot spectrum as follows. (i) In the ordered phase, the isolated peak corresponding to the maximum eigenvalue appears, indicating the classical nature of WL configurations. (ii) In the disordered phase, the spectrum converges to the dome-like distribution characterized by an effective aspect ratio of the snapshots. (iii) At the critical point, the power-law distribution is observed with an appropriate scaling analysis of the fixed aspect ratio. For the XXZ chain, we also obtain numerical results consistent with the above properties. In addition, the representation-basis dependence of the snapshot spectrum is also discussed. <P />...

    DOI: 10.7566/JPSJ.88.024003

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  • Sine-square deformation and supersymmetric quantum mechanics 査読

    Okunishi, Kouichi, Katsura, Hosho

    Journal of Physics A Mathematical General   48 ( 44 )   2015年11月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:IOP PUBLISHING LTD  

    We investigate the sine-square deformation (SSD) of free fermions in one-dimensional continuous space. On the basis of supersymmetric quantum mechanics, we prove the correspondence between the many-body ground state of the system with SSD and that of the uniform system with periodic boundary conditions. We also discuss the connection between the SSD in the continuous space and its lattice version, where the geometric correction due to the real-space deformation plays an important role in relating the eigenstates of the lattice SSD with those of the continuous SSD. <P />...

    DOI: 10.1088/1751-8113/48/44/445208

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  • High Field Magnetization of Single Crystals of the S=1/2 Quasi-1D Ising-like Antiferromagnet SrCo<SUB>2</SUB>V<SUB>2</SUB>O<SUB>8</SUB> 査読

    Okutani, Akira, Kida, Takanori, Usui, Tomoyasu, Kimura, Tsuyoshi, Okunishi, Kouichi, Hagiwara, Masayuki

    Physics Procedia   75   779 - 784   2015年

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    We report on the results of magnetic susceptibility and high field magnetization measurements on single crystals of SrCo<SUB>2</SUB>V<SUB>2</SUB>O<SUB>8</SUB>, which is regarded as an S = 1/2 quasi one-dimensional Ising-like antiferromagnet and is almost isostructural to BaCo<SUB>2</SUB>V<SUB>2</SUB>O<SUB>8</SUB> that has exhibited a field-induced order-to-disorder transition. The magnetization measurements have been performed at 1.4 K below the Néel temperature T<SUB>N</SUB> = 4.8 K in pulsed fields of up to 50 T. The temperature dependence of magnetic susceptibility parallel to the c-axis (chain axis) has a broad maximum near 40 K, which is typical of a low dimensional antiferromagnet, while the susceptibilities perpendicular to the c-axis do not show such a maximum. The magnetization at 1.9 K along the c-axis shows a steep increase at 3.8 T as in BaCo<SUB>2</SUB>V<SUB>2</SUB>O<SUB>8</SUB>, corresponding to the field-induced order-to-disorder transition. The magnetization curve along the c-axis up to the saturation magnetization is well fitted to the magnetization curve calculated by the exact solution based on the Bethe ansatz for an S = 1/2 XXZ model, and this analysis provides parameter values for the exchange constant J/k<SUB>B</SUB> = 78 K, the anisotropic exchange parameter ∊ = 0.50, and the g-value along the c-axis g<SUB>c</SUB> = 6.1. In contrast, the magnetization curves perpendicular to the c-axis show gradual increase up to 50 T. <P />...

    DOI: 10.1016/j.phpro.2015.12.101

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  • Snapshot Spectrum and Critical Phenomenon for Two-Dimensional Classical Spin Systems 査読

    Imura, Yukinari, Okubo, Tsuyoshi, Morita, Satoshi, Okunishi, Kouichi

    Journal of the Physical Society of Japan   83 ( 11 )   2014年11月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PHYSICAL SOC JAPAN  

    We investigate the eigenvalue distribution of the snapshot density matrix (SDM) generated by Monte Carlo simulation for two-dimensional classical spin systems. We find that the distribution in the high-temperature limit is well explained by the random-matrix theory, while that in the low-temperature limit can be characterized by the zero-eigenvalue condensation. At the critical point, we obtain the power-law distribution with a nontrivial exponent α ≡ (2 - η)/(1 - η) and the asymptotic form of the snapshot entropy, on the basis of the relationship of the SDM with the correlation function matrix. The aspect-ratio dependence of the SDM spectrum is also mentioned. <P />...

    DOI: 10.7566/JPSJ.83.114002

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  • Haldane State Formed by Oxygen Molecules Encapsulated in Single-Walled Carbon Nanotubes 査読

    Hagiwara, Masayuki, Ikeda, Masami, Kida, Takanori, Matsuda, Kazuyuki, Tadera, Shin, Kyakuno, Haruka, Yanagi, Kazuhiro, Maniwa, Yutaka, Okunishi, Kouichi

    Journal of the Physical Society of Japan   83 ( 11 )   2014年11月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PHYSICAL SOC JAPAN  

    We report on the results of X-ray diffraction (XRD), magnetic susceptibility, and high-field magnetization measurements of oxygen molecules, which are unique magnetic homonuclear diatomic molecules with spin-1, encapsulated in single-walled carbon nanotubes (SWCNTs) with diameters of about 0.8 nm. Antiferromagnetic interactions between neighboring oxygen molecules are expected in SWCNTs, resulting in the formation of a spin-1 one-dimensional Heisenberg antiferromagnet, known as a Haldane magnet. The XRD pattern can be predicted accurately by considering the expected oxygen molecule alignment. The temperature evolution of the magnetic susceptibility and the high-field magnetization curve are typical of those for a Haldane magnet with spin-1. The results indicate that the Haldane state has been realized in a nanospaced material for the first time. This provides an alternative to the conventional condensed matter approach to forming quantum spin systems. <P />...

    DOI: 10.7566/JPSJ.83.113706

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  • Symmetry-protected topological order and negative-sign problem for SO(N) bilinear-biquadratic chains 査読

    Okunishi, Kouichi, Harada, Kenji

    Physical Review B   89 ( 13 )   2014年4月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER PHYSICAL SOC  

    Using a generalized Jordan-Wigner transformation combined with the defining representation of the SO(N) spin, we map the SO(N) bilinear-biquadratic (BLBQ) spin chain into the N-color bosonic particle model. We find that, when the Jordan-Wigner transformation disentangles the symmetry-protected topological entanglement, this bosonic model becomes negative-sign-free in the context of quantum Monte Carlo simulation. For the SO(3) case, moreover, the Kennedy-Tasaki transformation for the S =1 BLBQ chain, which is also a topological disentangler, derives the same bosonic model through the dimer-R bases. We present the temperature dependence of the energy, entropy, and string order parameter for the SO(N =3,4,5) BLBQ chains by a world-line Monte Carlo simulation for the N-color bosonic particle model. <P />...

    DOI: 10.1103/PhysRevB.89.134422

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  • Fine-Tuning of Magnetic Interactions in Organic Spin Ladders 査読

    Yamaguchi, Hironori, Miyagai, Hirotsugu, Shimokawa, Tokuro, Iwase, Kenji, Ono, Toshio, Kono, Yohei, Kase, Naoki, Araki, Koji, Kittaka, Shunichiro, Sakakibara, Toshiro, Kawakami, Takashi, Okunishi, Kouichi, Hosokoshi, Yuko

    Journal of the Physical Society of Japan   83 ( 3 )   2014年3月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PHYSICAL SOC JAPAN  

    We have succeeded in synthesizing two types of new organic radical crystals 3-I-V [= 3-(3-iodophenyl)-1,5-diphenylverdazyl] and 3-Br-4-F-V [= 3-(3-bromo-4-fluorophenyl)-1,5-diphenylverdazyl]. Their crystal strucutures are found to be isomorphous to that of previously reported spin ladder 3-Cl-4-F-V. Through the quantitative analysis of their molecular arrangements and magnetic properties, we confirm that these materials form ferromagnetic chain-based spin ladders with slightly modulated magnetic interactions. These results present the first quantitative demonstration of the fine-tuning of magnetic interactions in the molecular-based materials. <P />...

    DOI: 10.7566/JPSJ.83.033707

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  • Doubling of entanglement spectrum in tensor renormalization group 査読

    Hiroshi Ueda, Kouichi Okunishi, Tomotoshi Nishino

    PHYSICAL REVIEW B   89 ( 7 )   2014年2月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER PHYSICAL SOC  

    We investigate the entanglement spectrum in HOTRG-tensor renormalization group (RG) method combined with the higher order singular value decomposition-for two-dimensional (2D) classical vertex models. In the off-critical region, it is explained that the entanglement spectrum associated with the RG transformation is described by "doubling" of the spectrum of a corner transfer matrix. We then demonstrate that the doubling actually occurs for the square-lattice Ising model by HOTRG calculations up to D = 64, where D is the cutoff dimension of tensors. At the critical point, we also find that a nontrivial D scaling behavior appears in the entanglement entropy. We mention about the HOTRG for the 1D quantum system as well.

    DOI: 10.1103/PhysRevB.89.075116

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  • Exotic Quantum Phase Transition of the Spin Nanotube 査読

    Toru Sakai, Hiroki Nakano, Kouichi Okunishi

    27TH INTERNATIONAL CONFERENCE ON LOW TEMPERATURE PHYSICS (LT27), PTS 1-5   568   2014年

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    記述言語:英語   掲載種別:研究論文(国際会議プロシーディングス)   出版者・発行元:IOP PUBLISHING LTD  

    The S=1/2 three-leg spin tube is investigated using the numerical diagonalization up to the 42-spin clusters. A precise finite-size scaling analysis, so called the level spectroscopy, is carried out. It indicates a quantum phase transition between the spin-gap and gapless phases, with respect to the ratio of the leg and rung exchange interactions. The present analysis gives a precise estimate for the critical value of the ratio.

    DOI: 10.1088/1742-6596/568/4/042024

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  • Quantum phase transitions in 1/3 plateau of the quantum spin tube 査読

    Kouichi Okunishi, Masahiro Sato, Toru Sakai, Kiyomi Okamoto, Chigaku Itoi

    JOURNAL OF THE KOREAN PHYSICAL SOCIETY   63 ( 3 )   596 - 600   2013年8月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:KOREAN PHYSICAL SOC  

    We study quantum phase transitions in the 1/3 plateau state of the three-leg spin-1/2 tube, where the low-energy effective chirality degree of freedom plays an essential role. Using the density matrix renormalization group and the effective chirality model, we find that, as the leg coupling increases, the chirality liquid, a novel spin imbalance phase and the vector-spin-chirality ordered phase emerge without closing the plateau spin gap. We also clarify the role of the S (3)-symmetry of the spin tube, behind these quantum phase transitions.

    DOI: 10.3938/jkps.63.596

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  • High-Field Magnetization of the S=1/2 Two-Leg Spin-Ladder Antiferromagnets CuAX(2) (A=Diazine, X=Halogen) 査読

    M. Ikeda, Z. Honda, M. Sakai, S. Kimura, K. Okunishi, M. Hagiwara

    JOURNAL OF LOW TEMPERATURE PHYSICS   170 ( 5-6 )   296 - 301   2013年3月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:SPRINGER/PLENUM PUBLISHERS  

    We have performed magnetic susceptibility and high-field magnetization measurements on powder samples of Cu(DEP)Br-2 (DEP=2,3-diethyl-pyrazine) and Cu(DMP)Cl-2 (DMP=2,3-dimethyl-pyrazine) which are regarded as spin-1/2 two-leg spin-ladders in which the exchange constants are given by J (ayen) along the leg and J (aSyen) along the rung. The temperature dependences of magnetic susceptibilities of these compounds show a round peak at 11 and 18 K, respectively, and are compared with those calculated with a density matrix renormalization group method for alpha=J (ayen)/J (aSyen)=0.45-0.55. The exchange constants of the former and the latter compounds are evaluated to be and , respectively. No magnetization at 1.3 K appears in magnetic fields of up to about 5 T for Cu(DEP)Br-2 and about 14 T for Cu(DMP)Cl-2, and then the magnetization increases monotonically to saturate at around 25 T and 38 T for respective compounds. Both magnetization curves are also compared with those calculated for alpha=0.45-0.55 using the same exchange parameter values as in the susceptibilities.

    DOI: 10.1007/s10909-012-0809-y

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  • Collapse of Magnetic Order of the Quasi One-Dimensional Ising-Like Antiferromagnet BaCo2V2O8 in Transverse Fields 査読

    Shojiro Kimura, Koichi Okunishi, Masayuki Hagiwara, Koichi Kindo, Zhangzhen He, Tomoyasu Taniyama, Mitsuru Itoh, Keiichi Koyama, Kazuo Watanabe

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   82 ( 3 )   2013年3月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PHYSICAL SOC JAPAN  

    We have performed magnetization measurements of the quasi one-dimensional (1D) S = 1/2 Ising-like anti-ferromagnet BaCo2V2O8 in transverse fields. Different from the expectation by the 1D S = 1/2 XXZ model, a steep suppression of the antiferromagnetic order is observed for H parallel to left perpendicular100right perpendicular, whereas the experimental results for H parallel to left perpendicular110right perpendicular agree with the XXZ model. We show that these anisotropic behaviors are caused by a characteristic screw chain structure in BaCo2V2O8, which gives rise to a peculiar angular dependence of effective fields upon application of uniform external magnetic fields.

    DOI: 10.7566/JPSJ.82.033706

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  • Magnetic Field-Induced Phase Transitions in the S=1/2 Two-Leg Spin-Ladder Material Cu(DEP)Br-2 査読

    Zentaro Honda, Hiroko Aruga Katori, Masami Ikeda, Masayuki Hagiwara, Kouichi Okunishi, Masamichi Sakai, Takeshi Fukuda, Norihiko Kamata

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   81 ( 11 )   2012年11月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PHYSICAL SOC JAPAN  

    From heat capacity (C-p) measurements on a single crystal sample of the spin (S) 1/2 two-leg spin-ladder material Cu(DEP)Br-2 above a critical magnetic field (H-c0) where the energy gap between the singlet ground state and the excited magnetic state closes, we have observed a peak anomaly, which is indicative of a magnetic long-range ordering. We have examined how the position of the anomaly in C-p changes with temperature (T) and magnetic field (H) to obtain the experimental H-T phase diagram of this S = 1/2 two-leg spin ladder material. From the low-temperature specific heat below H-c0, we have evaluated the field-dependent energy gap that vanishes at about 7.6 T.

    DOI: 10.1143/JPSJ.81.113710

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  • Spin-chirality separation and S-3 symmetry breaking in the magnetization plateau of the quantum spin tube 査読

    Kouichi Okunishi, Masahiro Sato, Toru Sakai, Kiyomi Okamoto, Chigak Itoi

    PHYSICAL REVIEW B   85 ( 5 )   2012年2月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER PHYSICAL SOC  

    We study the magnetization plateau state of the three-leg spin-1/2 tube in the strong rung coupling region, where S-3 symmetry breaking and the low-energy chirality degree of freedom play crucial roles. On the basis of the effective chirality model and density matrix renormalization group, we clarify that, as the leg coupling increases, the chirality liquid with gapless nonmagnetic excitations, the spin-imbalance phase, and the vector-spin-chirality ordered phase emerge without closing the plateau spin gap. The relevance of these results to experiments is also discussed.

    DOI: 10.1103/PhysRevB.85.054416

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  • Exotic Quantum Phase Transitions in Spin Nanotubes 査読

    Toru Sakai, Masahiro Sato, Kouichi Okunishi, Kiyomi Okamoto, Chigak Itoi

    26TH INTERNATIONAL CONFERENCE ON LOW TEMPERATURE PHYSICS (LT26), PTS 1-5   400   2012年

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    記述言語:英語   掲載種別:研究論文(国際会議プロシーディングス)   出版者・発行元:IOP PUBLISHING LTD  

    Recently some quantum spin systems on tube lattices, so called spin nanotubes, have been synthesized. They are expected to be interesting low-dimensional systems like the carbon nanotubes. As the first step of theoretical study on the spin nanotube, we investigate the S=1/2 three-leg spin tube, which is the simplest one, using the numerical exact diagonalization and the finite-size scaling analysis. In our previous works the quantum phase transition between the spin-gap and gapless phases was revealed to occur due to the asymmetric lattice distortion. In addition the transition between the magnetization plateau and gapless phases at 1/3 of the saturation magnetization induced by the same distortion. Recently we found that the Tomonaga-Luttinger liquid is realized in the 1/3 magnetization plateau phase for the symmetric three-leg spin tube.

    DOI: 10.1088/1742-6596/400/3/032075

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  • Novel Phase Transition Probed by Sound Velocity in Quasi-One-Dimensional Ising-Like Antiferromagnet BaCo2V2O8 査読

    Hironori Yamaguchi, Shadi Yasin, Sergei Zherlitsyn, Kumiko Omura, Shojiro Kimuray, Shunsuke Yoshii, Kouichi Okunishi, Zhangzhen He, Tomoyasu Taniyama, Mitsuru Itoh, Masayuki Hagiwaraz

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   80 ( 3 )   2011年3月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PHYSICAL SOC JAPAN  

    We have performed magnetostriction and ultrasound measurements on the quasi-one-dimensional S = 1/2 Ising-like antiferromagnet BaCo2V2O8 in high magnetic fields of up to 51 T. The magnetostriction shows lattice shrinking along the c-axis, and we interpret it as the lattice distortion caused by exchange striction and Van-Vleck paramagnetism. It is clarified that the field dependence of sound velocity is correlated to the magnetostriction. We observed a pronounced softening in the sound velocity of the longitudinal-acoustic c(33) mode and found a novel phase transition.

    DOI: 10.1143/JPSJ.80.033701

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  • Uniform Matrix Product State in the Thermodynamic Limit 査読

    Hiroshi Ueda, Isao Maruyama, Kouichi Okunishi

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   80 ( 2 )   2011年2月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PHYSICAL SOC JAPAN  

    We study a uniform matrix product state as a variational state for classical and quantum spin chains in the thermodynamic limit. Under careful treatment of the translational symmetry, eigenvalues of the transfer matrix defined in the calculation of expectation values can reflect the periodicity of the ground state and indicate the optimum periodicity of the matrix product state. We discuss the relationship between the periodicity and accuracy of magnetization curves. This approach is free from the error due to finite system size, and works well, especially for the magnetic plateau problem.

    DOI: 10.1143/JPSJ.80.023001

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  • Magnetization plateau of the quantum spin nanotube 査読

    Kiyomi Okamoto, Masahiro Sato, Kouichi Okunishi, Toru Sakai, Chigak Itoi

    PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES   43 ( 3 )   769 - 772   2011年1月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    The S = 1/2 three-leg spin nanotube is theoretically investigated, using the numerical diagonalization and the density matrix renormalization group (DMRG) calculation. The system has a magnetization plateau at 1/3 of the saturation magnetization for sufficiently large rung interaction. We mainly focus on the effect of the asymmetric distortion from the regular triangle to the isosceles one. We find an anomalous increase of the plateau width near the symmetric point (i.e., regular triangle) when the asymmetric parameter is varied. (C) 2010 Elsevier B.V. All rights reserved.

