Takaki Muramatsu

Superconductivity of CeRhIn5 under High Pressure

The pressure dependence of transport properties in CeRhIn5 was studied up to 8.5 GPa by using a diamond-anvil cell. The electrical resistivity does not follow the T 2 -dependecne of a Fermi liquid nature under any pressure. In this non-Fermi liquid state, superconductivity was observed in a wide pressure range of 1:5 GPa <P <7:6 GPa. The superconducting temperature TC has a double-maximum structure with a minimum at 5.2 GPa. A maximum of TC ¼ 2:2 K was observed at P ¼ 2:5 GPa, where the corresponding electrical resistivity just above TC has the maximum. The upper critical ;eld HC2 and the magnetoresistances were also investigated under pressure.

Second Anomaly in the Specific Heat of β-Pyrochlore Oxide Superconductor KOs2O6

Resistivity and specific heat have been measured on a single-crystalline sample of a β-pyrochlore oxide superconductor, KOs 2 O 6 . It is found that a second peak in specific heat, which may evidence an unknown phase transition, appears at around T p ∼7.5 K below the superconducting transition temperature T c =9.53 K. Applying magnetic fields up to 14 T, T c is reduced gradually down to 7.1 K, while T p is slightly raised and becomes even higher than T c at 14 T, which implies that the second anomaly is not associated directly with the superconductivity. It is demonstrated, however, that there is significant communication between the two anomalies, suggesting that they come from the same electrons. It is also reported that the Sommerfeld coefficient γ in KOs 2 O 6 is possibly much larger than those in other members of β-pyrochlore oxide superconductors, RbOs 2 O 6 ( T c =6.3 K) and CsOs 2 O 6 ( T c =3.3 K).

Electrical resistivity of CeTIn5 (T = Rh, Ir) under high pressure

Abstract We have studied the superconducting properties of CeTIn 5 (T=Rh, Ir) under high pressures by means of electrical resistivity measurements and determined the pressure–temperature phase diagrams for the superconducting transition. For both systems, the superconductivity exists in a wide pressure range, 1.5⩽ P ⩽6.5 GPa for CeRhIn 5 and 0⩽ P ⩽5.2 GPa for CeIrIn 5 .

Slow Magnetization Reversal in [Ni 4 (OMe) 4 (sal) 4 (MeOH) 4 ]

The magnetic behavior of the nickel cubane complex [Ni 4 (Ome) 4 -(sal) 4 (MeOH) 4 ](Hsal=salicylaldehyde) was studied by using a Hartshorn bridge equipped with a 3 He/ 4 He dilution refrigerator. This complex has long been known to have a well-isolated high-spin ground state with S =4 and it was suggested to have an easy-axis type anisotropy. AC magnetic susceptibility data clearly show a slow magnetization reversal below 0.3 K, characteristic of single-molecule magnets. The Arrhenius plot of the relaxation rate provides the activation energy of about 5 K.

High pressure effects on superconductivity in the β-pyrochlore oxides AOs2O6 (A = K, Rb, Cs)

Recently new pyrochlore oxides superconductors AOs2O6 (A=K,Rb,Cs) were found and we measured the pressure dependence of magnetization up to 1.2 GPa in order to deduce the pressure effect of Tc in the three beta-pyrochlore oxides. It is found that the initial pressure dependence of Tc is positive for all the compounds. Only KOs2O6 exhibits a saturation in Tc at 0.56 GPa and the downturn at higher pressure

Photocarrier Injection and Current–Voltage Characteristics of La0.8Sr0.2MnO3/SrTiO3:Nb Heterojunction at Low Temperature

An epitaxial heterojunction made of a p-type perovskite manganite La0.8Sr0.2MnO3 film and an n-type strontium titanate SrTiO3:Nb substrate has been fabricated by the pulsed laser deposition technique. The current?voltage (I?V) characteristics and photovoltaic properties are measured under UV light irradiation in a wide temperature range down to 10 K. It is found that the junction acts as an efficient UV photodiode with a large external quantum efficiency of 28%. It is also demonstrated that the manganite film can be doped with a certain amount of holes by photocarrier injection. The maximum surface hole density is estimated to be 3.0×1013 cm-2.

OBSERVATION OF WEAK FERROMAGNETISM IN ORGANIC VERDAZYL RADICAL CRYSTAL, P-CYDPTV

Abstract Magnetic property of 3-(4-cyanophenyl)-1,5-diphenyl-6-thioxoverdazyl (p-CyDpTV) radical crystal has been studied in the temperature range of 0.05–300 K in external magnetic fields 3, 11, 30, and 100 Oe by using SQUID magnetometer. Weak ferromagnetic behavior was observed at TN = 0.41 K. Below TN, the spontaneous magnetization Ms(T) increased rapidly, reaching a maximum of 31.1 emu/mol at 0.05 K. Spin-canting angle was estimated to be 0.32° at 0.05 K. Near TN, M s (T) ∝ [1 − ( T T N )] β with the critical exponent of β = 0.36, in agreement with that expected for an isotropic three-dimensional (3D) Heisenberg spin system.

Phase diagram of a new spin-Peierls compound:p-CyDOV

Abstract Susceptibility and magnetization measurements were performed to investigate the magnetic behavior of the verdazyl crystal p-CyDOV. The magnetic phase diagram was examined to check whether p-CyDOV is a real spin-Peierls material or not. Observed field dependence of the transition temperature shows good agreement with the theory for a spin-Peierls system. Magnetic phase diagram obtained agrees well with those of other spin-Peierls materials. As a result, it has been concluded that the p-CyDOV is a spin-Peierls compound.

Superconductivity under high pressure

Abstract In part 1, we review techniques developed in our laboratory for producing the complex extreme condition of very low temperature and ultra-high pressure and those for measuring electrical resistance and magnetization of the sample confined in the extremely small space of the used pressure cell. In part 2, we review our experimental results in search for pressure-induced superconductivity, which have been obtained by the use of developed techniques. Typical examples are shown in the case of simple inorganic and organic molecular crystals, ionic crystals, and magnetic metals.

Transport, AC-susceptibility and PQR measurements of strongly electron correlated compound YbInCu4 at high pressures

We have investigated physical properties of the valence fluctuating compound YbInCu4 at high pressures and low temperatures. The first-order valence transition temperature TV ~ 42 K at ambient pressure is completely suppressed for pressures above 2.49 GPa. Present electrical resistivity data in the pressure-stabilized high-temperature (HT) phase exhibit successive shoulders at around 20 and 2.4 K. The former can reasonably be compared with the Kondo temperature TK ~ 25 K estimated from the static-susceptibility data at ambient pressure. For pressures above 2.27 GPa, the ac-susceptibility showed a clear peak at 2.4 K, which can be explained as the onset of a long-range ferromagnetic ordering. 63Cu pure-quadrupole-resonance in the HT phase showed that the value of spin-lattice relaxation time T1 and the almost T-independent behavior are hardly affected with pressure up to 1.3 GPa, in contrast to the large increase in quadrupole frequency.