Shusuke Yomo

The Strontium Content Dependence of Pressure Effect in (La1-xSrx)2CuO4

The enhancement of the superconducting transition temperature by pressure was studied up to ~1.5 GPa for samples of (La1-xSrx)2CuO4 with varying strontium content x. Simultaneously measured Hall coefficients were found to be independent of pressure in the present range, ruling out a pressure induced carrier density change as an origin of the Tc enhancement. The pressure coefficient of Tc was found roughly the same for the two regimes. i.e. for x<0.08, where Tc increases with x. and for 0.08<x<0.15 where it decreases with x. These results are discussed in the context of the current controversy over the nature of the electronic state in this class of materials.

High Pressure Study and the Critical Current of High Tc Superconductor (La0.9Sr0.1)2CuO4-y

The superconductivity of the multi-phased sample of (La0.9Sr0.1)2CuO4-y has been studied under hydrostatic pressure to 2.0 GPa and quasihydrostatic pressure to 4.2 GPa. The superconducting transition temperature (Tc) increases initially with increasing pressure. The results of the critical current and the diamagnetic susceptibility measurements, and the high sensitivity of Tc to the hydrostaticity of pressure suggest that the superconductivity of the high Tc phase in this multi-phased material is of bulk and highly anisotropic in nature, while that of the low Tc phase is of interfacial nature.

Pressure Effect on the Lattice Constant and Compressibility of a Superconductor (La0.9Sr0.1)2CuO4-y

(La0.9Sr0.1)2CuO4-y with K2NiF4-type layered structure shows a superconducting transition at 37 K. The pressure effect on the lattice constants along a- and c-axis was measured at room temperature up to 70 kbar. Compression coefficients of a- and c-axis are found to be almost of the same value of (2.5±0.4)×10-4kbar.-1

Electrical resistivity of CeCu6 under high pressure

Abstract A smooth transition from the heavy-fermion to the intermediate-valence state has been observed in the electrical resistivity measurement of the paramagnetic system CeCu 6 with increasing pressure up to 120 kbar. The pressure dependences of some characteristic physical quantities and the scaling behavior are described.

High pressure effect on the electrical properties of NiS2

Abstract A semiconductor-metal transition in the electrical resistance of NiS 2 , which has been suggested to be a Mott transition, is observed with decreasing temperature under pressure up to 44 kbar. The transition temperature increases with pressure with a slope of dT dP = 6 ± 1 K/kbar . The activation energy in a semiconducting region is found to decrease with increasing pressure and to vanish at about 46 kbar. The critical pressure and temperature are predicted to be 46 ± 2 kbar and 350 ± 20 K.

High-Pressure X-Ray Study on (L1-xAx)aCubOcSystem

The (L1-xAx)aCubOc system have high Tc superconductivity, where L stands for La, Y and so on, A for the alkaline earth elements, like Ba, Sr and so on. The pressure effect on the lattice constants of YBa2Cu3O7-y has been measured at room temperature up to 60 kbar. Compression coefficients of a- and c-axis are found to be almost the same value of (3.3±0.3)×10-4 kbar-1, which is larger than the value for (La0.9Sr0.1)2CuO4 and the ratio of c/a is found to be almost constant, whose tendency was observed in (La0.9Sr0.1)2CuO4. But the ratio of a/b decreases slightly under pressure.

High pressure effect on the anomalous electrical resistivity of narrow-band ferromagnets CoS2 and Fe0.2Co0.8S2

Abstract The resistivity was measured between 4.2 and 300 K under high pressure in order to clarify the effect of electron correlation. With decreasing T c (by pressure) a hump anomaly of CoS 2 becomes sharper and an anomalous increase with lowering the temperature in Fe 0.2 Co 0.8 S 2 is suppressed above 60 kbar.

Pressure Study and the Critical Current of High-Tc Superconductors YBa2Cu3O7-y and GdBa2Cu3O7-y

The pressure dependences of the superconducting transitions of the title materials have been studied under hydrostatic pressure up to Gpa. The midpoints of the resistive transitions were 92.2 K and 93.2 K for YBa2Cu3O7-y and GdBa2Cu3O7-y, respectively, and increased with increasing pressure at a rate of (5–6)×10-3 GPa-1. The critical current of GdBa2Cu3O7-y was two to three times as large as that of YBa2Cu3O7-y; both were enhanced by the application of pressure.

Hole localization and superconductivity in La2CuO4+δ

Abstract Hall coefficient of the oxygen-intercalated La214 cuprate was measured at room temperature and 77.4 K for δ=0.015 – 0.110. The Hall number, defined as a quantity inversely proportional to the Hall coefficient, increased linearly with δ for δ 0.030, it was observed to be lower than the expected increase of hole carrier with increasing δ. This indicated the existence of localized holes in the electronically phase separated superconducting phases. Our resistivity and the thermopower measurements were also consistent with this conjecture.

Electrical resistivity of CoS2 in the paramagnetic phase

Abstract The resistivity of ferromagnetic CoS 2 has been measured from 4 K to 600 K, in order to investigate the effect of the temperature change in the local spin density amplitude on the resistivity in the paramagnetic phase. The temperature dependence of the resistivity was found to change at around 240 K and 400 K.