    DOI: 10.1016/j.physe.2010.07.047

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  • Numerical study of random correlation matrices: Finite-size effects 査読

    Yuta Arai, Kouichi Okunishi, Hiroshi Iyetomi

    Smart Innovation, Systems and Technologies   10   557 - 565   2011年

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    記述言語:英語   掲載種別:研究論文(国際会議プロシーディングス)  

    We report the numerical calculations of the distribution of maximal eigenvalue for various size of random correlation matrices. Such an extensive study enables us to work out empirical formulas for the average and standard deviation of the maximal eigenvalue, which are accurate in a wide range of parameters. As an application of those formulas, we propose a criterion to single out statistically meaningful correlations in the principal component analysis. The new criterion incorporates finite-size effects into the current method based on the random matrix theory, which gives the exact results in the infinite-size limit. © 2011 Springer-Verlag Berlin Heidelberg.

    DOI: 10.1007/978-3-642-22194-1_55

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  • Exotic Quantum Critical Phenomena of the Spin Nanotubes 査読

    Toru Sakai, Masahiro Sato, Kouichi Okunishi, Kiyomi Okamoto, Chigak Itoi

    INTERNATIONAL SYMPOSIUM: NANOSCIENCE AND QUANTUM PHYSICS 2011 (NANOPHYS'11)   302   2011年

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    記述言語:英語   掲載種別:研究論文(国際会議プロシーディングス)   出版者・発行元:IOP PUBLISHING LTD  

    Recently some quantum spin systems on tube lattices, so called spin nanotubes, have been synthesized. They are expected to be interesting low-dimensional systems like the carbon nanotubes. As the first step of theoretical study on the spin nanotube, we investigate the S=1/2 three-leg spin tube, which is the simplest one, using the numerical exact diagonalization (ED), combined with the recently developed level spectroscopy analysis. Particularly, we introduce the lattice distortion from the regular triangle unit to the isosceles one. We revealed that the S=1/2 isosceles triangle spin tube exhibits the quantum phase transition between the spin gapped and gapless spin liquid phases.

    DOI: 10.1088/1742-6596/302/1/012013

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  • Exotic quantum phase transitions in the spin nanotube 査読

    T. Sakai, M. Sato, K. Okunishi, K. Okamoto, C. Itoi

    INTERNATIONAL CONFERENCE ON FRUSTRATION IN CONDENSED MATTER (ICFCM)   320   2011年

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    記述言語:英語   掲載種別:研究論文(国際会議プロシーディングス)   出版者・発行元:IOP PUBLISHING LTD  

    We investigate the S=1/2 three-leg spin tube, which is the simplest spin nanotube, using the density matrix renormalization group (DMRG) and the numerical exact diagonalization (ED), conbined with a finite-size scaling analysis. Particularly, we intruduce the lattice distortion from the regular triangle unit to the isosceles one. In our previous works, we revealed that the S=1/2 isosceles triangle spin tube exhibits following exotic quantum phenomena: the quantum phase transition between the spin gapped and gapless spin liquid phases, the 1/3 magnetization plateau due to two different mechanisms, and a new phase between the two plateau phases where the translational symmetry is spontaneously broken. We briefly review these results and show our new calculation of the expectation value of the magnetization at each site to specify the spin structure in the new field induced phase.

    DOI: 10.1088/1742-6596/320/1/012022

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  • Scale-free property and edge state of Wilson's numerical renormalization group 査読

    Kouichi Okunishi, Tomotoshi Nishino

    Physical Review B - Condensed Matter and Materials Physics   82 ( 14 )   2010年10月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    We discuss the scale-free property of Wilson's numerical renormalization group (NRG) for the Kondo-impurity problem. The single-particle state of the effective Hamiltonian with a cutoff Λ is described by the wave-packet basis having the scale-free property. The energy scale of the system can be controlled by the lattice translation of the wave-packet basis with no reference of rescaling of the lattice space. We also analyze the role of the Kondo interaction in the context of wave-packet basis and then discuss the scaling and renormalization of the Kondo coupling. In addition, we clarify the role of the edge state in the lowest energy scale of Wilson NRG. © 2010 The American Physical Society.

    DOI: 10.1103/PhysRevB.82.144409

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  • Quantum spin nanotubes-frustration, competing orders and criticalities 査読

    Toru Sakai, Masahiro Sato, Kiyomi Okamoto, Kouichi Okunishi, Chigak Itoi

    JOURNAL OF PHYSICS-CONDENSED MATTER   22 ( 40 )   2010年10月

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    記述言語:英語   出版者・発行元:IOP PUBLISHING LTD  

    Recent developments of theoretical studies on spin nanotubes are reviewed, especially focusing on the S = 1/2 three-leg spin tube. In contrast to the three-leg spin ladder, the tube has a spin gap in the case of the regular-triangle unit cell when the rung interaction is sufficiently large. The effective theory based on the Hubbard Hamiltonian indicates a quantum phase transition to a gapless spin liquid due to the lattice distortion to an isosceles triangle. This is also supported by the numerical diagonalization and the density matrix renormalization group analyses. Furthermore, combining analytical and numerical approaches, we reveal several novel magnetic-field-induced phenomena: Neel, dimer, chiral and/or inhomogeneous orders, a new mechanism for the magnetization plateau formation, and others. The recently synthesized spin tube materials are also briefly introduced.

    DOI: 10.1088/0953-8984/22/40/403201

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  • Scale-free property and edge state of Wilson's numerical renormalization group 査読

    Kouichi Okunishi, Tomotoshi Nishino

    PHYSICAL REVIEW B   82 ( 14 )   2010年10月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER PHYSICAL SOC  

    We discuss the scale-free property of Wilson's numerical renormalization group (NRG) for the Kondo-impurity problem. The single-particle state of the effective Hamiltonian with a cutoff Lambda is described by the wave-packet basis having the scale-free property. The energy scale of the system can be controlled by the lattice translation of the wave-packet basis with no reference of rescaling of the lattice space. We also analyze the role of the Kondo interaction in the context of wave-packet basis and then discuss the scaling and renormalization of the Kondo coupling. In addition, we clarify the role of the edge state in the lowest energy scale of Wilson NRG.

    DOI: 10.1103/PhysRevB.82.144409

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  • Field Induced Exotic Phenomena of the S=1/2 Three-Leg Quantum Spin Nanotube 査読

    Toru Sakai, Kouichi Okunishi, Kiyomi Okamoto, Chigak Itoi, Masahiro Sato

    JOURNAL OF LOW TEMPERATURE PHYSICS   159 ( 1-2 )   55 - 59   2010年4月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:SPRINGER/PLENUM PUBLISHERS  

    The magnetization process of the S=1/2 three-leg spin tube with an asymmetric exchange interaction in each triangle unit is investigated by the numerical exact diagonalization and the density matrix renormalization group calculation. It is found that the 1/3 magnetization plateau appears due to two different mechanisms depending on the asymmetry. The phase diagrams of the 1/3 plateau is presented. In addition some cusp-like anomalies are found in the magnetization curve.

    DOI: 10.1007/s10909-009-0075-9

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  • Half Magnetization Plateau of a Frustrated S=1 Antiferromagnetic Chain 査読

    T. Tonegawa, H. Nakano, T. Sakai, K. Okamoto, K. Okunishi, K. Nomura

    INTERNATIONAL CONFERENCE ON MAGNETISM (ICM 2009)   200   2010年

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    記述言語:英語   掲載種別:研究論文(国際会議プロシーディングス)   出版者・発行元:IOP PUBLISHING LTD  

    We numerically investigate the magnetization process of a frustrated S = 1 chain with antiferromagnetic nearest-neighbor (nn) and next-nearest-neighbor (nnn) exchange interactions, which compete with each other, and also with uniaxial single-ion-type anisotropy energies. Special attention is paid to the half magnetization plateau which appears at the half of the saturation magnetization in the ground-state magnetization curve. We determine the parameter region where this plateau appears by employing the level spectroscopy method, and find that the plateau appears even in the absence of the single-ion-type anisotropy, if the ratio of the nnn to the nn interaction constant is in a specified region.

    DOI: 10.1088/1742-6596/200/2/022065

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  • Wang-Landau simulation of the quasi-one-dimensional spin systems 査読

    Kouichi Okunishi, Takayuki Tanabe

    INTERNATIONAL CONFERENCE ON MAGNETISM (ICM 2009)   200   2010年

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    記述言語:英語   掲載種別:研究論文(国際会議プロシーディングス)   出版者・発行元:IOP PUBLISHING LTD  

    Using Wang-Landau simulation, we investigate the nature of the quasi-one-dimensional(Q1D) Ising model in terms of energy density of states(DOS); In order to treat the large difference of the energy scales between intra-and inter-chain couplings, we particularly introduce DOS in the two dimensional energy space corresponding to the intra-and inter-chain directions, and then discuss size dependence of the peak structure of specific heat. We also analyze the inter-chain coupling dependence of the spin flop transition of the Q1D classical XXZ model with the Ising-like anisotropy using the Wang-Landau simulation.

    DOI: 10.1088/1742-6596/200/2/022043

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  • High-Field Magnetism of the Spin-Ladder Material Na2Fe2(C2O4)(3)(H2O)(2) 査読

    Hironori Yamaguchi, Shojiro Kimura, Zentaro Honda, Kouichi Okunishi, Synge Todo, Koichi Kindo, Masayuki Hagiwara

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   78 ( 12 )   2009年12月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PHYSICAL SOC JAPAN  

    We report the results of our magnetic susceptibility, magnetization, and multi-frequency electron spin resonance (ESR) measurements on single crystals of the two-leg spin-ladder material Na2Fe2(C2O4)(3)(H2O)(2). Magnetization processes in magnetic fields up 53 T at 1.3 K parallel (H parallel to a) and perpendicular (H perpendicular to a) to the leg direction show a gradual increase compared with those of the isomorphous compound Na2Co2(C2O4)(3)(H2O)(2), which was well explained by an isolated antiferromagnetic dinner model. Thus, we calculated the magnetic susceptibilities and magnetizations on the basis of a two-leg ladder model with a fictitious spin (S) one by a quantum Monte Carlo method and a density-matrix renormalization group method, respectively. These calculated results, however, did not agree well with the experimental ones. Therefore, we regarded Na2Fe2(C2O4)(3)(H2O)(2) as an isolated S = 1 antiferromagnetic dimer with not only the anisotropy of the exchange interactions due to the truncation of spin space but also single-ion anisotropy. Consequently, we obtained good agreement between the experimental and calculated results on the magnetic susceptibilities, magnetizations, ESR resonance modes, and specific heat.

    DOI: 10.1143/JPSJ.78.124701

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  • QUANTUM PHASE TRANSITION OF A TRIANGULAR LATTICE SPIN TUBE AND EDGE SPIN EFFECTS 査読

    Kouichi Okunishi, Shin-Ichiro Yoshikawa, Toru Sakai, Seiji Miyashita

    INTERNATIONAL JOURNAL OF MODERN PHYSICS C   20 ( 9 )   1423 - 1430   2009年9月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:WORLD SCIENTIFIC PUBL CO PTE LTD  

    We study the low-energy excitation of the quantum spin tube with the triangular lattice structure, using density matrix renormalization group. Taking account of the edge spin effect, we particularly investigate the spin gap behavior and the low-field magnetization curve near the quantum phase transition point in contrast with the usual free boundary condition. We then find that the bulk behavior of the spin tube can be extracted easier for the single spin termination.

    DOI: 10.1142/S0129183109014485

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  • Magnetic Field versus Temperature Phase Diagram of the Spin-1/2 Alternating-Bond Chain Compound F5PNN 査読

    Yasuo Yoshida, Tatsuya Kawae, Yuko Hosokoshi, Katsuya Inoue, Nobuya Maeshima, Kouichi Okunishi, Kiyomi Okamoto, Toru Sakai

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   78 ( 7 )   2009年7月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PHYSICAL SOC JAPAN  

    We have measured the specific heat of the S = 1/2 alternating-bond Heisenberg antiferromagnetic chain compound pentafluorophenyl nitronyl nitroxide in magnetic fields using a single crystal and powder. A sharp peak due to field-induced magnetic ordering (FIMO) is observed in both samples. The H-T phase boundary of the FIMO of the single crystal is symmetric with respect to the central field of the gapless field region H-C1 &lt;= H &lt;= H-C2, whereas it is distorted for the powder whose ordering temperatures are lower. We discuss possibility that an effective pressure caused by mixing the powder with grease, which is reported for various organic compounds, plays an important role for the distorted phase boundary. An analysis employing calculations based on the finite temperature density matrix renormalization group suggests that the pressure-induced frustration enhances incommensurate spin correlation leading to the distorted phase boundary for the powder.

    DOI: 10.1143/JPSJ.78.074716

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  • Wang-Landau Simulation for the Quasi-One-Dimensional Ising Model 査読

    Takayuki Tanabe, Kouichi Okunishi

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   78 ( 5 )   2009年5月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PHYSICAL SOC JAPAN  

    We revisit the nature of the quasi-one-dimensional Ising model on the basis of Wang-Landau simulation. We introduce the density of states in the two-dimensional energy space corresponding to the intra- and interchain directions. We then analyze the interchain coupling dependence of specific heat of the anistropic two-dimensional Ising model in the context of the density of states, and further discuss the size dependence of peak temperature. We also discuss the feature of the phase transition in the three-dimensional case.

    DOI: 10.1143/JPSJ.78.054002

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  • High field magnetism of the quasi one-dimensional anisotropic antiferromagnet BaCo2V2O8 査読

    S. Kimura, T. Takeuchi, M Hagiwara, K Okunishi, K Kindo, Z He, T Taniyama, M Itoh

    J. Phys.: Conf. Ser.   150   042090   2009年3月

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    記述言語:英語   掲載種別:研究論文(国際会議プロシーディングス)  

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  • Low temperature magnetism of the S=1/2 quasi one-dimensional Ising-like anitiferromagnet BaCo2V2O8 査読

    S. Kimura, T. Takeuchi, M. Hagiwara, K. Okunishi, Z. He, T. Taniyama, M. Itoh

    25TH INTERNATIONAL CONFERENCE ON LOW TEMPERATURE PHYSICS (LT25), PART 4   150   2009年

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    記述言語:英語   掲載種別:研究論文(国際会議プロシーディングス)   出版者・発行元:IOP PUBLISHING LTD  

    We report the results of thermodynamic measurements on the quasi one-dimensional (1D) Ising-like antiferromagnet BaCo2V2O8 in the magnetic fields for H//c down to 200 mK. An appearance of a long range magnetic ordered phase is observed in the field induced region above H-c similar or equal to 3.9 T at temperature below 1.8 K. A peculiar incommensurate ordering, which reflects a quantum critical nature of S = 1/2 1D Ising-like antiferromagnet, realizes in this ordered phase.

    DOI: 10.1088/1742-6596/150/4/042090

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  • Exotic incommensurate order of weakly coupled S=1/2 XXZ antiferromagnetic chains 査読

    Takafumi Suzuki, Naoki Kawashima, Kouichi Okunishi

    25TH INTERNATIONAL CONFERENCE ON LOW TEMPERATURE PHYSICS (LT25), PART 4   150   2009年

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    記述言語:英語   掲載種別:研究論文(国際会議プロシーディングス)   出版者・発行元:IOP PUBLISHING LTD  

    We study the ordered state of the quasi-one dimensional S = 1/2 XXZ spin model in magnetic fields by using quantum Monte Carlo simulations based on the directed-loop algorithm. The H-T phase diagram is illustrated and we find that an incommensurate long-range-ordered state is stable in a certain low-magnetized region. The result of the finite-size scaling analysis indicates that the finite temperature transition to the incommensurate order is of the first order.

    DOI: 10.1088/1742-6596/150/4/042197

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  • Quantum phase transitions of the asymmetric three-leg spin tube

    Tôru Sakai, Masahiro Sato, Kouichi Okunishi, Yuichi Otsuka, Kiyomi Okamoto, Chigak Itoi

    Physical Review B - Condensed Matter and Materials Physics   78 ( 18 )   2008年11月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    We investigate quantum phase transitions of the S= 1 2 three-leg antiferromagnetic spin tube with asymmetric interchain (rung) exchange interactions. On the basis of the electron tube system, we propose a useful effective theory to give the global phase diagram of the asymmetric spin tube. In addition, using other effective theories we raise the reliability of the phase diagram. The density-matrix renormalization-group and the numerical diagonalization analyses show that the finite spin gap appears in a narrow region around the rung-symmetric line, in contrast to a recent paper by Nishimoto and Arikawa, [Phys. Rev. B 78, 054421 (2008)]. The numerical calculations indicate that this global phase diagram obtained by use of the effective theories is qualitatively correct. In the gapless phase on the phase diagram, the numerical data are fitted by a finite-size scaling in the c=1 conformal field theory. We argue that all the phase transitions between the gapful and gapless phases belong to the Berezinskii-Kosterlitz-Thouless universality class. © 2008 The American Physical Society.

    DOI: 10.1103/PhysRevB.78.184415

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  • Quantum phase transitions of the asymmetric three-leg spin tube 査読

    Toru Sakai, Masahiro Sato, Kouichi Okunishi, Yuichi Otsuka, Kiyomi Okamoto, Chigak Itoi

    PHYSICAL REVIEW B   78 ( 18 )   2008年11月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER PHYSICAL SOC  

    We investigate quantum phase transitions of the S=1/2 three-leg antiferromagnetic spin tube with asymmetric interchain (rung) exchange interactions. On the basis of the electron tube system, we propose a useful effective theory to give the global phase diagram of the asymmetric spin tube. In addition, using other effective theories we raise the reliability of the phase diagram. The density-matrix renormalization-group and the numerical diagonalization analyses show that the finite spin gap appears in a narrow region around the rung-symmetric line, in contrast to a recent paper by Nishimoto and Arikawa, [Phys. Rev. B 78, 054421 (2008)]. The numerical calculations indicate that this global phase diagram obtained by use of the effective theories is qualitatively correct. In the gapless phase on the phase diagram, the numerical data are fitted by a finite-size scaling in the c=1 conformal field theory. We argue that all the phase transitions between the gapful and gapless phases belong to the Berezinskii-Kosterlitz-Thouless universality class.

    DOI: 10.1103/PhysRevB.78.184415

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  • Exotic finite-temperature phase diagram for weakly coupled S=1/2 XXZ chain in a magnetic field 査読

    Takafumi Suzuki, Naoki Kawashima, Kohichi Okunishi

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   76 ( 12 )   2007年12月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PHYSICAL SOC JAPAN  

    We investigate the ordered state in S = 1/2 XXZ spin chains with weak interchain couplings in a magnetic field by using quantum Monte Carlo simulations based on the directed-loop algorithm. The HT phase diagram is illustrated for a square lattice and we find that an exotic spin-liquid state with quasi long-range incommensurate correlations in both the spin-chain and interchain directions may appear in the low-magnetization region without a finite-temperature transition. We also show that the incommensurate long-range-ordered state is stable for a cubic lattice.

    DOI: 10.1143/JPSJ.76.123707

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  • Field-induced incommensurate order for the quasi-one-dimensional XXZ model in a magnetic field 査読

    Kouichi Okunishi, Takahumi Suzuki

    PHYSICAL REVIEW B   76 ( 22 )   2007年12月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER PHYSICAL SOC  

    We investigate phase transitions of the quasi-one-dimensional S=1/2 XXZ model in a magnetic field, using bosonization combined with a mean-field treatment of the interchain interaction. We then find that the field-induced incommensurate order is certainly realized in the low-field region, while transverse staggered order appears in the high-field region. On the basis of the result, we discuss the field-induced phase transition recently observed for BaCo2V2O8.

