Faculty Profiles - NAKANO Tomohito

Papers - NAKANO Tomohito

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Novel thermoelectric properties of complex transition-metal oxides Reviewed

Ichiro Terasaki, Manabu Iwakawa, Tomohito Nakano, Akira Tsukuda, Wataru Kobayashi

DALTON TRANSACTIONS 39 ( 4 ) 1005 - 1011 2010

Language：English Publishing type：Research paper (scientific journal) Publisher：ROYAL SOC CHEMISTRY

We report how the thermopower of complex transition-metal oxides is susceptible to small changes in material parameters. In the A-site ordered perovskite oxide R(2/3)Cu(3)Ti(3.6)Ru(0.4)O(12), the thermopower changes from 15 to -100 mu VK(-1) at 300 K in going from R = La to Er. We associate this with the hybridization between Cu 3d and Ru 4d electrons, which depends on R. For stronger hybridization, the Cu 3d electrons become more itinerant leading to positive thermopower. In the A-site ordered perovskite cobalt oxide Sr(3)YCo(4)O(10.5), the spin state of the Co(3+) ions determines the magnitude of the thermopower, where partial isovalent substitution (Ca for Sr and Rh for Co) enhances the thermopower whilst keeping the resistivity intact. These substitutions stabilize the low spin state of the Co(3+) ions, which affects the thermopower through the entropy of the background for the carriers. We propose that the control of the magnetism plays a pivotal role in determining the thermopower in a certain class of complex oxides.

DOI： 10.1039/b914661j

Pressure effect on magnetic properties of a weak ferromagnet BaIrO3 Reviewed

Takanori Kida, Atsuko Senda, Shunsuke Yoshii, Masayuki Hagiwara, Tomohito Nakano, Ichiro Terasaki

INTERNATIONAL CONFERENCE ON MAGNETISM (ICM 2009) 200 2010

Language：English Publishing type：Research paper (international conference proceedings) Publisher：IOP PUBLISHING LTD

We have investigated the pressure effect on magnetic properties of a weak itinerant ferromagnet BaIrO3, using a Cu-Be piston-cylinder type pressure cell. Under external pressure the ferromagnetic transition temperature T-c decreases linearly with pressure at a large rate of dT(c)/dP similar to -17 K/GPa. The critical exponent beta up to 0.86 GPa indicates the anomalous value (beta similar to 0.9) and is nearly pressure-independent. Anomalous pressure effects on conductivity and magnetism suggest the existence of a nontrivial coupling among charge, spin and lattice in this compound.

DOI： 10.1088/1742-6596/200/1/012084

Pressure-induced Quantum Phase Transition in Cr1-xVx Alloys

Oomi G., Fuchizaki Y., Ohashi M., Nakano T., Uwatoko Y.

Transactions of the Magnetics Society of Japan 34 ( 3 ) 263 - 266 2010

Language：English Publisher：The Magnetics Society of Japan

The electrical resistivities and Hall effect of Cr<sub>1-x</sub>V<sub>x</sub> alloys(x<0.04) were measured at high pressure. The Néel temperature T<sub>N</sub> of x = 0.015, 0.032 and 0.035 was found to disappear at corresponding P<sub>c</sub> of 5.6 GPa, 1.1 GPa and 0.7 GPa, respectively. The Hall coefficients of x = 0.032 and 0.035 demonstrated a discontinuous change around P<sub>c</sub>, which implies a quantum phase transition induced by high pressure. We found that the exponents in the power law of resistivity of x = 0.015 had an anomaly around P<sub>c</sub>, which means that the large quantum fluctuations affect the resistivity even at high temperature. A possibility of superconductivity is explored near P<sub>c</sub> for x = 0.032. A slight decrease in the ρ(T) curve was observed below 1 K at high pressure. The origins for that are briefly discussed.

