N. Môri

Metallization of magnetite (Fe3O4) under high pressure

Electrical resistivity measurements have been made on a good qualified single crystalline magnetite (Fe3O4) at temperatures from 300 down to 3.0 K under pressure up to 10 GPa. A steep change in resistivity at the Verwey transition temperature has been observed at pressure below 6.5 GPa, which shows a quite distinct result reported in prior work. Moreover, the Verwey transition temperature has been found to decrease nonlinearly with increasing pressure and it disappears at around 8 GPa. Above 8 GPa magnetite exhibits metallic behavior. The residual-resistivity ratio of the metallic state is observed to be more than 350. This finding of a metallic ground state in magnetite provides insight into the understanding of the Verwey transition in magnetite.

A newly developed high-pressure cell by using modified Bridgman anvils for precise measurements in magnetic fields at low temperatures

We have succeeded in developing a new high-pressure cell designed for measuring the absolute value of the electrical resistivity in magnetic fields at low temperatures. This pressure cell, based on a technique using modified Bridgman anvils with a Teflon capsule, can generate nearly hydrostatic pressure at least up to 6 GPa. The most important feature of this pressure cell is its compact body which can be easily combined with a dilution refrigerator in a superconducting magnet. One of its applications is for precise magneto-transport measurements under nearly hydrostatic pressure, which is demonstrated by the investigation into the pressure-induced superconducting state of the spin-ladder Sr14−xCaxCu24O41 (x=12) single crystal.

Is pressure dependence of Tc through change in carrier density

Abstract The pressure dependence of the Hall coefficient is studied for YBa 2 Cu 4 O 8 , YBa 2 Cu 3 O 7 , and Bi 2 Sr 2 CaCu 2 O 8 . The Hall coefficient decreases by ∼8–12%/GPa for these cuprates. The anomalous temperature dependence of the Hall coefficient is found to be intimately related with the occurrence of superconductivity.

A simple uniaxial pressure device for electrical resistivity measurements: Suitable for closed cycle refrigerator system

A simple uniaxial pressure device suitable for closed cycle refrigerator system (CCRS) has been built. This device in principle is applicable to any crystal. In this device the pressure can be varied smoothly and continuously to any desired temperature using a disc-micrometer and a spring – holder attachment, which are kept on the demountable top flange of the vacuum shroud of CCRS at room temperature. This device is not dependent on pressure calibration and the pressure calculation is obtained directly from the surface area of the crystal, the rotations of the disc-micrometer and the spring – constant value of the spring. Piezoresistance measurements were made on n-type Si to check the quality of data from the uniaxial pressure device. The performance of the uniaxial pressure device is illustrated by investigating the uniaxial pressure dependence of bilayer manganite La1.25Sr1.75Mn2O7 single crystals along the ab-plane and c-axis using electrical resistivity measurements down to 15 K. We found that the p...

Electronic phase diagram of the two-leg ladder compound Sr14−xCaxCu24O41 as investigated by the transport property measurements

Abstract We study the charge transport properties of the two-legged ladder compound, Sr 14− x Ca x Cu 24 O 41 , with various Ca concentrations. For 0 ≤ x ≤ 3, the charge dynamics of this compound is characterized by the strong suppression in the low-energy optical conductivity (σ(ω)) with decreasing temperature, which is not observed for higher x . The decrease in σ(ω) is responsible for the insulating behavior in these regions and may be associated with the ordering of hole carriers within the ladders. This possible charge ordering is suppressed by applying high pressure and the system becomes metallic for x ∼ 0, although the metallic state is distinct from that for higher Ca region where the superconducting ground state is realized under high pressure. It is considered that the charge ordered state observed for x = 0 is another ground state in this system which competes with superconductivity, and that the Ca substitution destabilizes the charge ordered state.

Superconductivity in quantum spin-ladder compound Ca13.6Sr0.4Cu24O41+δ at high pressure

Abstract Superconductivity has been observed in a spin-ladder compound Ca13.6Sr0.4Cu24O41+δ having Cu2O3 ladder layer. Electrical resistivity measurements show that Tc appears at 7 K at 1.5 GPa and disappears above 6 GPa after having the maximum Tc of 12 K at 3 GPa. The bulk superconductivity was suggested in this system from AC susceptibility measurements, the volume fraction of which was estimated to be about 5% at 4.2 K at 3.5 GPa. In contrast with the oxide superconductor having CuO2-layer, this system shows superconductivity in the very narrow pressure range.

Pressure-induced structural-phase transition of CeRu2

Abstract Electrical resistivity of cubic Laves-phase compound CeRu 2 has been measured from 3 to 300 K under the quasi-hydrostatics pressure P from 1.5 up to 8 GPa. X-ray diffraction was performed at room temperature under the pressure up to 15 GPa. Temperature dependence of electrical resistivity ρ ( T ) shows no anomaly at pressures below 5 GPa and resistivity decreases with increasing pressure. However, ρ ( T ) curves, measured under the pressure above 5.5 GPa, reveal an abrupt decrease at critical temperature T a . This indicates that a new phase is induced by the pressure. T a shows a large pressure dependence: Δ T a /Δ P =59.2 K/GPa. According to the X-ray data, the phase transition above 5.5 GPa is a structural-phase transition.

Transport properties of BaTiO3−δ under high pressure

Abstract We have measured the electrical resistivity of doped ferroelectric single crystal BaTiO3−δ under high pressure up to 8.0 GPa in the temperature range between 4.2 and 295 K. By applying pressure above 6.5 GPa metallic conduction was observed down to 4.2 K. A T2-dependence of resistivity was found in the pressure-induced metallic phase below 100 K, suggesting that electrons in the Ti 3d band of BaTiO3−δ have a nature similar to strongly correlated electron systems.

Two-band superconductivity and the effect of pressure in Ho1−xDyxNi2B2C

Abstract The effect of pressure on the superconductivity and magnetism in Ho1−xDyxNi2B2C is studied both theoretically and experimentally. At P=0 GPa , the breakdown of the de Gennes scaling in this compound for superconducting transition temperatures that are less than the Neel temperature (Tc x=0.1 (T c N ) , increasing pressure decreases Tc, but pressure has no influence on Tc of the x=0.4 sample (Tc>TN). The pressure dependence of the upper critical field also supports the phenomenological theory of multiband superconductivity in an antiferromagnetic state.

Anisotropy of the upper critical field of the spin-ladder Sr14−xCaxCu24O41

We report on the resistive upper critical field, H c2 (T) for the hole-doped two-leg ladder Sr 14-x Ca x Cu 24 O 41 (x = 12) single crystal, which becomes superconducting with T c ∼ 5 K in a pressure above ∼ 3.0 GPa. From the results of magneto-transport measurements on a single sample at a pressure of 3.5 GPa, H c2 (T) has been determined up to 20T along the a-axis (⊥ ladder) and up to 7T both along the b-axis (⊥ ladder-plane) and along the c-axis (|| ladder). A clear difference in H c2 (T) is observed among these three directions, indicating that this system has an anisotropic superconducting ground state. Also, it is found that H h c2 (T) shows non-linearity near T c at zero field and no saturation down to T/T c ∼0.03, exhibiting an upward curvature. Furthermore. H a c2 is found to exceed the Pauli limit by a factor of more than 2. This fact suggests a possibility of triplet superconductivity.