    DOI: 10.1103/PhysRevB.76.224411

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  • Field-induced order-disorder transition in antiferromagnetic BaCo2V2O8 driven by a softening of spinon excitation 査読

    S. Kimura, H. Yashiro, K. Okunishi, M. Hagiwara, Z. He, K. Kindo, T. Taniyama, M. Itoh

    PHYSICAL REVIEW LETTERS   99 ( 8 )   2007年8月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER PHYSICAL SOC  

    High field magnetization and ESR measurements on the quasi-one-dimensional (1D) antiferromagnet BaCo2V2O8 have been performed in magnetic fields up to 50 T along the chain. The experimental results are explained well in terms of a 1D S=1/2 antiferromagnetic XXZ model in longitudinal fields. We show that the quantum phase transition from the Neel ordered phase to the spin liquid one in the model is responsible for a peculiar order to disorder transition in BaCo2V2O8.

    DOI: 10.1103/PhysRevLett.99.087602

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  • Phase diagram and string-density plateau state of the anisotropic triangular antiferromagnetic Ising model 査読

    Hiromi Otsuka, Yutaka Okabe, Kouichi Okunishi

    JOURNAL OF PHYSICS-CONDENSED MATTER   19 ( 14 )   2007年4月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:IOP PUBLISHING LTD  

    We report a study on the ground states of the triangular antiferromagnetic Ising model with a spacial anisotropy in the magnetic field. By the use of the level- spectroscopy and the density- matrix renormalization- group methods, we precisely determine the field- induced Berezinskii - Kosterlitz Thouless transition point and the Pokrovski - Talapov transition points with an anisotropic nature. Then we provide the global phase diagram of the present model.

    DOI: 10.1088/0953-8984/19/14/145236

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  • Finite-field ground state of an S=1 zigzag ladder 査読

    T. Tonegawa, K. Okamoto, K. Okunishi, T. Sakai

    JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS   310 ( 2 )   E372 - E374   2007年3月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    Using numerical methods, we study the finite-field ground state of an S = 1 zigzag ladder. Special attention is paid to the magnetization plateau which appears at a half of the saturation magnetization in the ground-state magnetization curve. (c) 2006 Elsevier B.V. All rights reserved.

    DOI: 10.1016/j.jmmm.2006.10.316

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  • Quantum phase transition in the three-leg spin nanotube 査読

    T. Sakai, K. Okunishi, K. Okamoto, M. Sato, M. Matsumoto, Y. Otsuka

    JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS   310 ( 2 )   E423 - E425   2007年3月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    An S = (1)/(2) three-leg antiferromagnetic spin nanotube is investigated using the DMRG method and the level spectroscopy analysis combined with the numerical exact diagonalization. It is known that the system has a spin gap, when the three rung couplings are equivalent. In this paper, we see that as one of the three rung exchanges varies, a quantum phase transition occurs at a critical value of the varied rung exchange, and then the system is in a gapless Tomonaga-Luttinger-liquid phase. A ground-state phase diagram containing this phase transition is obtained by the phenomenological renormalization. (c) 2006 Published by Elsevier B.V.

    DOI: 10.1016/j.jmmm.2006.10.431

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  • Chemical potential-electron density curve of the zigzag Hubbard ladder for Pr247 査読

    Kouichi Okunishi

    JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS   310 ( 2 )   1108 - 1109   2007年3月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    We investigate the filling dependence of the zigzag Hubbard ladder, which is a relevant model to the recently synthesized quasi-one-dimensional super conductor Pr2Ba4Cu7O15-delta(Pr247). Using the density matrix renormalization group method, we calculate chemical potential-electron density (mu-rho) curve of the model and then discuss the relation between the oxygen-deficiency and transition temperature in Pr247. (c) 2006 Published by Elsevier B.V.

    DOI: 10.1016/j.jmmm.2006.10.300

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  • Possibility of field-induced incommensurate order in a quasi-one-dimensional frustrated spin system 査読

    N Maeshima, K Okunishi, K Okamoto, T Sakai, K Yonemitsu

    JOURNAL OF PHYSICS-CONDENSED MATTER   18 ( 20 )   4819 - 4826   2006年5月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:IOP PUBLISHING LTD  

    We study an incommensurate long-range order induced by an external magnetic field in a quasi-one-dimensional bond-alternating spin system, F5PNN, focusing on the role of the frustrating interaction which can be enhanced by a high-pressure effect. On the basis of the density matrix renormalization group analysis of a microscopic model for F5PNN, we present several H-T phase diagrams for typical parameters of the frustrating next-nearest-neighbour coupling and the interchain interaction, and then discuss how the field-induced incommensurate order develops by the frustration effect in such phase diagrams. A magnetization plateau at half the saturation moment is also mentioned.

    DOI: 10.1088/0953-8984/18/20/006

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  • Field-induced Berezinskii-Kosterlitz-Thouless transition and string-density plateau in the anisotropic triangular antiferromagnetic Ising model 査読

    H Otsuka, Y Okabe, K Okunishi

    PHYSICAL REVIEW E   73 ( 3 )   2006年3月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMERICAN PHYSICAL SOC  

    The field-induced Berezinskii-Kosterlitz-Thouless (BKT) transition in the ground state of the triangular antiferromagnetic Ising model is studied by the level-spectroscopy method. We analyze dimensions of operators around the BKT line, and estimate the BKT point H-c similar or equal to 0.5229 +/- 0.001, which is followed by the level-consistency check to demonstrate the accuracy of our estimate. Further we investigate the anisotropic case to clarify the stability of the field-induced string-density plateau against an incommensurate liquid state by the density-matrix renormalization-group method.

    DOI: 10.1103/PhysRevE.73.035105

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  • Product wave function renormalization group: Construction from the matrix product point of view 査読

    Kouji Ueda, Tomotoshi Nishino, Kouichi Okunishi, Yasuhiro Hieida, Rene Derian, Andrej Gendiar

    Journal of the Physical Society of Japan   75 ( 1 )   2006年1月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    We present a construction of a matrix product state (MPS) that approximates the largest-eigenvalue eigenvector of a transfer matrix T of a two-dimensional classical lattice model. A state vector created from the upper or the lower half of a finite size cluster approximates the largest-eigenvalue eigenvector. Decomposition of this state vector into the MPS gives a way of extending the MPS recursively. The extension process is a special case of the product wave function renormalization group (PWFRG) method, that accelerates the numerical calculation of the infinite system density matrix renormalization group (DMRG) method. As a result, we successfully give the physical interpretation of the PWFRG method, and obtain its appropriate initial condition. ©2006 The Physical Society of Japan.

    DOI: 10.1143/JPSJ.75.014003

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  • Field-induced incommensurate order in a quasi-1D S=1 antiferromagnet 査読

    Toru Sakai, Kiyomi Okamoto, Kouichi Okunishi, Takashi Tonegawa

    LOW TEMPERATURE PHYSICS, PTS A AND B   850   1065 - +   2006年

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    記述言語:英語   掲載種別:研究論文(国際会議プロシーディングス)   出版者・発行元:AMER INST PHYSICS  

    We investigate the S=1 frustrated antiferromagnetic quantum spin chain in magnetic field, using numerical exact diagonalization of finite-size clusters. The calculated critical exponents of the spin correlation functions indicate that the incommensurate spin correlation parallel to the external field can be stronger than the transverse antiferromagnetic one for some intermediate fields. When such an incommensurate correlation is dominant, the quasi-1D system with finite interchain interactions should exhibit long-range incommensurate order. We also discuss a possible mechanism for the recently observed field-induced phase of NDMAP within this framework.

    DOI: 10.1063/1.2355071

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  • High field magnetism of the quasi one-dimensional anisotropic antiferromagnet BaCO2V2O8 査読

    S. Kimura, H. Yashiro, M. Hagiwara, K. Okunishi, K. Kindo, Z. He, T. Taniyama, M. Itoh

    YAMADA CONFERENCE LX ON RESEARCH IN HIGH MAGNETIC FIELDS   51   99 - +   2006年

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    記述言語:英語   掲載種別:研究論文(国際会議プロシーディングス)   出版者・発行元:IOP PUBLISHING LTD  

    We have performed high field magnetization measurements on the quasi one-dimensional (1D) antiferromagnet BaCo2V2O8, which shows a curious field-induced order-disorder transition, in pulsed magnetic fields up to 55 T. The observed magnetization curves up to the saturation field show non-linear increases, suggesting a strong quantum fluctuation. We analyze the magnetization curves in terms of the S = 1/2 1D XXZ model.

    DOI: 10.1088/1/1742-6596/51/1/021

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  • Energy gap of spin nanotube 査読

    T Sakai, M Matsumoto, K Okunishi, K Okamoto, M Sato

    PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES   29 ( 3-4 )   633 - 636   2005年11月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    Recently some quantum spin systems on tube lattices, so-called spin nanotubes, have been synthesized. They are expected to be interesting low-dimensional systems like the carbon nanotubes. As a first step of theoretical study oil the spin nanotube, we investigate the S = 1/2 three-leg spin tube, which is the simplest one, using numerical analyses of finite clusters and a finite-size scaling technique. The spin gap, which is one of the most interesting quantities reflecting the macroscopic quantum effect, was revealed to be open for ally finite rung exchange couplings, in contrast to the three-leg spin ladder system which is gapless. We also found a quantum phase transition caused by an asymmetric rung interaction. When one of the three rung coupling constants is changed, the spin gap vanishes very rapidly. (c) 2005 Elsevier B.V. All rights reserved.

    DOI: 10.1016/j.physe.2005.06.044

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  • Finite-field ground state of an S=1 anisotropic antiferromagnetic chain 査読

    T Tonegawa, K Okunishi, T Sakai, M Kaburagi

    PROGRESS OF THEORETICAL PHYSICS SUPPLEMENT   ( 159 )   77 - 81   2005年

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PROGRESS THEORETICAL PHYSICS PUBLICATION OFFICE  

    We investigate, using numerical methods, the finite-field ground-state properties of an S = 1 anisotropic antiferromagnetic chain described by the Hamiltonian H=Sigma(l){S-l(x) S-l+1(x) + S-l(y) S-l+1(y) + Delta S-l(z) Sl+1} + d Sigma(l)(S-l(z)). It is found that the magnetization jump anomaly (a first-order phase transition) appears in the ground-state magnetization curve when Delta=5.0 and -1.50 less than or similar to D less than or similar to 1.05; in a certain region of D, the jump starts from a finite value of the magnetization m per spin. We also find that the a second-order phase transition between commensurate and incommensurate gapless phases takes place in the finite-field ground state. The critical point of this transition as well as the values of m at the onset and end of the magnetization jump is summarized in the m versus D phase diagram.

    DOI: 10.1143/PTPS.159.77

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  • Magnetization plateau of S=1 spin ladder BIP-TENO 査読

    M Kikuchi, K Okamoto, K Okunishi, T Sakai

    PROGRESS OF THEORETICAL PHYSICS SUPPLEMENT   ( 159 )   251 - 255   2005年

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PROGRESS THEORETICAL PHYSICS PUBLICATION OFFICE  

    A mechanism of the magnetization plateau observed in the high-field magnetizatiom measurement of the organic S = 1 spin ladder system BIP-TENO is investigated. Assuming possible lattice distortion, we propose a possible scenario of the plateau formation on the basis of the bond-alternating ladder. Some unsolved problems are also discussed.

    DOI: 10.1143/PTPS.159.251

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  • Low-energy excitations of the S=1/2 quantum spin tube with the triangular lattice structure 査読

    K Okunishi, S Yoshikawa, T Sakai, S Miyashita

    PROGRESS OF THEORETICAL PHYSICS SUPPLEMENT   ( 159 )   297 - 301   2005年

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PROGRESS THEORETICAL PHYSICS PUBLICATION OFFICE  

    In this paper, we investigate the low-energy properties of the S = 1/2 anriferromagnetic quantum spin tube with the triangular lattice structure. Using the density matrix renormalization group (DMRG) method, we calculate the singlet-triplet excitation gap up to 144 triangles along the leg direction. Analyzing the size dependence of the gap, we find that the system is gapful for alpha &lt; 1.2. A similar analysis of the low-field edge of the magnetization curve suggests that the system is gapless for alpha &gt; 1.2.

    DOI: 10.1143/PTPS.159.297

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  • Effect of staggered field on the magnetization process of S=1/2 antiferromagnetic chains 査読

    A Matsuo, K Hosokawa, Y Narumi, K Okunishi, N Maeshima, T Asano, Y Ajiro, N Ishida, T Nogami, K Kindo

    PROGRESS OF THEORETICAL PHYSICS SUPPLEMENT   ( 159 )   158 - 162   2005年

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PROGRESS THEORETICAL PHYSICS PUBLICATION OFFICE  

    We measured the magnetization processes of the S = 1/2 antiferromagnetic chain compound, Cu benzoate. From the inelastic neutron scattering and the specific heat measurements it was recently discovered that the substance exhibits an unexpected gap which varies approximately as the 2/3 power of an applied magnetic field. The primary mechanism of the gap formation is produced by an effective staggered field due to both the alternating g-tensor and the Dzyaloshinskii-Moriya interaction. We observed the effect of the effective staggered field on the magnetization process of Cu benzoate as a rounding behavior near the saturation field as well as an initial nonlinear increment near zero field. The experimental results are well reproduced by the numerical calculation on the Hamiltonian which includes the appropriate term of the staggered field, using the finite temperature density matrix renormalization group method.

    DOI: 10.1143/PTPS.159.158

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  • Bond-Alternating S=1 Spin Ladder in Magnetic Field 査読

    Munehisa Kikuchi, Kiyomi Okamoto, Kouichi Okunishi, Toru Sakai

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   74   2005年

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PHYSICAL SOC JAPAN  

    The magnetization process of the S = 1 spin ladder system with bond-alternation due to the static lattice distortion is investigated by the numerical diagonalization of finite clusters and the level spectroscopy method. We focus our attention mainly on the magnetization plateau caused by field-induced spin gap. The plateau phase diagram is presented at M-s/4, where M-s is the saturated magnetization. In addition we discuss the magnetic curves and the mechanisms of the plateau at M-s/4 observed in the high field magnetization measurement of organic S = 1 ladder compound BIP-TENO.

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  • Magnetization Process of an S=1 Frustrated Spin Ladder 査読

    Kiyomi Okamoto, Kouichi Okunishi, Toru Sakai

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   74   2005年

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PHYSICAL SOC JAPAN  

    We investigate, at T = 0, the magnetization process of the S = 1 frustrated two-leg spin ladder having the leg interaction (J), the rung interaction (J(0)) and the diagonal interaction (J(d)), especially focusing on the appearance and the disappearance of the magnetization plateaux. We use the degenerate perturbation theory, the level spectroscopy analysis of the numerical diagonalization data and the DMRG. When J(d) = 0, the M-s/2 plateau (M-s is the saturation magnetization) appears for J(0)/J &gt; 1.463. Then there is no M-s/2 plateau for the so-called isotropic case J = J(0). In case of J(d) &gt; 0, J(d) helps the M-s/2 plateau. And also, the M-s/4 and (3/4) Ms plateaux appear when Jd is sufficiently large. These two plateaux are associated with the spontaneous breaking of the translation symmetry along the leg direction. We show the plateau phase diagram on the J(d) versus J plane, and also the magnetic phase diagram on the H versus J(d) plane for fixed J.

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  • Fractional S-z Excitation and Its Boundstate for the S=1/2 Antiferromagnetic Zigzag Spin Chain in a Magnetic Field 査読

    Kouichi Okunishi, Takashi Tonegawa

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   74   2005年

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PHYSICAL SOC JAPAN  

    We demonstrate that domain-wall type excitations carrying the fractional magnetization S-z = +/- 1/3 and their bound state describe the low-energy physics around the 1/3 plateau of the antiferromagnetic zigzag spin chain in the strongly frustrated region. In particular we discuss the relevance of such domain-wall excitations to the characteristic properties of the magnetization curves: the 1/3 plateau accompanying the spontaneous symmetry breaking of the translation, the cusp singularities above and/or below the plateau, and the even-odd effect in the magnetization curve.

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  • Field-Induced Phase Transitions and Long-Range Orders in the S=1/2 Spin Bond-Alternating Chain with Frustrating Interaction 査読

    Nobuya Maeshima, Kouichi Okunishi, Kiyomi Okamoto, Toru Sakai, Kenji Yonemitsu

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   74   2005年

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PHYSICAL SOC JAPAN  

    We study field-induced phase transitions and long-range orders in the S = 1/2 spin bond-alternating chain with frustrating interaction. By using the inter-chain mean field theory combined with the finite temperature density matrix renormalization group, we investigate properties of field-induced long-range orders and phase transitions. We find that the inter-chain interaction strongly affects the realized phases of this system.

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  • High-field magnetization of S = 1 antiferromagnetic bond-alternating chain compounds 査読

    Yasuo Narumi, Koichi Kindo, Masayuki Hagiwara, Hiroki Nakano, Akira Kawaguchi, Kouichi Okunishi, Masanori Kohno

    Physical Review B - Condensed Matter and Materials Physics   69 ( 17 )   1 - 174405   2004年5月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    High-field magnetization and susceptibility measurements have been performed on S = 1 antiferromagnetic bond-alternating chain compounds: [Ni 2(dpt)2(μ-ox)(μ-N3)](PF6) [dpt=bis(3-aminopropyl)amine, ox= C2O4] (abbreviated as NDOAP) and [Ni(333-tet)(μ-tet)(μ-NO2)] ClO4 [333-tet=N,N′-bis(3-aminopropyl)-1,3-propanediamine] (abbreviated as NTENP). The magnetization of NDOAP shows two steep increases at about 200 and 500 kOe. A magnetization plateau is observed between about 300 and 500 kOe, corresponding to half the value of the saturation moment. Comparison between numerical calculations and the experimental results, including the magnetic susceptibility, shows that NDOAP has a singlet ground state due to dimerization with a bond-alternating ratio α=0.1. The half-saturation plateau is also observed around 700 kOe in the magnetization of NTENP. The magnetization process of NTENP is well reproduced by the numerical result calculated for the parameter of α=0.45 in the singlet-dimer phase. The a dependence of the magnetization processes is discussed.

    DOI: 10.1103/PhysRevB.69.174405

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  • Magnetization plateau and cusp in S=1 spin ladder 査読

    T Sakai, K Okamoto, K Okunishi, K Kindo, Y Narumi, Y Hosokoshi, K Katoh, K Inoue, T Goto

    PHYSICA B-CONDENSED MATTER   346   34 - 37   2004年4月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    Recently, the organic compound biphenyl (BIP-TENO) was synthesized and found to be the first S = 1 spin ladder. The present high-field measurement up to 70 T indicates that a plateau appears in the magnetization curve at M = M(s)/4, where Ms is the saturation magnetization. We propose possible mechanisms of the M = M(s)/4 plateau in BIP-TENO. The frustration in the first, second and third neighbor antiferromagnetic exchange interactions in the S = 1 ladder yields the M = M(s)/4 plateau, with a spontaneous breaking of the translational symmetry. This can be known by use of the degenerate perturbation theory from the strong rung coupling limit. By use of the numerical diagonalization method, we revealed that the most realistic mechanism of the M = M(s)/4 plateau in BIP-TENO is due to the third neighbor interaction. Further theoretical analyses suggest the present mechanism should also lead to a cusp in the magnetization curve. The cusp-like anomaly in the experimental magnetization curve is a considerable evidence for our mechanism. We also discuss the M(s)/2 plateau problem. We show that a small rung coupling cannot yield the M(s)/2 plateau, which is quite different from the M = 0 plateau problem of the S = 1/2 ladder. 2 (C) 2004 Elsevier B.V. All rights reserved.