DOI： 10.3379/msjmag.1003R045

Other Link： https://jlc.jst.go.jp/DN/JALC/00351380509?from=CiNii

Chemical and Physical Pressure Effects on the Magnetic and Transport Properties of the A-Site Ordered Perovskite Sr3YCo4O10.5 Reviewed

Shin Yoshida, Wataru Kobayashi, Tomohito Nakano, Ichiro Terasaki, Kazuyuki Matsubayashi, Yoshiya Uwatoko, Inga Grigoraviciute, Maarit Karppinen, Hisao Yamauchi

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 78 ( 9 ) 094711-1-5 - 94711 2009.9

Language：English Publishing type：Research paper (scientific journal) Publisher：PHYSICAL SOC JAPAN

We have measured and analysed the magnetization, resistivity and thermopower of polycrystalline samples of Ca-subsituted Sr3YCo4O10.5. While the transition temperature and thermopower systematically decrease with increasing Ca content, the resistivity is nearly independent of the Ca-substitution level. We have furhter found that the substitution of Ca for Sr systematically decreases the lattice paremeters while maintaining the oxygen content unchanged, and have explained the Ca-substitution effects in terms of the spin-state transition induced by chemical pressure. We have also measured the magnetization of one of the samples under hydrostatic pressure. The result shows that the physical-pressure effect is semiquantitatively consistent with the chemical-pressure effect.

DOI： 10.1143/JPSJ.78.094711

Ferromagnetism and spin-glass transitions in the Heusler compounds Ru2-xFexCrSi Reviewed

Masahiko Hiroi, Tsugumi Rokkaku, Kazuhisa Matsuda, Toru Hisamatsu, Iduru Shigeta, Masakazu Ito, Takuo Sakon, Keiichi Koyama, Kazuo Watanabe, Shintaro Nakamura, Tsutomu Nojima, Tomohito Nakano, Lin Chen, Tetsuya Fujiwara, Yoshiya Uwatoko, Hirotaka Manaka, Norio Terada

PHYSICAL REVIEW B 79 ( 22 ) 224423-1-10 2009.6

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER PHYSICAL SOC

The results of a comprehensive study of the structural, magnetic, and magnetotransport properties of the Heusler compounds Ru2-xFexCrSi are presented. The Fe-rich compounds (x >= 1.5) exhibit a usual ferromagnetic transition. The Ru-rich compound (x=0.1) does not show ferromagnetism but exhibits a peak in magnetic susceptibility at T-N(*)=30 K. Nevertheless, specific-heat measurements show that there is no antiferromagnetic transition at T-N(*) nor at any other temperatures. With further decreasing temperature strong irreversible behavior occurs below a temperature T-g. It is proposed that these results can be interpreted as successive spin-glass transitions at T-N(*) and T-g. The compounds with intermediate x (=0.3 and 0.5) are found to be ferromagnetic. Both the saturation magnetization and the Curie temperature T-C increase with increasing x. However, being different from the Fe-rich compounds, hysteresis of the magnetization is observed and this suggests that the ferromagnetism has glassy character. In lower-temperature range fairly large negative magnetoresistance are observed. These results are suggestive of a magnetically inhomogeneous ferromagnetic state, that is, the formation of ferromagnetic clusters.

DOI： 10.1103/PhysRevB.79.224423

Pressure-induced superconductivity in the orthorhombic Kondo compound CePtSi2 Reviewed

Tomohito Nakano, Masashi Ohashi, Gendo Oomi, Kazuyuki Matsubayashi, Yoshiya Uwatoko

PHYSICAL REVIEW B 79 ( 17 ) 172507-1-4 2009.5

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER PHYSICAL SOC

We have discovered a member of superconductivity induced by hydrostatic pressure in an antiferromagnetic (AF) concentrated Kondo compound CePtSi2 by measuring the resistivity and magnetoresistance. The AF transition temperature T-N of 1.8 K at ambient pressure is suppressed by pressure and disappears around 1.2 GPa. Above 1.4 GPa, a superconducting phase appears between 1.4 and 2.4 GPa, with the maximum T-c of 0.14 K at 1.7 GPa. The upper critical field of the superconductivity is evaluated to be 0.7 T, which is clearly larger than that of the Pauli limit predicted by the BCS theory.

