Masashi Ohashi

High-pressure apparatus for the measurement of thermal and transport properties at multi-extreme conditions

A high-pressure apparatus which we can use at low temperature and high magnetic field has been designed for making measurements of thermal and transport properties of condensed matter. The details of the apparatus and some examples of the measurements are described and briefly discussed. It is proved that the apparatus is suitable for use up to hydrostatic pressure of ~ 3.5 GPa, down to ~ 2.0 K and up to ~ 11 T.

Anomalous elastic softening of Sm Ru 4 P 12 under high pressure

The filled skutterudite compound SmRu_4P_{12} undergoes a complex evolution from a paramagnetic metal (phase I) to a probable multipolar ordering insulator (phase II) at T_{MI} = 16.5 K, then to a magnetically ordered phase (phase III) at T_{N} = 14 K. Elastic properties under hydrostatic pressures were investigated to study the nature of the ordering phases. We found that distinct elastic softening above T_{MI} is induced by pressure, giving evidence of quadrupole degeneracy of the ground state in the crystalline electric field. It also suggests that quadrupole moment may be one of the order parameters below T_{MI} under pressure. Strangely, the largest degree of softening is found in the transverse elastic constant C_{T} at around 0.5-0.6 GPa, presumably having relevancy to the competing and very different Gruneisen parameters \Omega of T_{MI} and T_{N}. Interplay between the two phase transitions is also verified by the rapid increase of T_{MI} under pressure with a considerably large \Omega of 9. Our results can be understood on the basis of the proposed octupole scenario for SmRu_4P_{12}.

Effect of pressure on the superconductivity of RuSm1.4Ce0.6Sr2Cu2O10

Abstract A new hybrid ruthenate–cuprate superconductor RuSm1.4Ce0.6Sr2Cu2O10 was synthesized. This compound also exhibits magnetic ordering around 130 K . Electrical resistance measurement under high pressure has been carried out in order to clarify the interplay between magnetism and superconductivity. It is found that Tc increases but the magnetic ordering temperature decreases with pressure, i.e., the superconductivity competes with magnetic ordering.

Studies of Pressure-Induced Mott Metal-Insulator Transition of BaCoS2.

Transport, thermal and elastic properties under the hydrostatic pressure p up to 21.2 kbar have been studied on the layered compound BaCoS 2 , which exhibits the Mott insulator→metal transition wit...

Thermal expansion anomaly near the martensitic transformation in GdCu under pressure

Abstract The thermal expansion and electrical resistivity of GdCu have been measured under pressure. A large anomaly in the temperature dependence of thermal expansion is observed at 239 K (=M s ) at ambient pressure. Ms is found to decrease rapidly by applying pressure at a rate of d M s / d P∼−160 K / GPa .

Anomalous pressure dependence of the electrical resistance and thermal expansion of UGe2 in the paramagnetic state

The electrical resistivity of single-crystalline UGe 2 has been measured under high pressure. It is found that the pressure dependence of the electrical resistivity shows an anomaly around 1 GPa even in the paramagnetic temperature range above T C = 52 K. But the anomaly disappears above 200 K. The thermal expansion of UGe 2 was also measured at high pressure to clarify the lattice properties. The thermal expansion coefficients show a peak near 1 GPa below T C but a similar peak is also found near 1 GPa at 80 K, i.e., in the paramagnetic region. The compressibility shows an anomalous increase with decreasing temperature even in the paramagnetic region. These facts suggest that the paramagnetic state of UGe 2 is not only in the anomalous electronic state but also in the anomalous lattice state.

Relation between the Superconducting Transition Temperature and the Axial Ratio in R Ni 2 B 2 C ( R =Y, Ho, Er, Tm, Lu) Under High Pressure

The effect of pressure on the lattice parameters of R Ni 2 B 2 C ( R = Y, Ho, Er, Tm, Lu) has been measured in order to investigate the relation of T C to axial ratio c/a in the tetragonal lattice or the cohesive properties of these materials. The large anisotropies are observed in the lattice compression curves especially for R =Ho and Tm; the c -axis is more compressible than the a -axis. On the other hand, the anisotropy in the lattice compression is found to be relatively small for R =Y and Lu. The close relation between T C and c/a is pointed out particularly for R =Ho.

Single Crystal Growth of RNiX2 (R = U and Ce, X = Si and Ge) Ternary Compounds

We have attempted to grow single crystal of RNiX 2 (R = U and Ce, X = Si and Ge) which crystallizes in orthorhombic CeNiSi 2 -type layered structure. Although RNiX 2 compounds do not melt congruently, CeNiGe 2 and UNiSi 2 single crystals can be grown by the Czochralsky method from Ge- or Si-rich sample as the initial one. As for CeNiGe 2 , the nominal composition of each compounds are refined to be CeNi x Ge 2 ( x < 1) from the result of SEM analysis. Although it is well-known that CeNiGe 2 orders antiferromagnetically at T N1 = 3.8 K and T N2 = 3.2 K, both T N1 and T N2 depend on the concentration of Ni. It may comes from the fact that the magnetic property relies on a structural homogeneity and a stoichiometry. UNiSi 2 , which orders ferromagnetically at T C = 95 K, displays the anisotropic behavior of the magnetization. The easy magnetization direction is in the ac plane. From the result of the magnetic susceptibility along the ac plane, the effective magnetic moment is obtained to be µ eff ∼2.47 µ B ,...

Precursor phenomena to pressure- induced superconductivity in the thermal expansion of UGe2

Thermal expansions of single crystalline UGe2 have been measured at high pressure up to 2 GPa. It is found at ambient pressure that there is a large anisotropy in the α(T) curve depending on the ciystal axis and discontinuous change corresponding to ferromagnetic ordering near 50 K(= TC). Furthermore a small change in α (T) is observed near 25 K (= T*) below TC, which may be related with a phase transition. We studied the pressure dependence of TC and T*, which are strongly dependent on pressure and diappear near 1.8 GPa and 1.2 GPa, respectively. Anomalous pressure dependence is also found in the α (T) and ρ(T) still above TC. The origin of these anomalies will be discussed briefly in connection with the ferromagnetism and pressure-induced superconductivity.

Anomalous elastic softening of SmRu4P12 under high pressure

The filled skutterudite compound SmRu_4P_{12} undergoes a complex evolution from a paramagnetic metal (phase I) to a probable multipolar ordering insulator (phase II) at T_{MI} = 16.5 K, then to a magnetically ordered phase (phase III) at T_{N} = 14 K. Elastic properties under hydrostatic pressures were investigated to study the nature of the ordering phases. We found that distinct elastic softening above T_{MI} is induced by pressure, giving evidence of quadrupole degeneracy of the ground state in the crystalline electric field. It also suggests that quadrupole moment may be one of the order parameters below T_{MI} under pressure. Strangely, the largest degree of softening is found in the transverse elastic constant C_{T} at around 0.5-0.6 GPa, presumably having relevancy to the competing and very different Gruneisen parameters \Omega of T_{MI} and T_{N}. Interplay between the two phase transitions is also verified by the rapid increase of T_{MI} under pressure with a considerably large \Omega of 9. Our results can be understood on the basis of the proposed octupole scenario for SmRu_4P_{12}.