    DOI: 10.1016/j.physb.2004.01.015

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  • Magnetization plateaux in an anisotropic S=1/2 antiferromagnetic chain with frastration and bond-alternation 査読

    T Tonegawa, K Okamoto, K Okunishi, K Nomura, M Kaburagi

    PHYSICA B-CONDENSED MATTER   346   50 - 54   2004年4月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    Using numerical methods, we study the finite-field ground state of an anisotropic S = 1/2 antiferromagnetic chain with bond-alternating nearest-neighbor and uniform next-nearest-neighbor interactions. Special attention is paid to the magnetization plateaux at a half and one-third of the saturation magnetization, which appear in the ground-state magnetization curve. (C) 2004 Elsevier B.V. All rights reserved.

    DOI: 10.1016/j.physb.2004.01.018

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  • Anomalous magnetization process in frustrated spin ladders 査読

    Tôru Sakai, Kiyomi Okamoto, Kouichi Okunishi, Masahiro Sato

    Journal of Physics Condensed Matter   16 ( 11 )   S785 - S789   2004年3月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    We study, at T = 0, the anomalies in the magnetization curve of the S = 1 two-leg ladder with frustrated interactions. We focus mainly on the existence of the M = Ms/2 plateau, where Ms is the saturation magnetization. We report the results using the degenerate perturbation theory and the density matrix renormalization group, which lead to consistent conclusion with each other. We also touch on the M = Ms/4 and (3/4)Ms plateaux and cusps.

    DOI: 10.1088/0953-8984/16/11/030

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  • The magnetization process of an Ising-type frustrated S=1 spin chain 査読

    M Kaburagi, M Kang, T Tonegawa, K Okunishi

    JOURNAL OF PHYSICS-CONDENSED MATTER   16 ( 11 )   S765 - S772   2004年3月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:IOP PUBLISHING LTD  

    The magnetization process of an Ising-type S = 1 spin chain with nearest-neighbour interaction (J(1)), next-nearest-neighbour interaction (J(2)) and single-ion anisotropy (D) is investigated on the basis of the ground state phase diagram of the chain in a magnetic field (H greater than or equal to 0). The 49 possible spin structures for the ground state are obtained using Morita's theorem. The magnetization process is obtained from the ground state phase diagram, giving J(2)/J(1) versus H/J(1), determined by comparing the energies of the 49 spin structures. It is shown that (1) among the 49 spin structures, 14 structures with unit cells of &lt;+&gt;, &lt;0&gt;, &lt;+-&gt;, &lt;+0&gt;, &lt;+ + -&gt;, &lt;+ + 0&gt;, &lt;+ - 0&gt;, &lt;+00&gt;, &lt;+ + - -&gt;, &lt;+ + 0-&gt;, &lt;+ + 00&gt;, &lt;+ + -0&gt;, &lt;+ - +00&gt; and &lt;+ + -0 + 0-&gt; appear in the phase diagram; (2) the magnetization process is equivalent to that of the usual Ising model for D less than or equal to 0; (3) a long period spin structure, &lt;+ + -0 + 0-&gt;, is realized for the intermediate ranges of J(2)/J(1) and D/J(1) due to the frustration of the system.

    DOI: 10.1088/0953-8984/16/11/027

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  • Fractional S-z excitation and its bound state around the 1/3 plateau of the S=1/2 Ising-like zigzag XXZ chain 査読

    K Okunishi, T Tonegawa

    PHYSICAL REVIEW B   68 ( 22 )   2003年12月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMERICAN PHYSICAL SOC  

    We present the microscopic view for the excitations around the 1/3 plateau state of the Ising-like zigzag XXZ chain. We analyze the low-energy excitations around the plateau with the degenerating perturbation theory from the Ising limit, combined with the Bethe-form wave function. We then find that the domain-wall particles carrying S-z=+/-1/3 and its bound state of S-z=+/-2/3 describe well the low-energy excitations around the 1/3 plateau state. The formation of the bound state of the domain walls clearly provides the microscopic mechanism of the cusp singularities and the even-odd behavior in the magnetization curve.

    DOI: 10.1103/PhysRevB.68.224422

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  • Magnetization curves of quasi-one-dimensional haldane systems 査読

    A Kawaguchi, A Koga, N Kawakami, K Okunishi

    INTERNATIONAL JOURNAL OF MODERN PHYSICS B   16 ( 20-22 )   3339 - 3342   2002年8月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:WORLD SCIENTIFIC PUBL CO PTE LTD  

    We study the magnetization process of a quasi-one-dimensional S=1 antiferromagnet with bond alternation by using the density matrix renormalization group method combined with interchain mean-field approximation. Particularly, we discuss how the interchain couplings affect the magnetization curve around a plateau structure in both of the Haldane phase and the dimer phase. It is shown that the antiferromagnetic correlation induced by the interchain couplings reduces the region of the plateau significantly, which will play an important role when the theoretical results are compared with actual measurements of the magnetization.

    DOI: 10.1142/S021797920201436X

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  • Magnetization process for a quasi-one-dimensional S=1 antiferromagnet 査読

    A Kawaguchi, A Koga, K Okunishi, N Kawakami

    PHYSICAL REVIEW B   65 ( 21 )   2002年6月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMERICAN PHYSICAL SOC  

    We investigate the magnetization process for a quasi-one-dimensional S=1 antiferromagnet with bond alternation. By combining the density matrix renormalization group method with the interchain mean-field theory, we discuss how the interchain coupling affects the magnetization curve. It is found that the width of the magnetization plateau is considerably reduced upon introducing the interchain coupling. We obtain the phase diagram in a magnetic field. The effect of single-ion anisotropy is also addressed.

    DOI: 10.1103/PhysRevB.65.214405

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  • Hole-doping effects on orthogonal-dimer spin chain 査読

    A Kawaguchi, A Koga, K Okunishi, N Kawakami

    JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS   63 ( 6-8 )   1431 - 1434   2002年6月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PERGAMON-ELSEVIER SCIENCE LTD  

    We study the hole-doping effects on the frustrated orthogonal-dimer spin model with linked-tetrahedra chain structure. By using the density matrix renormalization group method we discuss how the metal-insulator transition is affected by geometrical frustration. When the system is in the dimer-singlet phase, hole-doping smoothly drives the system to a metallic phase. In contrast when the system is in the plaquette-singlet phase strong frustration induces the first-order transition to a metallic phase upon hole-doping which is characterized by the discontinuity in the electron density. We compute the spin correlation function to clarify the origin of the first-order phase transition. (C) 2002 Elsevier Science Ltd. All rights reserved.

    DOI: 10.1016/S0022-3697(02)00106-3

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  • Matrix product state approximation for the maximum-eigenvalue eigenstate of the quantum transfer matrix 査読

    N Maeshima, Y Hieida, T Nishino, K Okunishi

    PROGRESS OF THEORETICAL PHYSICS SUPPLEMENT   ( 145 )   204 - 207   2002年

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PROGRESS THEORETICAL PHYSICS PUBLICATION OFFICE  

    We propose a matrix product state (MPS) formulation to calculate thermodynamic quantities of one dimensional (1D) quantum systems. The maximum-eigenvalue eigenstate of the quantum transfer matrix is represented as the product, of local matrices, which are obtained by the DMRG method for the two dimensional (2D) classical system mapped from the original ID quantum system. This MPS formulation is successfully applied to the S = 1/2 XY model.

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  • Cusp singularity in the magnetization curve of 1D quantum spin systems 査読

    K Okunishi

    PROGRESS OF THEORETICAL PHYSICS SUPPLEMENT   ( 145 )   119 - 124   2002年

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PROGRESS THEORETICAL PHYSICS PUBLICATION OFFICE  

    We give a short review of the cusp singularity in the magnetization curve of ID quantum spin systems. We show that the cusp singularity appears for a certain class of nonintegrable frustrated spin systems, using the density matrix renormaziation group method. We also discuss the cusp appearance mechanism on the basis of the one particle dispersion curve of the elementary excitation, where the shape change of the dispersion curve induced by the frustrated interaction plays an essential role.

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  • First-order phase transitions in frustrated spin systems 査読

    A Koga, A Kawaguchi, K Okunishi, N Kawakami

    PROGRESS OF THEORETICAL PHYSICS SUPPLEMENT   ( 145 )   52 - 57   2002年

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PROGRESS THEORETICAL PHYSICS PUBLICATION OFFICE  

    We give a short review of our recent works on the first-order quantum phase transitions in frustrated spin chains with orthogonal-dimer structure. When the ratio of the competing antiferromagnetic exchange couplings is varied, a first-order transition occurs between the dimer phase and the plaquette phase, which is accompanied by the discontinuity in the spin excitation gap. We further show that strong frustration triggers the phase transitions in a magnetic field, which exhibit plateaus and jumps in the magnetization curve. The hole-doping effect is also addressed for the orthogonal-dimer chain with linked-tetrahedra structure. It is found that the competing antiferromagnetic interactions result in a first-order metal-insulator transition upon hole doping.

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  • Anisotropic antiferromagnetic spin chains in a transverse field: Reentrant behavior of the staggered magnetization 査読

    Yasuhiro Hieida, Kouichi Okunishi, Yasuhiro Akutsu

    Physical Review B - Condensed Matter and Materials Physics   64 ( 22 )   2244221 - 2244226   2001年12月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    We investigate one-dimensional S = 1/2 and S = 1 antiferromagnetic quantum-spin chains with easy-axis anisotropies in a transverse field. We calculate both the uniform magnetization and the staggered magnetization, using a variant of the density-matrix renormalization-group method. We find that the staggered magnetizations exhibit reentrant behavior as functions of the transverse field, where the competition between the classical Néel order and the quantum fluctuations plays an important role. We also discuss the critical behavior associated with the staggered magnetizations.

    DOI: 10.1103/PhysRevB.64.224422

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  • Spinon excitation and Mobius boundary condition in S=1/2 antiferromagnetic Heisenberg spin ladder with zigzag structure 査読

    K Okunishi, N Maeshima

    PHYSICAL REVIEW B   64 ( 21 )   2001年12月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER PHYSICAL SOC  

    We investigate the low-lying excitation of the antiferromagnetic zigzag spin ladder with Mobius boundary condition. Using the Lanczos and Householder diagonalization methods, we calculate the excitation spectrum of the zigzag ladder in the momentum space. We then show that the topological defect generated by the Mobius boundary provides the clear evidence of the single-spinon excitation. On the basis of the obtained single-spinon dispersion curve, we analyze the two-spinon scattering states under the usual periodic boundary condition. We further discuss the interaction effect between the spinons, and the connection to the cusp singularity in the magnetization process.

    DOI: 10.1103/PhysRevLett.87.212406

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  • Vertical density matrix algorithm: A higher-dimensional numerical renormalization scheme based on the tensor product state ansatz 査読

    N Maeshima, Y Hieida, Y Akutsu, T Nishino, K Okunishi

    PHYSICAL REVIEW E   64 ( 1 )   2001年7月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMERICAN PHYSICAL SOC  

    We present a new algorithm to calculate the thermodynamic quantities of three-dimensional (3D) classical statistical systems, based on the ideas of the tensor product state and the density matrix renormalization group. We represent the maximum-eigenvalue eigenstate of the transfer matrix as the product of local tensors that are iteratively optimized by the use of the "vertical density matrix" formed by cutting the system along the transfer direction. This algorithm, which we call vertical density matrix algorithm (VDMA), is successfully applied to the 3D Ising model. Using the Suzuki-Trotter transformation we can also apply the VDMA to 2D quantum systems, which we demonstrate for the 2D transverse field Ising model.

    DOI: 10.1103/PhysRevE.64.016705

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  • High-field magnetization of an S=1/2 zigzag chain compound (N2H5)CuCl3 査読

    M Hagiwara, Y Narumi, K Kindo, N Maeshima, K Okunishi, T Sakai, M Takahashi

    PHYSICA B   294   83 - 86   2001年1月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    The results of magnetic susceptibility and high held magnetization measurements on a single crystal sample of copper zigzag chain compound (N2H5)CuCl3 are reported. Magnetic susceptibility of this compound shows a round maximum around 10K and decreases gradually. High-field magnetization up to 300kOe of this compound shows a monotonic increase with a concave curvature. We compared the experimental results with those of calculation by the temperature-dependent density-matrix-renormalization-group method and obtained fairly good agreement between them when the ratio of the next-nearest-neighbor exchange constant to the nearest neighbor one is 0.2 similar to 0.3. (C) 2001 Elsevier Science B.V. All rights reserved.

    DOI: 10.1016/S0921-4526(00)00614-1

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  • Spinon excitation and Möbius boundary condition in (formula presented) antiferromagnetic Heisenberg spin ladder with zigzag structure 査読

    Kouichi Okunishi, Nobuya Maeshima

    Physical Review B - Condensed Matter and Materials Physics   64 ( 21 )   2001年

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    We investigate the low-lying excitation of the antiferromagnetic zigzag spin ladder with Möbius boundary condition. Using the Lanczos and Householder diagonalization methods, we calculate the excitation spectrum of the zigzag ladder in the momentum space. We then show that the topological defect generated by the Möbius boundary provides the clear evidence of the single-spinon excitation. On the basis of the obtained single-spinon dispersion curve, we analyze the two-spinon scattering states under the usual periodic boundary condition. We further discuss the interaction effect between the spinons, and the connection to the cusp singularity in the magnetization process. © 2001 The American Physical Society.

    DOI: 10.1103/PhysRevB.64.212406

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  • First-order quantum phase transition in the orthogonal-dimer spin chain 査読

    A Koga, K Okunishi, N Kawakami

    PHYSICAL REVIEW B   62 ( 9 )   5558 - 5563   2000年9月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER PHYSICAL SOC  

    We investigate the low-energy properties of the orthogonal-dimer spin chain characterized by a frustrated dimer-plaquette structure. When the competing antiferromagnetic couplings are varied, the first-order quantum phase transition occurs between the dimer and the plaquette phases, which is accompanied by nontrivial features due to frustration. besides the discontinuity in the lowest excitation gap at the transition point, a sharp level crossing occurs for the spectrum in the plaquette phase. We further reveal that the plateau in the magnetization curve at 1/4 of the full moment dramatically changes its character in the vicinity of the critical point. It is argued that the first-order phase transition in this system captures some essential properties found in the two-dimensional orthogonal-dimer model proposed for SrCu2(BO3)(2).

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  • Antiferromagnetic zigzag spin chain in magnetic fields at finite temperatures 査読

    N Maeshima, K Okunishi

    PHYSICAL REVIEW B   62 ( 2 )   934 - 939   2000年7月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER PHYSICAL SOC  

    We study thermodynamic behaviors of the antiferromagnetic zigzag spill chain in magnetic fields, using the density-matrix renormalization-group method for the quantum transfer matrix. We focus on the thermodynamics of the system near the critical fields in the ground-state magnetization process (M-H curve): the saturation field, the lower critical field associated with excitation gap, and the field at the middle-field cusp singularity. We calculate magnetization. susceptibility, and specific heat of the zigzag chain in magnetic fields at finite temperatures. and then discuss how the calculated quantities reflect the low-lying excitations of the system related with the critical behaviors in the M-H curve.

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  • Middle-field cusp singularities in the magnetization process of one-dimensional quantum antiferromagnets 査読

    K Okunishi, Y Hieida, Y Akutsu

    PHYSICAL REVIEW B   60 ( 10 )   R6953 - R6956   1999年9月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER PHYSICAL SOC  

    We study the zero-temperature magnetization process (M-H curve) of one-dimensional quantum antiferromagnets using a variant of the density-matrix renormalization group method. For both the S=1/2 zig-zag spin ladder and the S=1 bilinear-biquadratic chain, we find clear cusp-type singularities in the middle-field region of the M-H curve. These singularities are successfully explained in terms of the double-minimum shape of the energy dispersion of the low-lying excitations. For the S=1/2 zig-zag spin ladder, we find that the cusp formation accompanies the Fermi-liquid to non-Fermi-liquid transition. [S0163-1829(99)50834-5].

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  • Magnetization process of bilinear-biquadratic spin chains at finite temperature 査読

    K Okunishi

    PHYSICAL REVIEW B   60 ( 6 )   4043 - 4052   1999年8月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER PHYSICAL SOC  

    We investigate magnetization process (M-H curve) of S = 1 bilinear-biquadratic (BLBQ) spin chain near critical fields at finite temperatures. We use the density matrix renormalization group (DMRG) for the two-dimensional classical lattice model mapped by the Trotter decomposition, with a help of the Baxter's variational principle. By comparing the DMRG result of S = 1/2 XY chain with the exact solution, we show that the DMRG is an efficient tool to calculate the M-H curve at finite temperatures. Further, we compute the M-H curve of the BLBQ chain. We compare the DMRG curve of the magnetization process of the BLBQ chain with those obtained by analytic approaches: correctly mapped delta-function. Bose-gas approach acid "Bethe-ansatz approximation" approach. Near the saturation field, we show that the delta-function Bose gas and the Bethe-ansatz approximation describe the M-H curve well in both at zero temperature and finite temperatures. Near the lower critical field, we find that the delta-function Bose gas. is a good effective model at the Heisenberg point. We further find that the delta-function Bose gas cannot describe the M-H curve at finite temperatures, near the special point where the M-H curve changes qualitatively. [S0163-1829(99)02730-7].

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  • Universal asymptotic eigenvalue distribution of density matrices and corner transfer matrices in the thermodynamic limit 査読

    K Okunishi, Y Hieida, Y Akutsu

    PHYSICAL REVIEW E   59 ( 6 )   R6227 - R6230   1999年6月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMERICAN PHYSICAL SOC  

    We study the asymptotic behavior of the eigenvalue distribution of the corner transfer matrix (M-CTM) and the density matrix (M-DM) in the density-matrix renormalization group. We utilize the relationship MDM = M-CTM(4), which holds for noncritical systems in the thermodynamic limit. We derive the exact and universal asymptotic form of the hl,M-DM eigenvalue distribution for a class of integrable models in the massive regime. For nonintegrable models, the universal asymptotic form is also verified by numerical renormalization group calculations. [S1063-651X(99)50806-4].

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  • delta-function Bose-gas picture of S=1 antiferromagnetic quantum spin chains near critical fields 査読

    K Okunishi, Y Hieida, Y Akutsu

    PHYSICAL REVIEW B   59 ( 10 )   6806 - 6812   1999年3月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER PHYSICAL SOC  

    We study the zero-temperature magnetization curve (M-H curve) of the one-dimensional quantum anti-ferromagnet of spin one. The Hamiltonian H we consider is of the bilinear-biquadratic form: H = Sigma(i)f((s) over right arrow(i).s(i+1)) (+Zeeman term) where (s) over right arrow(i) is the spin operator at site i and f(X) = X + beta X-2 with 0 less than or equal to beta &lt; 1. We focus on validity of the delta-function Bose-gas picture near the two critical fields: upper-critical field H-s above which the magnetization saturates and the lower-critical field H-c associated with the Haldane gap. As for the behavior near H-s, we take ''low-energy effective S matrix" approach, where the correct effective Bose-gas coupling constant c is extracted from the two down-spin S matrix in its low-energy limit. We find that the resulting value of c differs from the spin-wave value. We draw the M-H curve by using the resultant Bose gas, and compare it with numerical calculation where the product-wave-function renormalization-group (PWFRG) method, a variant of White's density-matrix renormalization group method, is employed. Excellent agreement is seen between the PWFRG calculation and the correctly mapped Bose-gas calculation. We also test the validity of the Bose-gas picture near the lower-critical field H-c. Comparing the PWFRG-calculated M similar to H curves with the Bose-gas prediction, we find that there are two distinct regions, I and II, of beta separated by a critical value beta(c)(approximate to 0.41). In region I, 0 &lt; beta &lt; beta(c), the effective Bose coupling c is positive but rather small. The small value of c makes the "critical region" of the square-root behavior M similar to root H-H-c very narrow. Further, we find that in the beta --&gt; beta(c) - 0, the square-root behavior transmutes to a different one, M similar to (H - H-c)(theta) with theta approximate to 1/4. In region II, beta(c) &lt; beta &lt; 1, the square-root behavior is more pronounced as compared with region I, but the effective coupling c becomes negative. [S0163-1829(99)09009-8].