DOI： 10.1103/PhysRevB.79.172507

Anisotropic Lattice Compression and Possible Valence Change in Kondo Compound CeAu_2Si_2

OHMURA Mio, SAKAI Kumiko, NAKANO Tomohito, MIYAGAWA Hidenori, OOMI Gendo, SATO Isamu, KOMATSUBARA Takemi, AOKI Haruyoshi, MATSUMOTO Yuji, UWATOKO Yoshiya

Journal of the Magnetics Society of Japan 33 ( 2 ) 31 - 34 2009.3

Language：English Publisher：The Magnetics Society of Japan (MSJ)

Lattice constants and crystal structure of tetragonal Kondo compound CeAu<sub>2</sub>Si<sub>2</sub> have been investigated at high pressure in order to examine pressure-induced change of crystal structure and gradual valence change of Ce atom. It is found that the lattice constants a and c decrease with increasing pressure without any discontinuity up to 13 GPa at room temperature. The bulk modulus <i>B</i><sub>0</sub> and its pressure derivative <i>B</i><sub>0</sub>' were obtained to be 112 GPa and 3.2. These values are comparable with other Kondo compounds. From the ρ(T) curve at high pressure which does not change so much under high pressure, it is suggested that the valence of Ce changes sluggishly with pressure and the Kondo state of CeAu<sub>2</sub>Si<sub>2</sub> is very stable at high pressure.

DOI： 10.3379/msjmag.0902LC8069

Partial Gap of SDW in Ce(Ru1-xRhx)2Si2 Reviewed

T. Nakano, S. Murayama, G. Oomi

Journal of the Magnetics Society of Japan 33 ( 2 ) 35-37 2009

Thermal expansion and magnetostriction of the concentrated Kondo compound CePtSi2 Reviewed

Tomohito Nakano, Gendo Oomi, Tetsuya Takeuchi

25TH INTERNATIONAL CONFERENCE ON LOW TEMPERATURE PHYSICS (LT25), PART 4 150 ( 4 ) 042139-1-4 2009

Language：English Publishing type：Research paper (international conference proceedings) Publisher：IOP PUBLISHING LTD

We measured the thermal expansion and magnetostriction of a polycrystalline concentrated Kondo compound CePtSi2, which has an orthorhombic CeNiGe2-type structure and the characteristic Kondo temperature T-K of 3 K. The linear thermal expansion dl/l decreases smoothly with decreasing temperature down to 2.5 K, and shows a rapid increase below 2 K due to antiferromagnetic ordering. The magnetostriction at 0.7 K shows an anomaly below 0.5 T, suggesting the disappearance of antiferromagnetic ordering.

DOI： 10.1088/1742-6596/150/4/042139

Hall effect in CeAl2 under high pressure Reviewed

Hidenori Miyagawa, Masashi Ohashi, Tomohito Nakano, Gendo Oomi, Isamu Satoh, Takemi Komatsubara

25TH INTERNATIONAL CONFERENCE ON LOW TEMPERATURE PHYSICS (LT25), PART 4 150 ( 4 ) 042127-1-4 2009

Language：English Publishing type：Research paper (international conference proceedings) Publisher：IOP PUBLISHING LTD

We have measured the Hall effect of the single crystalline CeAl2 under high pressure up to 3 GPa. An anomalous Hall effect was observed at high pressures. The Hall coefficient R-H at 0.1 GPa is positive (R-H similar to 1.7 x 10(-9) m(3)/As). It decreases smoothly with increasing pressure below 2 GPa, but rapidly above 2 GPa. The sign of the Hall coefficient is found to change from positive to negative as pressure increases. These results suggest a pressure-induced crossover in the electronic state around 2.7 GPa.

DOI： 10.1088/1742-6596/150/4/042127

Nanoscale spin-dependent transport in a weak itinerant ferromagnet BaIrO3 Reviewed

Takanori Kida, Shunsuke Yoshii, Masayuki Hagiwara, Tomohito Nakano, Ichiro Terasaki

25TH INTERNATIONAL CONFERENCE ON LOW TEMPERATURE PHYSICS (LT25), PART 2 150 ( 2 ) 022037-1-4 2009

Language：English Publishing type：Research paper (international conference proceedings) Publisher：IOP PUBLISHING LTD

We have performed magnetoresistance (MR) measurements for a BaIrO3 single crystal. As the magnetic field changes from 0 T to 14 T and back to 0 T, the longitudinal MR at 1.5 K traces out a butterfly-shaped hysteresis curve and the sign of the MR ratio for the field-ascending sweep changes from negative to positive near 6 T. The MR ratios for field-ascending and descending sweeps overlap each other around 13 T. A large anisotropic magnetoresistance (AMR) ratio per mu B, (rho parallel to - rho perpendicular to) rho(0) similar to 400 %/mu B at 1.5 K, is obtained, where rho parallel to, rho perpendicular to, and rho(0) are the longitudinal resistivity, the transverse resistivity, and the zero- field resistivity. The MR effects are interpreted based on a spin valve model considering a novel crystal structure and a local magnetic-anisotropy related to the spin-orbit coupling of this compound.