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  • Density matrix renormalization group: Introduction from a variational point of view 査読

    T Nishino, T Hikihara, K Okunishi, Y Hieida

    INTERNATIONAL JOURNAL OF MODERN PHYSICS B   13 ( 1 )   1 - 24   1999年1月

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    記述言語:英語   出版者・発行元:WORLD SCIENTIFIC PUBL CO PTE LTD  

    The density matrix renormalization group theory is reviewed as a numerical variational method. The variational state, expressed as a product of local tensors, is improved through locally tuning each tensor. The first section is a tutorial with simplified discussions. Details are discussed in the subsequent sections. The review concludes with some recent developments and future directions.

    DOI: 10.1142/S0217979299000023

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  • Numerical renormalization approach to two-dimensional quantum antiferromagnets with valence-bond-solid type ground state 査読

    Yasuhiro Hieida, Kouichi Okunishi, Yasuhiro Akutsu

    NEW JOURNAL OF PHYSICS   1   1999年

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:IOP PUBLISHING LTD  

    We study the ground-state properties of two-dimensional quantum spin systems having the valence-bond-solid (VBS) type ground states. The 'product-of-tensors' form of the ground-state wavefunction of the system is utilized to associate it with an equivalent classical lattice statistical model which can be treated by the transfer-matrix method. For diagonalization of the transfer matrix, we employ the product-wavefunction renormalization group method, which is a variant of the density-matrix renormalization group method. We obtain the correlation length and the sublattice magnetization accurately. For the anisotropically 'deformed' S = 3/2 VBS model on the honeycomb lattice, we find that the correlation length as a function of the deformation parameter behaves very much as that in the S = 3/2 VBS chain.

    DOI: 10.1088/1367-2630/1/1/007

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  • Transfer-matrix approach to classical systems 査読

    T Nishino, K Okunishi, Y Hieida, T Hikihara, H Takasaki

    DENSITY-MATRIX RENORMALIZATION   528   127 - 148   1999年

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    記述言語:英語   掲載種別:研究論文(国際会議プロシーディングス)   出版者・発行元:SPRINGER-VERLAG BERLIN  

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  • Density matrix and renormalization for classical lattice models 査読

    T Nishino, K Okunishi

    STRONGLY CORRELATED MAGNETIC AND SUPERCONDUCTING SYSTEMS   478   167 - 183   1997年

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    記述言語:英語   掲載種別:研究論文(国際会議プロシーディングス)   出版者・発行元:SPRINGER-VERLAG BERLIN  

    The density matrix renormalization group is a variational approximation method that maximizes the partition function - or minimize the ground state energy - of quantum lattice systems. The variational relation is expressed as Z = Tr rho greater than or equal to Tr (&lt;(1) over tilde&gt;rho), where rho is the density submatrix of the system, and (1) over tilde is a projection operator. In this report we apply the variational relation to two-dimensional (2D) classical lattice models, where the density submatrix rho is obtained as a product of the corner transfer matrices. The obtained renormalization group method for 2D classical lattice model, the corner transfer matrix renormalization group method, is applied to the q = 2 similar to 5 Potts models. With the help of the finite size scaling, critical exponents (q = 2, 3) and the latent heat (q = 5) are precisely obtained.

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▶ 全件表示

書籍等出版物

  • Transfer matrix approach to classical systems(共著)

    Springer, Berlin  1999年 

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  • density matrix renormalization for classical lattice models(共著)

    Springer, Berlin  1997年 

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MISC

  • Corner transfer matrix renormalization group analysis of the two-dimensional dodecahedron model 査読

    Hiroshi Ueda, Kouichi Okunishi, Seiji Yunoki, Tomotoshi Nishino

    arXiv:2004.08669   102 ( 3 )   2020年9月

  • Finite- m scaling analysis of Berezinskii-Kosterlitz-Thouless phase transitions and entanglement spectrum for the six-state clock model 査読

    Hiroshi Ueda, Kouichi Okunishi, Kenji Harada, Roman Krčmár, Andrej Gendiar, Seiji Yunoki, Tomotoshi Nishino

    Physical Review E   2020年6月

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    記述言語:英語  

    We investigate the Berezinskii-Kosterlitz-Thouless transitions for the
    square-lattice six-state clock model with the corner-transfer matrix
    renormalization group (CTMRG). Scaling analyzes for effective correlation
    length, magnetization, and entanglement entropy with respect to the cutoff
    dimension $m$ at the fixed point of CTMRG provide transition temperatures
    consistent with a variety of recent numerical studies. We also reveal that the
    fixed point spectrum of the corner transfer matrix in the critical intermediate
    phase of the six-state clock model is characterized by the scaling dimension
    consistent with the $c=1$ boundary conformal field theory associated with the
    effective $Z_6$ dual sine-Gordon model.

    DOI: 10.1103/physreve.101.062111

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  • Phase Transitions for Cuboc Orders in Stacked Kagome Heisenberg Systems 査読

    Seki, Kouichi, Okunishi, Kouichi

    Journal of the Physical Society of Japan   87 ( 2 )   2018年2月

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    記述言語:英語   出版者・発行元:Physical Society of Japan  

    Using an event-chain Monte Carlo (MC) algorithm, we investigate phase transitions of the stacked kagome Heisenberg systems up to the third-nearest-neighbor couplings with classical vector spins. In particular, we focus on two types of non-coplanar spin orders -cuboc1 and cuboc2-, both of which have 12 sublattice structures accompanied by translational symmetry breaking. We perform event-chain MC simulations up to L = 72, where L represents the linear dimension of the stacked kagome lattice, and find that the cuboc1 transition shows second-order-transition behaviors with a tendency of a weak-first-order transition up to L = 72, while the cuboc2 transition is basically described by the first-order transition. We then discuss the above transitions in connection with the effective Landau-Ginzburg-Wilson theory with the O(3) × O(3) symmetry. <P />...

    DOI: 10.7566/JPSJ.87.023001

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  • Snapshot spectra in the world-line quantum Monte Carlo for one-dimensional quantum spin systems 査読

    Seki, Kouichi, Okunishi, Kouichi

    2017年6月

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    出版者・発行元:arXiv e-prints  

    We introduce a snapshot density matrix and snapshot spectrum for world-line (WL) quantum Monte Carlo simulations, by integrating out the continuous-imaginary-time index of WL snapshots. For the transverse-field Ising chain, we reveal fundamental properties of the snapshot spectrum as follows. (i) In the ordered phase, the isolated peak corresponding to the maximum eigenvalue appears, indicating the classical nature of WL configurations. (ii) In the disordered phase, the spectrum converges to the dome-like distribution characterized by an effective aspect ratio of the snapshots. (iii) At the critical point, the power-law distribution is observed with an appropriate scaling analysis of the fixed aspect ratio. For the XXZ chain, we also obtain numerical results consistent with the above properties. In addition, the representation-basis dependence of the snapshot spectrum is also discussed. <P />...

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  • Sine-square deformation and Mobius quantization of two-dimensional conformal field theory 査読

    Kouichi Okunishi

    PROGRESS OF THEORETICAL AND EXPERIMENTAL PHYSICS   ( 6 )   2016年3月

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    記述言語:英語   出版者・発行元:arXiv e-prints  

    Motivated by sine-square deformation (SSD) for quantum critical systems in 1+1-dimension, we discuss a Mobius quantization approach to the two-dimensional conformal field theory (CFT), which bridges the conventional radial quantization and the dipolar quantization recently proposed by Ishibashi and Tada. We then find that the continuous Virasoro algebra of the dipolar quantization can be interpreted as a continuum limit of the Virasoro algebra for scaled generators in the SSD limit of the Mobius quantization approach. <P />...

    DOI: 10.1093/ptep/ptw060

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  • 擬一次元反強磁性体SrCo<sub>2</sub>V<sub>2</sub>O<sub>8</sub>の強磁場ESR

    奥谷顕, 木田孝則, 薄井智靖, 木村剛, 奥西巧一, 萩原政幸

    電子スピンサイエンス学会年会講演要旨集   54th   158 - 159   2015年11月

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    記述言語:日本語  

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  • 擬一次元反強磁性体SrCo<sub>2</sub>V<sub>2</sub>O<sub>8</sub>の強磁場磁性

    奥谷顕, 木田孝則, 薄井智靖, 木村剛, 奥西巧一, 萩原政幸

    日本物理学会講演概要集(CD-ROM)   70 ( 2 )   ROMBUNNO.16APS-35   2015年9月

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  • Magnetic phase diagram of the coupled triangular spin tubes for CsCrF<SUB>4</SUB> 査読

    Seki, Kouichi, Okunishi, Kouichi

    Physical Review B   91 ( 22 )   2015年6月

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    記述言語:英語   出版者・発行元:AMER PHYSICAL SOC  

    Using Monte Carlo simulations, we explore the magnetic phase diagram of triangular spin tubes coupled with a ferromagnetic intertube interaction for CsCrF<SUB>4</SUB>. The planar structure of the coupled tubes is topologically equivalent to the kagome-triangular lattice, which induces nontrivial frustration effects in the system. We particularly find that, depending on the intertube coupling, various ordered phases are actually realized, such as incommensurate order, ferromagnetic order, and cuboc order, which is characterized by the noncoplanar spin structure of the 12 sublattices accompanying the spin chirality breaking. We also discuss the relevance of the results to recent experiments on CsCrF<SUB>4</SUB>. <P />...

    DOI: 10.1103/PhysRevB.91.224403

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  • Field-induced incommensurate phase in the strong-rung spin ladder with ferromagnetic legs 査読

    Yamaguchi, H., Miyagai, H., Yoshida, M., Takigawa, M., Iwase, K., Ono, T., Kase, N., Araki, K., Kittaka, S., Sakakibara, T., Shimokawa, T., Okubo, T., Okunishi, K., Matsuo, A., Hosokoshi, Y.

    Physical Review B   89 ( 22 )   2014年6月

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    記述言語:英語   出版者・発行元:AMER PHYSICAL SOC  

    We report magnetization, specific heat, and NMR measurements of 3-Br-4-F-V [=3-(3-bromo-4-fluorophenyl)-1,5-diphenylverdazyl], a strong-rung S =1/2 Heisenberg spin ladder with ferromagnetic leg interactions. We explain the magnetic and thermodynamic properties based on the strong-rung regime. Furthermore, we find a field-induced successive phase transition in the specific heat and the nuclear spin-lattice relaxation rate 1/T<SUB>1</SUB>. <SUP>19</SUP>F-NMR spectra for higher- and lower-temperature phases indicate partial magnetic order and incommensurate long-range order, respectively, evidencing the presence of frustration due to weak interladder couplings. <P />...

    DOI: 10.1103/PhysRevB.89.220402

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  • 24aPS-69 擬スピン一次元系BaCo_2V_2O_8の弾性ソフト化(24aPS 領域3ポスターセッション,領域3(磁性,磁気共鳴))

    赤津 光洋, 木村 尚次郎, 萩原 政幸, 早坂 千明, 三本 啓輔, 荒木 幸治, 根本 祐一, 奥西 巧一, 後藤 輝孝, 大村 公美子, 山口 博則

    日本物理学会講演概要集   67 ( 0 )   2012年

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    記述言語:日本語   出版者・発行元:一般社団法人 日本物理学会  

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  • Topological disentangler for the valence-bond-solid chain

    Kouichi Okunishi

    PHYSICAL REVIEW B   83 ( 10 )   2011年3月

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    記述言語:英語   掲載種別:機関テクニカルレポート,技術報告書,プレプリント等   出版者・発行元:AMER PHYSICAL SOC  

    We discuss the topological disentangler for S = 1 quantum spin chains in the Haldane phase. We first point out that theKennedy-Tasaki (KT) nonlocal unitary transformation is the perfect disentangler for the Affleck-Kennedy-Lieb-Tasaki model. We then demonstrate that the KT transformation can be reconstructed as an assembly of pair disentanglers. Finally, we show that the KT transformation can be regarded as a topological disentangler, which selectively disentangles the twofold degeneracy in the entanglement spectrum of the S = 1 Heisenberg chain.

    DOI: 10.1103/PhysRevB.83.104411

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  • 27aTG-5 擬一次元Ising型反強磁性体BaCo_2V_2O_8の強磁場中放射光X線回折(27aTG 量子スピン系(一次元系及びクラスター系),領域3(磁性,磁気共鳴))

    木村 尚次郎, 金道 浩一, 何 長振, 谷山 智康, 伊藤 満, 豊川 秀訓, 石川 哲也, 北村 英男, 萩原 政幸, 吉居 俊輔, 鳴海 康雄, 寺田 典樹, 田中 良和, 岩城 雅裕, 勝又 紘一, 奥西 功一

    日本物理学会講演概要集   64 ( 0 )   2009年

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    記述言語:日本語   出版者・発行元:一般社団法人 日本物理学会  

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  • Longitudinal Spin Density Wave Order in a Quasi-1D Ising-like Quantum Antiferromagnet

    S. Kimura, M. Matsuda, T. Masuda, S. Hondo, K. Kaneko, N. Metoki, M. Hagiwara, T. Takeuchi, K. Okunishi, Z. He, K. Kindo, T. Taniyama, M. Itoh

    PHYSICAL REVIEW LETTERS   101 ( 20 )   2008年11月

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    記述言語:英語   掲載種別:機関テクニカルレポート,技術報告書,プレプリント等   出版者・発行元:AMER PHYSICAL SOC  

    From neutron diffraction measurements on a quasi-1D Ising-like Co2+ spin compound BaCo2V2O8, we observed an appearance of a novel type of incommensurate ordering in magnetic fields. This ordering is essentially different from the Neel-type ordering, which is expected for the classical system, and the peculiar spin structure is caused by quantum fluctuation inherent in the quantum spin chain. A Tomonaga-Luttinger liquid nature characteristic of the gapless quantum 1D system is responsible for the realization of the incommensurate ordering.

    DOI: 10.1103/PhysRevLett.101.207201

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  • On Calculation of Vector Spin Chirality for Zigzag Spin Chains

    Kouichi Okunishi

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   77 ( 11 )   2008年11月

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    記述言語:英語   掲載種別:機関テクニカルレポート,技術報告書,プレプリント等   出版者・発行元:PHYSICAL SOC JAPAN  

    We calculate the vector spin chirality for S = 1/2 zigzag spin chains having U(l) symmetry, using the density matrix renormalization group combined with unitary transformation. We then demonstrate the occurrence of the chiral order for the zigzag XY chain and discuss the associated phase transition. The results are consistent with the analysis based on the bosonization and the long distance behaviour of the chirality correlation function. For the S = 1/2 zigzag Heisenberg chain in a magnetic field, we also verify the chiral order that is predicted by the effective field theory and the chirality correlation function, and then determine its magnetic phase diagram.

    DOI: 10.1143/JPSJ.77.114004

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  • Novel ordering of an S=1/2 quasi-1D Ising-like antiferromagnet in magnetic field

    S. Kimura, T. Takeuchi, K. Okunishi, M. Hagiwara, Z. He, K. Kindo, T. Taniyama, M. Itoh

    PHYSICAL REVIEW LETTERS   100 ( 5 )   2008年2月

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    記述言語:英語   掲載種別:機関テクニカルレポート,技術報告書,プレプリント等   出版者・発行元:AMER PHYSICAL SOC  

    High-field specific heat measurements on BaCo2V2O8, which is a good realization of an S=1/2 quasi-one-dimensional (1D) Ising-like antifferomagnet, have been performed in magnetic fields up to 12 T along the chain and at temperature down to 200 mK. We have found a new magnetic ordered state in the field-induced phase above H-c similar or equal to 3.9 T. We suggest that a novel type of the incommensurate order, which is caused by the quantum effect inherent in the S = 1/2 quasi-1D Ising-like antiferromagnet, appears in the field-induced phase.

    DOI: 10.1103/PhysRevLett.100.057202

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  • Wilson-like real-space renormalization group and low-energy effective spectrum of the XXZ chain in the critical regime

    Kouichi Okunishi

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   76 ( 6 )   2007年6月

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    記述言語:英語   掲載種別:機関テクニカルレポート,技術報告書,プレプリント等   出版者・発行元:PHYSICAL SOC JAPAN  

    We present a novel real-space renormalization group (RG) for the one-dimensional XXZ model in the critical regime, reconsidering the role of the cutoff parameter in Wilson's RG for the Kondo impurity problem. We then demonstrate the RG calculation for the XXZ chain with the free boundary. Comparing the hierarchical structure of the obtained low-energy spectrum with the Bethe ansatz result, we find that the proper scaling dimension is reproduced as a fixed point of the RG transformation.

    DOI: 10.1143/JPSJ.76.063001

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  • Filling dependence of the zigzag Hubbard ladder for the quasi-one-dimensional superconductor Pr2Ba4Cu7O15-delta

    Kouichi Okunishi

    PHYSICAL REVIEW B   75 ( 17 )   2007年5月

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    記述言語:英語   掲載種別:機関テクニカルレポート,技術報告書,プレプリント等   出版者・発行元:AMERICAN PHYSICAL SOC  

    We investigate filling dependence of the zigzag Hubbard ladder using density-matrix renormalization group method. We illustrate the chemical-potential vs electron-density and spin gap vs electron-density curves, which reflect characteristic properties of the electron state. On the basis of the obtained phase diagram, we discuss the connection to a quasi-one-dimensional superconductor Pr2Ba4Cu7O15-delta.

    DOI: 10.1103/PhysRevB.75.174514

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  • 30aSB-6 純有機磁性体F_5PNNの磁場誘起秩序相(30aSB 量子スピン系(一次元系),領域3(磁性,磁気共鳴))

    吉田 靖雄, 河江 達也, 竹田 和義, 細越 裕子, 井上 克也, 前島 展也, 奥西 巧一, 岡本 清美, 坂井 徹

    日本物理学会講演概要集   61 ( 0 )   2006年

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    記述言語:日本語   出版者・発行元:一般社団法人 日本物理学会  

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  • Real-space renormalization group approach for the corner Hamiltonian

    K Okunishi

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   74 ( 12 )   3186 - 3192   2005年12月

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    記述言語:英語   掲載種別:機関テクニカルレポート,技術報告書,プレプリント等   出版者・発行元:PHYSICAL SOC JAPAN  

    We present a real-space renormalization group approach for the corner Hamiltonian, which is relevant to the reduced density matrix in the density matrix renormalization group. A set of self-consistent equations that the renormalized Hamiltonian should satisfy in the thermodynamic limit is also derived from the fixed point of the recursion relation for the corner Hamiltonian. We demonstrate the renormalization group algorithm for the S = 1/2 XXZ spin chain and show that the results are consistent with the exact solution. We further examine the renormalization group for the S = 1 Heisenberg spin chain and then discuss the nature of the eigenvalue spectrum of the corner Hamiltonian for the nonintegrable model.

    DOI: 10.1143/JPSJ.74.3186

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  • 24pYP-5 擬一次元純有機磁性体F_5PNNの磁場誘起格子不安定性(量子スピン系(一次元),領域3(磁性,磁気共鳴))

    吉田 靖雄, 坂井 徹, 河江 達也, 竹田 和義, 細越 裕子, 井上 克也, 後藤 恒昭, 前島 展也, 奥西 巧一, 岡本 清美

    日本物理学会講演概要集   60 ( 0 )   2005年

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    記述言語:日本語   出版者・発行元:一般社団法人 日本物理学会  

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  • Frustration-induced η inversion in the S = 1/2 bond-alternating spin chain

    Nobuya Maeshima, Kouichi Okunishi, Kiyomi Okamoto, Tôru Sakai

    Physical Review Letters   93 ( 12 )   1 - 127203   2004年9月

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    記述言語:英語  

    The frustration-induced enhancement of the incommensurate correlation for a bond-alternating spin chain in a magnetic field, associated with a quasi-one-dimensional organic compound F5PNN was studied. The temperature dependence of the staggered susceptibilities were investigated using the density matrix renormalization group. It was found that the incommensurate correlation becomes dominant in a certain range of the magnetic field. The mechanism of this enhancement were discussed on the basis of mapping to the effective S=1/2 XYZ chain and a possibility of the field-induced incommensurate long-range order.

    DOI: 10.1103/PhysRevLett.93.127203

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  • Frustration-induced eta inversion in the S=1/2 bond-alternating spin chain

    N Maeshima, K Okunishi, K Okamoto, T Sakai

    PHYSICAL REVIEW LETTERS   93 ( 12 )   2004年9月

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    記述言語:英語   出版者・発行元:AMERICAN PHYSICAL SOC  

    We study the frustration-induced enhancement of the incommensurate correlation for a bond-alternating quantum spin chain in a magnetic field, which is associated with a quasi-one-dimensional organic compound F5PNN. We investigate the temperature dependence of the staggered susceptibilities by using the density matrix renormalization group, and then find that the incommensurate correlation becomes dominant in a certain range of the magnetic field. We also discuss the mechanism of this enhancement on the basis of the mapping to the effective S=1/2 XXZ chain and a possibility of the field-induced incommensurate long-range order.

    DOI: 10.1103/PhysRevLett.93.127203

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  • Quantum fluctuation-induced phase transition in S=1/2 XY-like Heisenberg antiferromagnets on the triangular lattice

    S Yoshikawa, K Okunishi, M Senda, S Miyashita

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   73 ( 7 )   1798 - 1804   2004年7月

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    記述言語:英語   出版者・発行元:PHYSICAL SOC JAPAN  

    The selection of the ground state among nearly degenerate states due to quantum fluctuations is studied for the S = 1/2 XY-like Heisenberg antiferromagnets on the triangular lattice in the magnetic field applied along the hard axis, which was first pointed out by Nikuni and Shiba. We find that the selected ground state sensitively depends on the degree of the anisotropy and the magnitude of the magnetic field. This dependence is similar to that in the corresponding classical model at finite temperatures where various types of field induced phases appear due to the entropy effect. It is also found that the similarities of the selected states in the classical and quantum models are not the case in a two-leg ladder lattice, although the lattice consists of triangles locally and the ground state of this lattice in the classical case is the same as that of the triangular lattice.

    DOI: 10.1143/JPSJ.73.1798

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  • Quantum fluctuation-induced phase transition in S=1/2 XY-like Heisenberg antiferromagnets on the triangular lattice

    S Yoshikawa, K Okunishi, M Senda, S Miyashita

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   73 ( 7 )   1798 - 1804   2004年7月

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    記述言語:英語   出版者・発行元:PHYSICAL SOC JAPAN  

    The selection of the ground state among nearly degenerate states due to quantum fluctuations is studied for the S = 1/2 XY-like Heisenberg antiferromagnets on the triangular lattice in the magnetic field applied along the hard axis, which was first pointed out by Nikuni and Shiba. We find that the selected ground state sensitively depends on the degree of the anisotropy and the magnitude of the magnetic field. This dependence is similar to that in the corresponding classical model at finite temperatures where various types of field induced phases appear due to the entropy effect. It is also found that the similarities of the selected states in the classical and quantum models are not the case in a two-leg ladder lattice, although the lattice consists of triangles locally and the ground state of this lattice in the classical case is the same as that of the triangular lattice.

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  • High-field magnetization of S=1 antiferromagnetic bond-alternating chain compounds

    Y Narumi, K Kindo, M Hagiwara, H Nakano, A Kawaguchi, K Okunishi, M Kohno

    PHYSICAL REVIEW B   69 ( 17 )   2004年5月

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    記述言語:英語   出版者・発行元:AMERICAN PHYSICAL SOC  

    High-field magnetization and susceptibility measurements have been performed on S=1 antiferromagnetic bond-alternating chain compounds: [Ni-2(dpt)(2)(mu-ox)(mu-N-3)](PF6) [dpt=bis(3-aminopropyl)amine, ox= C2O4] (abbreviated as NDOAP) and [Ni(333-tet)(mu-NO2)]ClO4 [333-tet=N,N-'-bis(3-aminopropyl)-1,3propanediamine] (abbreviated as NTENP). The magnetization of NDOAP shows two steep increases at about 200 and 500 kOe. A magnetization plateau is observed between about 300 and 500 kOe, corresponding to half the value of the saturation moment. Comparison between numerical calculations and the experimental results, including the magnetic susceptibility, shows that NDOAP has a singlet ground state due to dimerization with a bond-alternating ratio alpha=0.1. The half-saturation plateau is also observed around 700 kOe in the magnetization of NTENP. The magnetization process of NTENP is well reproduced by the numerical result calculated for the parameter of alpha=0.45 in the singlet-dimer phase. The alpha dependence of the magnetization processes is discussed.

    DOI: 10.1103/PhysRevB.69.174405

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  • High-field magnetization of S=1 antiferromagnetic bond-alternating chain compounds

    Y Narumi, K Kindo, M Hagiwara, H Nakano, A Kawaguchi, K Okunishi, M Kohno

    PHYSICAL REVIEW B   69 ( 17 )   2004年5月

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    記述言語:英語   出版者・発行元:AMERICAN PHYSICAL SOC  

    High-field magnetization and susceptibility measurements have been performed on S=1 antiferromagnetic bond-alternating chain compounds: [Ni-2(dpt)(2)(mu-ox)(mu-N-3)](PF6) [dpt=bis(3-aminopropyl)amine, ox= C2O4] (abbreviated as NDOAP) and [Ni(333-tet)(mu-NO2)]ClO4 [333-tet=N,N-'-bis(3-aminopropyl)-1,3propanediamine] (abbreviated as NTENP). The magnetization of NDOAP shows two steep increases at about 200 and 500 kOe. A magnetization plateau is observed between about 300 and 500 kOe, corresponding to half the value of the saturation moment. Comparison between numerical calculations and the experimental results, including the magnetic susceptibility, shows that NDOAP has a singlet ground state due to dimerization with a bond-alternating ratio alpha=0.1. The half-saturation plateau is also observed around 700 kOe in the magnetization of NTENP. The magnetization process of NTENP is well reproduced by the numerical result calculated for the parameter of alpha=0.45 in the singlet-dimer phase. The alpha dependence of the magnetization processes is discussed.

    DOI: 10.1103/PhysRevB.69.174405

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  • Fractional S^z excitation and its bound state around the 1/3 plateau of the Ising-like zigzag XXZ chain

    Phys. Rev. B   69, 174405   2004年

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  • 13aWG-2 S=1/2 反強磁性ボンド交代鎖 F_5PNN の磁場中物性(量子スピン系 : 一次元, 領域 3)

    吉田 靖雄, 河江 達也, 竹田 和義, 細越 裕子, 井上 克也, 前島 展也, 奥西 巧一, 岡本 清美, 坂井 徹

    日本物理学会講演概要集   59 ( 0 )   2004年

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    記述言語:日本語   出版者・発行元:一般社団法人 日本物理学会  

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  • Field-induced spin liquids, orders, and gaps in frustrated bond-alternating quantum spin chain

    T. Sakai, N. Maeshima, K. Okunishi, K. Okamoto

    Bull. Am. Phys. Soc.   49 ( 1-part 1 )   491   2004年

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  • S=1 Spin Ladder in Magnetic Field

    Toru Sakai, Kiyomi Okamoto, Kouichi Okunishi, Masahiro Sato

    News Letter: Field-Induced New Quantum Phenomena in Magnetic Systems   ( 3 )   16   2004年

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  • Fractional S^z excitation and its bound state around the 1/3 plateau of the Ising-like zigzag XXZ chain

    Phys. Rev. B   69, 174405   2004年

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  • Exchange interaction via crystal-field excited states and its importance in CsCoCl3

    H Shiba, Y Ueda, K Okunishi, S Kimura, K Kindo

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   72 ( 9 )   2326 - 2333   2003年9月

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    記述言語:英語   出版者・発行元:PHYSICAL SOC JAPAN  

    The low-energy effective Hamiltonian is derived for a typical quasi-one-dimensional Ising-like antiferromagnet CsCoCl3. In contrast to previous treatments, in which only the processes within the crystal-field ground Kramers doublet are considered, second-order processes via crystal-field excited states are also taken into account in the present paper. They result in a new term, which is essentially a next-nearest neighbor ferromagnetic exchange whose anisotropy is different from the nearest-neighbor exchange. By including the new term the magnetization curve and ESR in CsCoCl3 are analyzed to show that those experiments can be explained in a way consistent with other experiments. Related theories on CsCoCl3 are discussed in this connection.

    DOI: 10.1143/JPSJ.72.2326

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  • Exchange interaction via crystal-field excited states and its importance in CsCoCl3

    H Shiba, Y Ueda, K Okunishi, S Kimura, K Kindo

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   72 ( 9 )   2326 - 2333   2003年9月

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    記述言語:英語   出版者・発行元:PHYSICAL SOC JAPAN  

    The low-energy effective Hamiltonian is derived for a typical quasi-one-dimensional Ising-like antiferromagnet CsCoCl3. In contrast to previous treatments, in which only the processes within the crystal-field ground Kramers doublet are considered, second-order processes via crystal-field excited states are also taken into account in the present paper. They result in a new term, which is essentially a next-nearest neighbor ferromagnetic exchange whose anisotropy is different from the nearest-neighbor exchange. By including the new term the magnetization curve and ESR in CsCoCl3 are analyzed to show that those experiments can be explained in a way consistent with other experiments. Related theories on CsCoCl3 are discussed in this connection.

    DOI: 10.1143/JPSJ.72.2326

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  • Exchange interaction via crystal-field excited states and its importance in CsCoCl3

    H Shiba, Y Ueda, K Okunishi, S Kimura, K Kindo

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   72 ( 9 )   2326 - 2333   2003年9月

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    記述言語:英語   出版者・発行元:PHYSICAL SOC JAPAN  

    The low-energy effective Hamiltonian is derived for a typical quasi-one-dimensional Ising-like antiferromagnet CsCoCl3. In contrast to previous treatments, in which only the processes within the crystal-field ground Kramers doublet are considered, second-order processes via crystal-field excited states are also taken into account in the present paper. They result in a new term, which is essentially a next-nearest neighbor ferromagnetic exchange whose anisotropy is different from the nearest-neighbor exchange. By including the new term the magnetization curve and ESR in CsCoCl3 are analyzed to show that those experiments can be explained in a way consistent with other experiments. Related theories on CsCoCl3 are discussed in this connection.

    DOI: 10.1143/JPSJ.72.2326

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  • Magnetic properties of a S = 1/2 zigzag spin chain compound (N2H5)CuCl3

    N Maeshima, M Hagiwara, Y Narumi, K Kindo, TC Kobayashi, K Okunishi

    JOURNAL OF PHYSICS-CONDENSED MATTER   15 ( 21 )   3607 - 3618   2003年6月

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    記述言語:英語   掲載種別:機関テクニカルレポート,技術報告書,プレプリント等   出版者・発行元:IOP PUBLISHING LTD  

    We present a theoretical and experimental study of a quasi-one-dimensional zigzag antiferromagnet (N2H5)CuCl3, which can be viewed as weakly coupled Heisenberg chains with a frustrated interaction. We first discuss generic features of the magnetic properties of the zigzag spin chain between the nearly single chain case and the nearly double chain case, on the basis of the finite temperature density-matrix renormalization group (DMRG) calculations. We next show the experimental results for the magnetic susceptibility and the high-field magnetization of a single crystal of (N2H5)CuCl3 above the Neel temperature T-N = 1.55 K. By comparing the experimental data with the DMRG results carefully, we finally obtain the ratio of the nearest and next-nearest exchange couplings as J(1)/J(2) = 0.25 with J(2)/k(B) = 16.3 K. We also investigate the three-dimensional (3D) coupling J' effect by using mean-field theory combined with the DMRG calculations. The estimated value J' similar to 0.04J(2) supports the one-dimensionality of (N2H5)CuCl3 above T-N.

    DOI: 10.1088/0953-8984/15/21/309

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  • Hole-doping effects on a frustrated spin ladder

    A Kawaguchi, A Koga, K Okunishi, N Kawakami

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   72 ( 2 )   405 - 410   2003年2月

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    記述言語:英語   出版者・発行元:PHYSICAL SOC JAPAN  

    Hole-doping effects are investigated on the t-J ladder model with the linked-tetrahedra structure. We discuss how a metal-insulator transition occurs upon hole doping with particular emphasis on the effects of geometrical frustration. By computing the electron density and the spin correlation function by the density matrix renormalization group, we show that strong frustration triggers a first-order transition to a metallic phase, when holes are doped into the plaquette-singlet phase. By examining spin excitations in a metallic case in detail, we discuss whether the spin-gap phase persists upon hole doping according to the strength of frustration. It is further shown that the lowest excited state in a spin-gap metallic phase can be described in two independent quasiparticles.

    DOI: 10.1143/JPSJ.72.405

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  • Hole-doping effects on a frustrated spin ladder

    A Kawaguchi, A Koga, K Okunishi, N Kawakami

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   72 ( 2 )   405 - 410   2003年2月

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    記述言語:英語   出版者・発行元:PHYSICAL SOC JAPAN  

    Hole-doping effects are investigated on the t-J ladder model with the linked-tetrahedra structure. We discuss how a metal-insulator transition occurs upon hole doping with particular emphasis on the effects of geometrical frustration. By computing the electron density and the spin correlation function by the density matrix renormalization group, we show that strong frustration triggers a first-order transition to a metallic phase, when holes are doped into the plaquette-singlet phase. By examining spin excitations in a metallic case in detail, we discuss whether the spin-gap phase persists upon hole doping according to the strength of frustration. It is further shown that the lowest excited state in a spin-gap metallic phase can be described in two independent quasiparticles.

    DOI: 10.1143/JPSJ.72.405

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  • Magnetic phase diagram of the S=1/2 antitlerromagnetic zigzag spin chain in the strongly frustrated region : cusp and plateau "jointly worked"

    J. phys. soc. Jpn.   72, 479   2003年

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  • Magnetic phase diagram of the S=1/2 antitlerromagnetic zigzag spin chain in the strongly frustrated region : cusp and plateau "jointly worked"

    J. phys. soc. Jpn.   72, 479   2003年

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  • Magnetization Process for a Quasi-one-dimensional S=1 Antiferromagnet.(共著)

    Physical Review B   65 ( 21 )   214405   2002年6月

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  • Magnetization Process for a Quasi-one-dimensional S=1 Antiferromagnet.(共著)

    Physical Review B   65   214405   2002年

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  • Anisotropic antiferromagnetic spin chains in a transverse field: Reentrant behavior of the staggered magnetization

    Y Hieida, K Okunishi, Y Akutsu

    PHYSICAL REVIEW B   64 ( 22 )   224422   2001年12月

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    記述言語:英語   出版者・発行元:AMER PHYSICAL SOC  

    We investigate one-dimensional S = 1/2 and S=1 antiferromagnetic quantum-spin chains with easy-axis anisotropies in a transverse field. We calculate both the uniform magnetization and the staggered magnetization, using a variant of the density-matrix renormalization-group method. We find that the staggered magnetizations exhibit reentrant behavior as functions of the transverse field, where the competition between the classical Neel order and the quantum fluctuations plays an important role. We also discuss the critical behavior associated with the staggered magnetizations.

    DOI: 10.1103/PhysRevB.64.224422

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  • Spinon Excitation and Möbius Boundary Condition in S=1/2 Antiferromagnetic Heisehberg Spin Ladder with ZigZag Structure.(共著)

    Physical Review B   64 ( 21 )   224422   2001年12月

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  • Antisotropic Antiferromagnetic Spin Chain in a Transversefield : Reentrant Behavior of the Staggerd Magnetization (共著)

    Physical Review B   64 ( 22 )   224422   2001年12月

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  • Vertical density matrix algorithm: A higher-dimensional numerical renormalization scheme based on the tensor product state ansatz

    N Maeshima, Y Hieida, Y Akutsu, T Nishino, K Okunishi

    PHYSICAL REVIEW E   64 ( 1 )   016705   2001年7月

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    記述言語:英語   出版者・発行元:AMERICAN PHYSICAL SOC  

    We present a new algorithm to calculate the thermodynamic quantities of three-dimensional (3D) classical statistical systems, based on the ideas of the tensor product state and the density matrix renormalization group. We represent the maximum-eigenvalue eigenstate of the transfer matrix as the product of local tensors that are iteratively optimized by the use of the "vertical density matrix" formed by cutting the system along the transfer direction. This algorithm, which we call vertical density matrix algorithm (VDMA), is successfully applied to the 3D Ising model. Using the Suzuki-Trotter transformation we can also apply the VDMA to 2D quantum systems, which we demonstrate for the 2D transverse field Ising model.

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  • Ventical Density Matrix Algorithn : A Higher Dimensional Numevical Renormalization Scheme Based on the Tensor Product State Ansats (共著)

    Physical Review E   64 ( 1 )   016705   2001年7月

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    記述言語:英語  

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  • Two-dimensional tensor product variational formulation

    T Nishino, Y Hieida, K Okunishi, N Maeshima, Y Akutsu, A Gendiar

    PROGRESS OF THEORETICAL PHYSICS   105 ( 3 )   409 - 417   2001年3月

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    記述言語:英語   出版者・発行元:PROGRESS THEORETICAL PHYSICS PUBLICATION OFFICE  

    We propose a numerical self-consistent method for 3D classical lattice models, which optimizes the variational state written as a two-dimensional product of tensors. The variational partition function is calculated using the corner transfer matrix renormalization group (CTMRG), which is a variant of the density matrix renormalization group (DMRG). The numerical efficiency of the method is exemplified in its application to the 3D Ising model.