DOI： 10.1088/1742-6596/150/2/022037

Resistivity of antiferromagnetic compounds Ce0.8La0.2(Ru 1-xRhx)2Si2 Reviewed

S. Ishiuchi, H. Naruse, S. Murayama, Y. Amakai, T. Nakano, Y. Uwatoko

Journal of Physics: Conference Series 150 ( 4 ) 042072-1-4 2009

Language：English Publishing type：Research paper (international conference proceedings) Publisher：Institute of Physics Publishing

We have measured the susceptibility χ and resistivity ρ on single-crystalline Ce0.8La0.2(Ru1-xRh x)2Si2 in directions parallel and perpendicular () to the c-axis. The χ for each Rh concentration increases with decreasing temperature, and then rapidly decreases for x ≤ 0.2 and x ≥ 0.8, indicating the AF transition at the Néel temperature TN. The ρ for x 0 shows a cusp at around TN 5.5 K, however, ρ indicates a broad maximum at higher temperature than TN and a rapid decrease at much lower temperature than TN, without evidence of a gap-type AF ordering. The ρ for x 0.2 indicates a clear cusp at about T N. Such sharp anomaly in ρ once disappears for x 0.6 and is again observed at TN for x ≥0.8 probably due to another type of the AF ordering. © 2009 IOP Publishing Ltd.

DOI： 10.1088/1742-6596/150/4/042072

Ferromagnetism and spin-glass transitions in the Heusler compounds Ru2-x Fex CrSi

Hiroi, M., Rokkaku, T., Matsuda, K., Hisamatsu, T., Shigeta, I., Ito, M., Sakon, T., Koyama, K., Watanabe, K., Nakamura, S., Nojima, T., Nakano, T., Chen, L., Fujiwara, T., Uwatoko, Y., Manaka, H., Terada, N.

Physical Review B - Condensed Matter and Materials Physics 79 ( 22 ) 2009

強相関酸化物BaIrO3 におけるモット絶縁体−電荷移動型絶縁体転移の圧力効果 Reviewed

中野智仁

高圧力の科学と技術 18 ( 1 ) 62-68 - 62 2008

Nonlinear thermoelectric properties of layered cobalt oxides Reviewed

Tomohito Nakano, Wataru Kobayashi, Ryoji Funahashi, Kenji Sugiura, Hiromichi Ohta, Kunihito Koumoto, Ichiro Terasaki

PROCEEDINGS ICT 07: TWENTY-SIXTH INTERNATIONAL CONFERENCE ON THERMOELECTRICS 121 - 123 2008

Language：English Publishing type：Research paper (international conference proceedings) Publisher：IEEE

We synthesized whisker-crystal and thin-film samples of the layered cobalt oxide (Bi2Sr2O4)(x)CoO2 well known as a candidate of a thermoelectric material, and measured their resistivity, voltage-current characteristics and magnetoresistance with pulsed currents. Resistivity of the whisker sample with a small current density of 2 mA/cm(2) increases with decreasing temperature below 100 K, suggesting development of a pseudogap on the density of states. This increase is suppressed by a large current density of 110 A/cm(2). The nonlinear voltage-current characteristics are observed both for the whisker and thin-film samples. The negative magnetoresistance of the thin-film sample is suppressed by current density. These suggest that the density of states recovers owing to the suppression of the pseudogap by applying large current density.

DOI： 10.1109/ICT.2007.4569438

Unconventional critical behavior in the weak ferromagnet BaIrO<sub>3</sub> Reviewed

Kida, T., Senda, A., Yoshii, S., Hagiwara, M., Takeuchi, T., Nakano, T., Terasaki, I.