    DOI: 10.1143/PTP.105.409

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  • Spinon Excitation and Möbius Boundary Condition in S=1/2 Antiferromagnetic Heisehberg Spin Ladder with ZigZag Structure.(共著)

    Physical Review B   64   224422   2001年

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  • Universal Relation Between the Dispersion Curve and the Ground-state Correlation Length in 1 D Antiferromagnetic Quantum Spin Systems. (共著)

    Physical Review B   64   104332   2001年

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  • Universal relation between the dispersion curve and the ground-state correlation length in one-dimensional antiferromagnetic quantum spin systems

    K. Okunishi, Y. Akutsu, N. Akutsu, T. Yamamoto

    Physical Review B - Condensed Matter and Materials Physics   64 ( 10 )   2001年

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    記述言語:英語   掲載種別:機関テクニカルレポート,技術報告書,プレプリント等  

    We discuss a universal relation (formula presented) with (formula presented) in 1D quantum spin systems with an excitation gap, where (formula presented) is the dispersion curve of the low-energy excitation and (formula presented) is the correlation length of the ground state. We first discuss this relation for integrable models such as the Ising model in a transverse filed and the (formula presented) model. We secondly make a derivation of the relation for general cases, in connection with the equilibrium crystal shape in the corresponding 2D classical system. We finally verify the relation for the (formula presented) bilinear-biquadratic spin chain and the (formula presented) zigzag spin ladder numerically. © 2001 The American Physical Society.

    DOI: 10.1103/PhysRevB.64.104432

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  • Universal Relation Between the Dispersion Curve and the Ground-state Correlation Length in 1 D Antiferromagnetic Quantum Spin Systems. (共著)

    Physical Review B   64   104332   2001年

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  • Self-consistent tensor product variational approximation for 3D classical models

    T Nishino, K Okunishi, Y Hieida, N Maeshima, Y Akutsu

    NUCLEAR PHYSICS B   575 ( 3 )   504 - 512   2000年6月

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    記述言語:英語   出版者・発行元:ELSEVIER SCIENCE BV  

    We propose a numerical variational method for three-dimensional (3D) classical lattice models. We construct the variational state as a product of local tensors, and improve it by use of the corner transfer matrix renormalization group (CTMRG), which is a variant of the density matrix renormalization group (DMRG) applied to 2D classical systems. Numerical efficiency of this approximation is investigated through trial applications to the 3D Ising model and the 3D 3-state Potts model. (C) 2000 Elsevier Science B.V. All rights reserved.

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  • Kramers-Wannier approximation for the 3D Ising model

    K Okunishi, T Nishino

    PROGRESS OF THEORETICAL PHYSICS   103 ( 3 )   541 - 548   2000年3月

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    記述言語:英語   出版者・発行元:PROGRESS THEORETICAL PHYSICS PUBLICATION OFFICE  

    We investigate the Kramers-Wannier approximation for the three-dimensional (3D) Ising model. The variational state is represented by an effective 2D Ising model, which contains two variational parameters. We numerically calculate the variational partition function using the corner transfer matrix renormalization group (CTMRG) method, and find its maximum with respect to the variational parameters. The value of the calculated transition point, K-c = 0.2184, is only 1.5% less than the true K-c. This result is better than that obtained using the corner transfer tensor renormalization group (CTTRG) approach. The calculated phase transition is mean-held like.

    DOI: 10.1143/PTP.103.541

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  • Self-Consistent Tensor Product Variational Approximation for 3D Classical Models

    T. Nishino, K. Okunishi, Y. Hieida, N. Maeshima, Y. Akutsu

    Nucl. Phys. B575 (2000) 504-512   2000年1月

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    掲載種別:機関テクニカルレポート,技術報告書,プレプリント等  

    We propose a numerical variational method for three-dimensional (3D)<br />
    classical lattice models. We construct the variational state as a product of<br />
    local tensors, and improve it by use of the corner transfer matrix<br />
    renormalization group (CTMRG), which is a variant of the density matrix<br />
    renormalization group (DMRG) applied to 2D classical systems. Numerical<br />
    efficiency of this approximation is investigated through trial applications to<br />
    the 3D Ising model and the 3D 3-state Potts model.

    DOI: 10.1016/S0550-3213(00)00133-4

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  • First order quantum phase transition in the orthogonal dimer spin chain(共著)

    Physical Review B   62 ( 9 )   5558 - 5563   2000年

  • Anti ferromagnetic zigzag spin chain in magnetic fields at finite temperatures(共著)

    Physical Review B   62 ( 2 )   934 - 939   2000年

  • Density matrix renormalization group: Introduction from a variational point of view

    T Nishino, T Hikihara, K Okunishi, Y Hieida

    INTERNATIONAL JOURNAL OF MODERN PHYSICS B   13 ( 1 )   1 - 24   1999年1月

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    記述言語:英語   掲載種別:書評論文,書評,文献紹介等   出版者・発行元:WORLD SCIENTIFIC PUBL CO PTE LTD  

    The density matrix renormalization group theory is reviewed as a numerical variational method. The variational state, expressed as a product of local tensors, is improved through locally tuning each tensor. The first section is a tutorial with simplified discussions. Details are discussed in the subsequent sections. The review concludes with some recent developments and future directions.

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  • delta-function base gas picture of S=1 antiferromagnetic spin chains near critical field(共著)

    Physical Review B   59 ( 10 )   6806 - 6812   1999年

  • Numerical renormalization approach to two-dimensional quantum antiferromagnets with valence-bond-solid type ground state

    Yasuhiro Hieida, Kouichi Okunishi, Yasuhiro Akutsu

    NEW JOURNAL OF PHYSICS   1   7   1999年

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    記述言語:英語   出版者・発行元:IOP PUBLISHING LTD  

    We study the ground-state properties of two-dimensional quantum spin systems having the valence-bond-solid (VBS) type ground states. The 'product-of-tensors' form of the ground-state wavefunction of the system is utilized to associate it with an equivalent classical lattice statistical model which can be treated by the transfer-matrix method. For diagonalization of the transfer matrix, we employ the product-wavefunction renormalization group method, which is a variant of the density-matrix renormalization group method. We obtain the correlation length and the sublattice magnetization accurately. For the anisotropically 'deformed' S = 3/2 VBS model on the honeycomb lattice, we find that the correlation length as a function of the deformation parameter behaves very much as that in the S = 3/2 VBS chain.

    DOI: 10.1088/1367-2630/1/1/007

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  • Universal asymptotic eigenvalue distribution of density matrix and corner transfer matrix in the thermodynamic limit(共著)

    physical review E   59 ( 6 )   R6227 - R6230   1999年

  • magnetization curve of bilinear-biquadratic spin chain at finite temperature

    physical review B   60 ( 6 )   4043 - 4052   1999年

  • Middle field cusp singularities in the magnetization process of one-dimensional quantum antiferromagnets(共著)

    physical Review B   60 ( 10 )   R6953 - R6956   1999年

  • A density matrix algorithm for 3D classical models

    T Nishino, K Okunishi

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   67 ( 9 )   3066 - 3072   1998年9月

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    記述言語:英語   出版者・発行元:PHYSICAL SOC JAPAN  

    We generalize the corner transfer matrix renormalization group, which consists of White's density matrix algorithm and Baxter's method of the corner transfer matrix, to three dimensional (3D) classical models. The renormalization group transformation is obtained through the diagonalization of density matrices for a cubic cluster. A trial application to 3D Ising model with m = 2 is shown as the simplest case.

    DOI: 10.1143/JPSJ.67.3066

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  • Numerical latent heat observation of the q = 5 Potts model

    T Nishino, K Okunishi

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   67 ( 4 )   1492 - 1493   1998年4月

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    記述言語:英語   出版者・発行元:PHYSICAL SOC JAPAN  

    DOI: 10.1143/JPSJ.67.1492

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  • Corner transfer matrix algorithm for classical renormalization group

    T Nishino, K Okunishi

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   66 ( 10 )   3040 - 3047   1997年10月

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    記述言語:英語   出版者・発行元:PHYSICAL SOC JAPAN  

    We report a real-space renormalization group (RSRG) algorithm, which is formulated through Baxter's corner transfer matrix (CTM), for two-dimensional (d = 2) classical lattice models. The new method performs the renormalization group transformation according to White's density matrix algorithm, so that variational free energies are minimized within a restricted degree of freedom. As a consequence of the renormalization, spin variables on each corner of CTM are replaced by a m-state block spin variable. It is shown that the thermodynamic functions and critical exponents of the q = 2, 3 Potts models can be precisely evaluated by use of the renormalization group method.

    DOI: 10.1143/JPSJ.66.3040

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  • Magnetization process of a one-dimensional quantum antiferromagnet: The product-wave-function renormalization group approach

    Y Hieida, K Okunishi, Y Akutsu

    PHYSICS LETTERS A   233 ( 4-6 )   464 - 470   1997年9月

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    記述言語:英語   出版者・発行元:ELSEVIER SCIENCE BV  

    The product-wave-function renormalization group method, a new numerical renormalization group scheme proposed recently, is applied to one-dimensional quantum spin chains in a magnetic field. We find the zero-temperature magnetization curve of the spin chains, which excellently agrees with the exact solution in the whole range of the field. (C) 1997 Published by Elsevier Science B.V.

    DOI: 10.1016/S0375-9601(97)00498-2

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  • Density matrix and renormalization for classical lattice models

    T Nishino, K Okunishi

    STRONGLY CORRELATED MAGNETIC AND SUPERCONDUCTING SYSTEMS   478   167 - 183   1997年

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    記述言語:英語   掲載種別:機関テクニカルレポート,技術報告書,プレプリント等   出版者・発行元:SPRINGER-VERLAG BERLIN  

    The density matrix renormalization group is a variational approximation method that maximizes the partition function - or minimize the ground state energy - of quantum lattice systems. The variational relation is expressed as Z = Tr rho greater than or equal to Tr (&lt;(1) over tilde&gt;rho), where rho is the density submatrix of the system, and (1) over tilde is a projection operator. In this report we apply the variational relation to two-dimensional (2D) classical lattice models, where the density submatrix rho is obtained as a product of the corner transfer matrices. The obtained renormalization group method for 2D classical lattice model, the corner transfer matrix renormalization group method, is applied to the q = 2 similar to 5 Potts models. With the help of the finite size scaling, critical exponents (q = 2, 3) and the latent heat (q = 5) are precisely obtained.

    DOI: 10.1007/BFb0104638

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  • Numerical Renormalization Group at Criticality

    T. Nishino, K. Okunishi, M. Kikuchi

    213   69   1996年1月

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    掲載種別:機関テクニカルレポート,技術報告書,プレプリント等  

    We apply a recently developed numerical renormalization group, the<br />
    corner-transfer-matrix renormalization group (CTMRG), to 2D classical lattice<br />
    models at their critical temperatures. It is shown that the combination of<br />
    CTMRG and the finite-size scaling analysis gives two independent critical<br />
    exponents.

    DOI: 10.1016/0375-9601(96)00128-4

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  • Corner Transfer Matrix Renormalization Group Method

    T. Nishino, K. Okunishi

    65 ( 4 )   891 - 894   1995年7月

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    掲載種別:機関テクニカルレポート,技術報告書,プレプリント等  

    We propose a new fast numerical renormalization group method,the corner<br />
    transfer matrix renormalization group (CTMRG) method, which is based on a<br />
    unified scheme of Baxter&#039;s corner transfer matrix method and White&#039;s density<br />
    matrix renormalization groupmethod. The key point is that a product of four<br />
    corner transfer matrices gives the densitymatrix. We formulate the CTMRG method<br />
    as a renormalization of 2D classical models.

    DOI: 10.1143/JPSJ.65.891

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  • Product Wave Function Renormalization Group

    Tomotoshi Nishino, Kouichi Okunishi

    Journal of the Physical Society of Japan   64 ( 11 )   4084 - 4087   1995年

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    記述言語:英語   掲載種別:機関テクニカルレポート,技術報告書,プレプリント等  

    We propose a fast numerical renormalization group method —the product wave function renormalization group (PWFRG) method —for ID quantum lattice models and 2D classical ones. A variational wave function, which is expressed as a matrix product, is improved through a self-consistent calculation. The new method has the same fixed point as the density matrix renormalization group method. © 1995, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.

    DOI: 10.1143/JPSJ.64.4084

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講演・口頭発表等

  • A low-energy effective model for the S=1/2 Kagome-triangular-strip Heisenberg model 国際会議

    奥西 巧一

    ICM2018 SanFrancisco  2018年7月 

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    記述言語:英語   会議種別:ポスター発表  

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  • サイン2乗変形と運動量

    奥西 巧一

    日本物r学会年次大会  2018年3月 

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    会議種別:ポスター発表  

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  • Sine-square deformation and Mobius quantization of 2D CFTs

    奥西 巧一

    理研iTHEMS研究会  2017年6月 

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    会議種別:口頭発表(一般)  

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  • Sine-square defomrantion and Mobius quantization for 2D CFT 国際会議

    奥西 巧一

    TNSAA Taiwan  2016年12月 

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    記述言語:英語   会議種別:口頭発表(一般)  

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  • Sine-Square deformation and Mobius qunatization in 2D CFT 国際会議

    奥西 巧一

    Quantum Information in String Theory and Many-body Systems, Yukawa Institute  2016年6月 

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    記述言語:英語   会議種別:口頭発表(招待・特別)  

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  • サイン2乗変形と共形場理論のメビウス量子化

    奥西 巧一

    日本物学会年次大会 大阪大学  2016年3月 

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    会議種別:ポスター発表  

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受賞

  • 日本物理学会領域3若手奨励賞

    2010年   日本物理学会  

    奥西 巧一

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共同研究・競争的資金等の研究

  • 量子多体系のエネルギースケール制御機構とエンタングルメントくりこみ群

    研究課題/領域番号:17H02931  2017年4月 - 2021年3月

    日本学術振興会  科学研究費助成事業 基盤研究(B)  基盤研究(B)

    奥西 巧一, 引原 俊哉

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    担当区分:研究代表者  資金種別:競争的資金

    配分額:18720000円 ( 直接経費:14400000円 、 間接経費:4320000円 )

    1次元臨界系のエンタングルメントスペクトルが、端のある共形場理論のスペクトルにより記述されることは知られていたが、予想される有限サイズの依存性と数値計算とのずれが大きいことが問題であった。そこで、境界のある共形場理論とメビウス変換を用いて、エンタングル点の幾何学的配置状況を取り入れてスペクトルの有限サイズ補正の導出を行った。その結果、スペクトルの一部については数値計算結果との一致が大きく改善されることが示された。一方、有意なずれを有するスペクトルも観察され、他の要因による補正も検証する必要があることが分かった。次に、corner Hamiltonianに対する有限温度の量子モンテカルロ計算が特任助教の関により実施され、corner Hamiltonianの有限温度の相関関数と一様系の基底状態の相関関数が同一視できることが確認された。これにより重力理論におけるUnruh効果と等価な現象が量子スピン系でも実現可能であることが明らかになった。
    一方、2次元ボンドランダムスピン系に対して、分担者の引原と特任助教の関が、Tree型テンソルネットワーク数値くりこみ群の新しい定式化を行なった。Treeの構成法をエンタングルメントと励起ギャップにもとづくもので比較検討を行った。また、くりこみ変換行列の構成に対しても同様の精度比較を行い、さらに、対角化や(可能な場合は)量子モンテカルロ法による計算精度の検証も行った。その結果、励起ギャップにもとづく数値くりこみ群の構成が、計算精度および簡便性においても性能がよいことが明らかになった。また、連携研究者の上田と奥西は、2次元6状態クロック模型のKT型相転移を大規模角転送行列くりこみ群計算と有限mスケーリング解析を組み合わせておこない、通常の共形場理論による予想と異なる結果が現れることを明らかにした。原因の解明は今後の課題である。

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  • 非局所変換による量子多体系のエンタングルメント制御と負符号問題

    研究課題/領域番号:26400387  2014年4月 - 2018年3月

    日本学術振興会  科学研究費助成事業 基盤研究(C)  基盤研究(C)

    奥西 巧一

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    担当区分:研究代表者  資金種別:競争的資金

    配分額:4160000円 ( 直接経費:3200000円 、 間接経費:960000円 )

    物理学で基本的な課題である量子多体系のエンタングルメントに関連し、本研究の主要な成果は以下の3点である。(1)SO(N)双1次双2次型の量子スピン鎖のエンタングルメントを解く一般化非局所Jordan-Wigner変換を構成し、その基底で量子モンテカルロ法の負符号を消去できることを示した。(2)量子モンテカルロ法において、スナップショットの虚時間方向を縮約したスナップショット密度行列の概念を提案し、スペクトル分布のスケーリング性などの基本的性質を明らかにした。(3)サイン2乗型の相互作用変調のある系に対し、厳密に解ける模型を構成し、また、共形場理論やエンタングルメントとの関係も明らかにした。

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  • スピンナノチューブにおけるスピン・電荷・カイラリティが創る新しい量子現象の理論

    研究課題/領域番号:23340109  2011年4月 - 2015年3月

    日本学術振興会  科学研究費助成事業 基盤研究(B)  基盤研究(B)

    坂井 徹, 中野 博生, 肘井 敬吾, 岡本 清美, 礒田 誠, 奥西 巧一, 利根川 孝, 野村 拓司

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    配分額:16770000円 ( 直接経費:12900000円 、 間接経費:3870000円 )

    スピンナノチューブと呼ばれる系について、大規模数値対角化と密度行列繰り込み群を用いて理論的に研究した。その結果、スピン異方性を変化させるとスピンギャップ相からギャップレス相への新しい量子相転移が起きることが判明した。また、量子効果による多スピン交換相互作用により、別のメカニズムに基づくスピンギャップ相が実現することもわかった。さらに、我々の理論的研究により、外部磁場によって新しいカイラル液体相が実現することが予言された。

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  • テンソル積状態の量子エンタングルメント制御と数値くりこみ群

    2011年4月 - 2014年3月

    日本学術振興会  科研費・基盤研究(C) 

    奥西 巧一

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    担当区分:研究代表者  資金種別:競争的資金

    配分額:4550000円 ( 直接経費:3500000円 、 間接経費:1050000円 )

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  • テンソル積状態の量子エンタングルメント制御と数値くりこみ群

    研究課題/領域番号:23540442  2011年 - 2013年

    日本学術振興会  科学研究費助成事業 基盤研究(C)  基盤研究(C)

    奥西 巧一

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    配分額:4550000円 ( 直接経費:3500000円 、 間接経費:1050000円 )

    量子多体系の基底状態を研究する上で、テンソル積型の状態を用いた数値くりこみ群が注目を集めている。まず、Wilson型の数値くりこみ群の成功のメカニズムが、エネルギースケールのスケールフリー性を持つことが鍵であることを明らかにした。次に、テンソル積状態のエンタングルメント構造に注目し、高次特異値分解を用いたテンソルくりこみ群の固定点が、量子および古典に共通の構造を持ち、それが対応する角転送行列の倍化現象により説明されることを初めて明らかにした。また、応用として擬1次元量子磁性体BaCo2V2O4の磁場誘起の微細秩序構造の決定なども行った。

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  • 強磁場の創る新奇スピン秩序状態の理論解析 研究課題

    2008年4月 - 2010年3月

    文部科学省  科研費・特定領域研究 

    奥西 巧一

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    担当区分:研究代表者  資金種別:競争的資金

    配分額:2200000円 ( 直接経費:2200000円 )

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  • スピンナノチューブの異常量子現象の理論的・計算科学的研究

    研究課題/領域番号:20340096  2008年 - 2010年

    日本学術振興会  科学研究費助成事業 基盤研究(B)  基盤研究(B)

    坂井 徹, 中野 博生, 野村 拓司, 妹尾 仁嗣, 利根川 孝, 岡本 清美, 奥西 巧一, 佐藤 正寛

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    配分額:17290000円 ( 直接経費:13300000円 、 間接経費:3990000円 )

    S=1/2三本鎖スピンナノチューブを数値的対角化と密度行列繰り込み群を用いた計算科学的解析により、理論的に研究した。その結果、この系はユニットセルが正三角形から二等辺三角形に歪むような格子ひずみにより、スピンギャップによって特徴づけられる量子相転移を示すことがわかった。

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  • 強磁場の創る新奇スピン秩序状態の理論解析

    研究課題/領域番号:20030003  2008年 - 2009年

    日本学術振興会  科学研究費助成事業 特定領域研究  特定領域研究

    奥西 巧一

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    配分額:2200000円 ( 直接経費:2200000円 )

    BaCo2V2O8における磁場誘起新奇非整合秩序の発見をうけ、疑1次元量子スピン系における磁場誘起相転移と、それにともなう新奇スピン秩序状態の解明が重要な論点となっている。本研究課題では、とくに、スピンのイジング異方性の役割と1次元性との兼ね合いの観点かち、疑1次元古典スピン系の熱相転移の特徴をモンテカルロシミュレーションなど計算物理的手法を用いて明らかにした。
    具体的には拡張アンサンブルにもとついたWang-Landau法を、鎖方向と鎖間方向に対応した2次元状態密度に拡張し、シミュレーションを行った。この2次元状態密度の状態分布から疑1次元系特有の比熱のピーク構造を抽出することができた。さらにこの結果より疑1次元系のスケーリング解析における一般的な注意点を明示的にまとめた。次に、連続スピン自由度をもつ疑1次元古典XXZ模型の磁場中相転移についても同様の解析を行った。通常の3次元での古典XXZ模型は磁場によりスピンフロップ転移を起こすことが知られている。しかし、スピンフロップ転移は1次相転移であり、通常のモンテカルロシミュレーションで扱うことは困難であったため、鎖間結合により弱く結合したXXZ鎖がどのような相転移を起こすのかはながらく非自明な問題として残されてきた。本研究では磁化についての状態密度に対してWang-Landauシミュレーションを行うことで問題の解決をはかり、系統的に磁場中相転移を調べることが可能にした。最終的に疑1次元古典XXZ模型の磁気相図の鎖間結合依存性を明らかにし、3次元的な特徴が疑1次元古典XXZ系でも保持されることが分かった。この結果、BaCo2V2O8で見られた新奇秩序は純粋に量子効果により誘起されていることが確認できた。

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  • corner Hamiltonianのくりこみ群的解析

    2006年4月 - 2009年3月

    日本学術振興会  科研費・若手研究(B) 

    奥西 巧一

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    担当区分:研究代表者  資金種別:競争的資金

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  • 密度行列くりこみ群を用いた強磁場スピン物性の研究

    2006年4月 - 2007年3月

    文部科学省  科研費・特定領域研究 

    奥西 巧一

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    担当区分:研究代表者  資金種別:競争的資金

    配分額:1700000円 ( 直接経費:1700000円 )

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  • corner Hamiltonianのくりこみ群的解析

    研究課題/領域番号:18740230  2006年 - 2008年

    日本学術振興会  科学研究費助成事業 若手研究(B)  若手研究(B)

    奥西 巧一

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    配分額:3010000円 ( 直接経費:2800000円 、 間接経費:210000円 )

    低次元量子系の高精度数値計算手法として定着しているDMRG 法をcorner Hamiltonianに対するWilson数値くりこみ群と再解釈することに成功した。また、Wilsonくりこみ群のカットオフが赤外発散の正則化に対応することを見出し、一般の量子系へ拡張によりXXZスピン鎖の低エネルギー励起を固定点として求め、朝永-ラッティンジャー液体の別証明を与えた。一方、1次元XXZ 模型の相関振幅を計算し、鎖間結合系の磁場誘起非整合秩序相を示した。また、スピン回転変換を通じてジグザグスピン系のスピンカイラリティー秩序を直接計算し、その転移がイジング普遍性であることを明らかにした。

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  • 密度行列くりこみ群を用いた強磁場スピン物性の研究

    研究課題/領域番号:18044004  2006年 - 2007年

    日本学術振興会  科学研究費助成事業 特定領域研究  特定領域研究

    奥西 巧一

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    配分額:1700000円 ( 直接経費:1700000円 )

    本年度は、前年度に引き続いて擬1次元量子スピン系のエキゾチックな磁場誘起秩序と三角形量子スピンチューブにおけるフラストレーションに起因する1次量子相転移の研究を行った。(1)昨年までに擬1次元XXZスピン鎖BaCO_2V_2O_8のXXZの異方性が0.5程度であることを決定したが、本年度はそのパラメーターに基づき、磁場中における1次元朝永-ラッティンジャー液体論と密度行列繰り込み群を組み合わせて臨界指数および相関振幅を厳密に計算し、さらに鎖間平均場近似とボソン化法を組み合わせることで、磁場誘起相転移の転移温度を定量的に決定することに成功した。また、量子モンテカルロ法も用いて非整合秩序が実現されることを非摂動的に示した。とくに、低磁場領域で新奇磁場誘起非整合秩序が起こることを示し、実験の相転移ラインと大変よい一致を示すことがわかった。この結果は、Phys.Rev.Bに出版し、本特定領域の国際シンポジウムでも発表した。(2)昨年度までに三角形スピンチューブは三角形をつなぐ相互作用の比が1.2程度で並進対称性の自発的に破れた状態とギャップレスの状態の間で量子相転移がおこることが解明されたが、そのメカニズムは謎であった。ユニット三角内の合成スピンに着目して合成スピンが厳密に保存する模型を構築し、そこからの連続性を用いて、量子相転移がユニットあたりの合成スピンが不連続に飛ぶことによる1次転移であることを明らかにした。

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  • スピンナノチューブの異常量子現象の理論的・計算科学的研究

    研究課題/領域番号:17340100  2005年 - 2007年

    日本学術振興会  科学研究費助成事業 基盤研究(B)  基盤研究(B)

    坂井 徹, 奥西 巧一, 岡本 清美, 佐藤 正寛, 大塚 雄一, 利根川 孝, 野村 拓司, 松本 宗久

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    配分額:6750000円 ( 直接経費:6300000円 、 間接経費:450000円 )

    スピンナノチューブの中でも最も簡単な系である3本足スピンチューブを出発点として、この系の理論的研究から取り掛かった。まず口径が正三角形の場合について、この系のスピン励起のギャップ(スピンギャップ)に起因した量子相転移の性質を、有限サイズの系に対する行列の数値的厳密対角化と密度行列繰り込み郡により数値的に計算し、有限サイズスケーリングを適用して解析した。その結果、従来から予想されていたように、桁方向の反強磁性的相互作用が少しでもあるとスピンギャップが開くこと、また桁方向の相互作用が強磁性的である場合にはギャップレスであることが判明した。また、口径が正三角形から二等辺三角形になるような歪みが加わった場合の、この系の応答を同様の方法で解析した結果、正三角形からのずれに対して非常に急激にスピンギャップが消失する量子相転移が起き得ることが示された。
    また、磁化過程に現れる新しい現象を理論的に研究した。まず飽和磁化付近における摂動論による解析的な研究により、従来予測されていたギャップレスの朝永・ラッティンジャー液体相の中に、新しいカイラル秩序相、及び不均一磁化秩序相が現れる場合があることが示された。いずれの相も、従来の2成分朝永・ラッティンジャー液体相のうちの1成分が存続しているため、長距離秩序が存在すると同時にギャップレスの励起スペクトルを伴うもので、まだ発見されていない新しい磁場誘起秩序相である。

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  • コバルト酸化物における多元要素の量子多体効果が導く異常物性と新奇超伝導機構の理論

    研究課題/領域番号:17540317  2005年 - 2007年

    日本学術振興会  科学研究費助成事業 基盤研究(C)  基盤研究(C)

    大野 義章, 奥西 巧一

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    配分額:3770000円 ( 直接経費:3500000円 、 間接経費:270000円 )

    層状コバルト酸化物Na_xCoO_2や水を挿入した超伝導体Na_xCoO_2・yH_2Oでは、CoO_2面が2次元三角格子を形成して幾何学的にフラストレートしており、またフェルミ面を切るバンドが3個のd(t_<2g>)軌道から構成され、多軌道、多バンドの特徴を持つ。本研究では、Coのd軌道5個とCo面の上下にある0のp軌道6個を全て考慮に入れ、バンド計算を再現する現実的な2次元三角格子llバンドd-p模型を導出し、動的平均場理論、スレーブボソン法、乱雑位相近似、Hartree-Fock近似を相補的に用いてCoO_2面の電子状態を調べ、以下のことを明らかにした。
    1.Coのd電子の強相関効果によりalgバンドが繰り込まれて狭くなると同時に、eg'バンドがフェルミレベルよりも下に押し下げられホールポケットが消失する。これはARPESの実験結果とコンシステントである。
    2.水の挿入によりtrigonal歪みが増大し、eg'バンドのホールポケットが出現する。このとき、alg軌道とeg'軌道のギャップ関数が逆符号となる一重項s波の超伝導が実現する。これは、ナイトシフトや不純物効果の実験とコンシステントである。
    3.Na量xが0.75に近づくと、x>0.75で実現する面内強磁性・面間反強磁性の磁気揺らぎの効果により電子比熱係数が急増し、x〜0.75では実験とコンシステントな巨大熱起電力が現れる。
    4.x=0.5のとき、Naイオンの1次元整列の効果によりフェルミ面のネスティングが増加し、反強磁性がTc1で実現する。さらに、Tc2以下では反強磁性秩序に加えて軌道秩序と電荷秩序が重ねて起こり、絶縁体に転移する。これは、Na_<0.5>CoO_22段転移の実験をよく説明する。

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  • 自発的な対称性の破れが創るハイスピン鎖の新たな量子状態

    研究課題/領域番号:16540332  2004年 - 2005年

    日本学術振興会  科学研究費助成事業 基盤研究(C)  基盤研究(C)

    利根川 孝, 奥西 巧一

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    配分額:3000000円 ( 直接経費:3000000円 )

    本科学研究費の補助のもとで,我々は,種々のハィスピン(主に,S=1)鎖の有限磁場中基底状態を,数値的方法および解析的方法を用いて研究した。得られた主な研究実績は,以下の通りである。
    1.1軸性の1イオン異方性エネルギーをもつ1次元S=1強磁性反強磁性ボンド交代鎖:この系の有限磁場基底状態を密度行列繰り込み群法を用いて数値的に調べ,基底状態磁化曲線中に2種類の1/2-プラトー(飽和磁化の半分の位置のプラトー)が出現することを明らかにした。更に,有限系の厳密対角化法に基づいたレベルスペクトロスコピー法および解析的方法を用いて,相互作用定数平面上の1/2-プラトー相図や磁場対相互作用定数平面上の磁化相図を決定した。
    2.異方的1次元S=1反強磁性鎖:イジング型の異方的交換相互作用と1軸性の1イオン型異方性エネルギーをもつ1次元S=1反強磁性鎖の有限磁場基底状態を,密度行列繰り込み群法を用いて調べた。その結果,i)基底状態磁化曲線に,磁化のジャンプ異常性(1次転移)が出現すること,ii)有限磁場基底状態中でcommensurate-incommensurate転移(2次転移)が起こること,などが明らかになった。
    3.ボンド交代最近接相互作用と一様な次近接相互作用をもつS=1鎖:まず,強磁性状態からのスピン波励起エネルギーの分散関係を計算し,その結果から,相互作用定数平面における強磁性状態相と非磁性1重項状態相との間の相境界線を厳密に決定した。次に,非磁性1重項状態相が安定な相互作用定数領域で,密度行列繰り込み群法を用いて基底状態磁化曲線を計算し,1/2-プラトーが出現することを明らかにした。更に,レベルスペクトロスコピー法を用いて,相互作用定数平面上の1/2-プラトー相図を決定した。

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  • インターネットを利用した計算物理に関する国際共同研究の試み

    研究課題/領域番号:14658041  2002年

    日本学術振興会  科学研究費助成事業 萌芽研究  萌芽研究

    小林 迪助, 奥西 巧一, 草部 浩一, 家富 洋

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    配分額:1300000円 ( 直接経費:1300000円 )

    本研究は新潟大学理学部および大学院自然科学研究科とデルフト工科大学応用科学部(オランダ)との間の交流協定を背景にした.内容については計算物理における基本的手法から多数のコンピューターの同時利用によって大規模計算を可能にする先端的並列計算技術までカバーすることを目的とした.参加する学生は単に受動的に講義を聴講するばかりではなく,学生同士が国際的チームをつくり講義を補完する様々な演習課題を共同で遂行した.これは学生に対して課題解決能力の獲得ばかりではなく,英語を用いたコミュニケーション技術を向上させる機会を与えるためである.通常のコミュニケーションはインターネットを使って行われた.合わせて相互に訪問する機会を計画に組み入れた.まず2002年9月28日から約1週間,デルフト工科大学側から教員2名および大学院生3名が新潟を訪問し,共同講義が開始された.PCの組み立て,並列計算用PCクラスターの構築および経路積分量子モンテカルロ(PlQMC)シミュレーションの講義が実施された.10月中旬から12月初旬までPlQMCの応用として液体ヘリウム中の電子拡散の問題が演習課題として行われた.12月14日から約1週間,新潟大学側から教員1名および大学院生4名がデルフト工科大学を訪問し,密度汎関数理論に基づく第一原理電子状態計算に関する講義が行われた.訪問後,2003年3月末までシリコン中の電子状態を求めるための計算コード作成が行われた.いくつかの問題点(インターネットテレビ会議の利用がネットワークの制限で実施できなかったこと,博士論文や修士論文の研究との両立のむずかしさ,学生の英語力の不足など)が浮かび上がってきたが,このような試みが我が国における物理高等教育の改善につながるものと自負している.本共同講義の途中経過が2002年12月21日の日本物理教育学会新潟支部会で報告されたことを付記する.

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  • 低次元多体問題の理論的研究と数値くり込み群の拡張

    研究課題/領域番号:99J01800  1999年 - 2000年

    日本学術振興会  科学研究費助成事業 特別研究員奨励費  特別研究員奨励費

    奥西 巧一

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    担当区分:研究代表者  資金種別:競争的資金

    配分額:2400000円 ( 直接経費:2400000円 )

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  • 角転送行列を用いた2次元古典格子系、1次元量子系の研究

    研究課題/領域番号:96J02250  1998年

    日本学術振興会  科学研究費助成事業 特別研究員奨励費  特別研究員奨励費

    奥西 巧一

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    配分額:900000円 ( 直接経費:900000円 )

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  • 角転送行列を用いた2次元古典格子系、1次元量子系の研究 研究課題

    1996年4月 - 1999年3月

    日本学術振興会  特別研究員奨励費 

    奥西 巧一

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    担当区分:研究代表者  資金種別:競争的資金

    配分額:2700000円 ( 直接経費:2700000円 )

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担当経験のある授業科目(researchmap)

  • 統計力学I

    機関名:新潟大学

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  • 物理学基礎AII

    機関名:新潟大学

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  • 統計力学II

    機関名:新潟大学

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  • 物性物理学I

    機関名:新潟大学

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  • 物理学基礎ゼミ

    機関名:新潟大学

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  • 現代物理学

    機関名:新潟大学

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  • 課題探索特講

    機関名:新潟大学

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  • 物性物理学II

    機関名:新潟大学

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  • 専門力アクティブラーニング

    機関名:新潟大学

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  • 基礎物理学演習II

    機関名:新潟大学

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  • 基礎物理学演習I

    機関名:新潟大学

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  • 多体系物理学

    機関名:新潟大学

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  • 統計力学演習I

    機関名:新潟大学

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  • 統計力学III

    機関名:新潟大学

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  • 物理数学演習I

    機関名:新潟大学

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  • 物理学演習III

    機関名:新潟大学

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  • 物理学演習II

    機関名:新潟大学

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  • 理学・スタディースキルズ

    機関名:新潟大学

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  • 多体問題

    機関名:新潟大学

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担当経験のある授業科目

  • 日本事情自然系A

    2021年
    -
    現在
    機関名:新潟大学

  • 領域概説 B (理学)

    2021年
    -
    現在
    機関名:新潟大学

  • 課題研究C

    2020年
    -
    現在
    機関名:新潟大学

  • 課題研究D

    2020年
    -
    現在
    機関名:新潟大学

  • 物性物理学

    2020年
    -
    現在
    機関名:新潟大学

  • 課題研究A

    2020年
    -
    現在
    機関名:新潟大学

  • 現代物理学セミナーB

    2020年
    -
    現在
    機関名:新潟大学

  • 統計力学IB

    2020年
    -
    現在
    機関名:新潟大学

  • 課題研究B

    2020年
    -
    現在
    機関名:新潟大学

  • 現代物理学セミナーA

    2020年
    -
    現在
    機関名:新潟大学

  • 統計力学IA

    2020年
    -
    現在
    機関名:新潟大学

  • 専門力アクティブ・ラーニング

    2018年
    -
    現在
    機関名:新潟大学

  • 理学スタディ・スキルズ

    2018年
    機関名:新潟大学

  • 物性物理学I

    2017年
    機関名:新潟大学

  • 課題探索特講

    2015年
    機関名:新潟大学

  • 計算物性学

    2013年
    -
    2017年
    機関名:新潟大学

  • 化学実験

    2013年
    機関名:新潟大学

  • 課題研究a

    2013年
    機関名:新潟大学

  • 課題研究b

    2013年
    機関名:新潟大学

  • 現代物理学

    2013年
    機関名:新潟大学

  • 物理学基礎ゼミ

    2013年
    機関名:新潟大学

  • 多体系物理学

    2012年
    -
    現在
    機関名:新潟大学

  • 統計力学I

    2012年
    -
    現在
    機関名:新潟大学

  • 数理物質科学演習Ⅰ(物理学)

    2012年
    -
    2015年
    機関名:新潟大学

  • 数理物質科学特定研究Ⅰ(物理学)

    2012年
    -
    2015年
    機関名:新潟大学

  • コラボレーション演習

    2012年
    -
    2015年
    機関名:新潟大学

  • 数理物質科学特定研究ⅡA(物理学)

    2012年
    -
    2014年
    機関名:新潟大学

  • 数理物質科学演習Ⅱ(物理学)

    2012年
    -
    2014年
    機関名:新潟大学

  • 数理物質科学特定研究ⅡB(物理学)

    2012年
    -
    2014年
    機関名:新潟大学

  • 物理学基礎A II

    2011年
    -
    現在
    機関名:新潟大学

  • 統計力学III

    2008年
    -
    2016年
    機関名:新潟大学

  • 物性物理学II

    2008年
    -
    2016年
    機関名:新潟大学

  • 物理数学演習I

    2008年
    -
    2010年
    機関名:新潟大学

  • 統計力学演習I

    2007年
    -
    現在
    機関名:新潟大学

  • 基礎物理学演習II

    2007年
    機関名:新潟大学

  • 基礎物理学演習I

    2007年
    機関名:新潟大学

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社会貢献活動

  • 日本物理学会・領域3運営委員

    役割:運営参加・支援

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  • 日本物理学会領域11世話人

    役割:運営参加・支援

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