EPL 84 ( 2 ) 27004-1-4 2008

Dielectric constant and ac conductivity of the layered cobalt oxide Bi(2)Sr(2)CoO(6+delta): A possible metal-dielectric composite made by self-organization of Co(2+) and Co(3+) ions Reviewed

Y. Nagao, I. Terasaki, T. Nakano

PHYSICAL REVIEW B 76 ( 14 ) 144203-1-5 2007.10

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER PHYSICAL SOC

The dielectric constant and ac conductivity of a single-crystal sample of the layered cobalt oxide Bi(2)Sr(2)CoO(6+delta) were measured along the c axis from 10(2) to 10(9) Hz from 110 to 300 K, the frequency dependence of which is found to be similar to that for metal-dielectric composites. The ac conductivity obeys the same scaling relation as that of disordered solids, which is quantitatively consistent with a theoretical calculation for spatially inhomogeneous conductive medium. This implies that the charge carrier on the Co(2+) site is electronically segregated in the CoO(2) plane. This compound can work as a metal-dielectric composite even in a form of single crystal, a possible mechanism of which is suggested on the basis of the electronic configuration of Co ions.

DOI： 10.1103/PhysRevB.76.144203

Positive and negative pressure effects on the magnetic ordering and the Kondo effect in the compound Ce2RhSi3 Reviewed

T. Nakano, Kausik Sengupta, Sudhindra Rayaprol, M. Hedo, Y. Uwatoko, E. V. Sampathkumaran

JOURNAL OF PHYSICS-CONDENSED MATTER 19 ( 32 ) 326205-1-13 2007.8

Language：English Publishing type：Research paper (scientific journal) Publisher：IOP PUBLISHING LTD

The competition between magnetic ordering and the Kondo effect in Ce2RhSi3, ordering antiferromagnetically at 7 K, is investigated by the measurements of magnetization, heat capacity and electrical resistivity on the solid solutions, Ce2-xLaxRhSi3, Ce2-yYyRhSi3, and Ce2RhSi3-zGez, as well as by high-pressure studies on this compound. The trends in the Kondo and Neel temperature variations among these alloys are compared to infer the roles of unit-cell volume and electronic structure changes. On the basis of the results, we infer that this compound lies at the peak of Doniach's magnetic phase diagram. The high-pressure electrical resistivity data indicate that the quantum critical point for this compound is in the vicinity of 4 GPa.

DOI： 10.1088/0953-8984/19/32/326205

Specific heat of superstoichiometric samarium dihydride (SmH2+delta) Reviewed

O. Nakamura, Y. Tanaka, K. Nakazawa, T. Nakano, M. Sakai, S. Orimo, Y. Uwatoko

JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS 310 ( 2 ) E65 - E67 2007.3

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

We have investigated the specific heat and magnetic properties of superstoichiometric samarium dihydride (SmH2.16). The temperature (T) dependence of the specific heat (C-p) has three peaks at 7.1, 10.8 and 15.9 K. Below 7.1 K, C/ T is an almost linear function of T-2. The Sommerfeld coefficient gamma(LT) of the electronic specific heat below 7.1K is 150mJ= (K-2 mole). SmH2.16 exhibits coexistence of heavy- fermion behavior and magnetic ordering. The temperature dependence of the magnetic susceptibility can be explained by a valence ratio of 75% Sm3+, while the Shottky contribution of the specific heat can be explained by a valence ratio of 60% Sm3+. The reason why the amount of Sm3+ and the lattice constant of SmH2.16 are small compared with SmH2 is not yet clear. (c) 2006 Elsevier B. V. All rights reserved.

DOI： 10.1016/j.jmmm.2006.10.244

Chemical pressure effect in the heavy fermion compound Ce1yLay(Ru0.85Rh0.15)(2)Si-2 Reviewed

Tsuyoshi Horii, Satomi Shimada, Tomohito Nakano, Shigeyuki Murayama, Hideaki Takano, Masato Hedo, Yoshiya Uwatoko

JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS 310 ( 2 ) 313 - 315 2007.3

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

We have measured the susceptibility and resistivity on single-crystalline Ce1-yLa(y)(Ru0.85Rh0.15)(2)Si-2 along the tetragonal c-axis. The susceptibility increases with decreasing temperature, and then rapidly decreases, indicating the AF transition in each La concentration y. The resistivity also shows an anomaly in each La concentration y at around the T-N. For y <= 0.1, we observe a jump in resistivity just below TN as is seen for Ce( Ru0.85Rh0.15)(2)Si-2. This is interpreted in terms of the gap opening on the Fermi- surface along the c-axis. However, for y >= 0.2 the resistivity shows a downward bend at TN. These experimental facts suggest that the crossover from the spin density wave (SDW) with the gap opening to the localized AF order occurs at around y = 0.1. (c) 2006 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jmmm.2006.10.013

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