Hedo Masato

Pressure-Induced Superconductivity in the Quasi-One-Dimensional Organic Conductor (TMTTF)2AsF6

We have investigated the transport properties of the quasi-one-dimensional (1-D) organic conductor (TMTTF) 2 AsF 6 at high pressures. A superconducting transition was observed, for the first time, at 2.38 K from a resistance measurement at 4.5 GPa. The presence of zero resistance was also confirmed at ∼2.3 K in the pressure range of 4.65≤ P ≤5 GPa. We discuss the electron correlation in (TMTTF) 2 AsF 6 based on its pressure–temperature phase diagram.

Pressure Effect on Magnetic Short Range Ordering of LuFe2Ge2

Thermal neutron elastic scattering experiments revealed that iron localized moments form in an antiferromagentic short range ordering in LuFe 2 Ge 2 below 9 K. A long range type antiferromagnetic ordering grows rapidly under high pressure above 1.6 GPa.

Split Fermi Surface Properties in Ullmannite NiSbS and PdBiSe with the Cubic Chiral Crystal Structure

We grew single crystals of ullmannite NiSbS and PbBiSe with the cubic chiral structure and carried out electrical resistivity, specific heat, and de Haas–van Alphen (dHvA) experiments to clarify their Fermi surface properties. The Fermi surfaces were found to split into two, reflecting the non-centrosymmetric crystal structure. The splitting energies between the two nearly spherical electron Fermi surfaces named α and α′ were determined as 220 K in NiSbS and 1050 K in PdBiSe for H || [100] or [001]. This difference in splitting energies between the two compounds originates mainly from the fact that the spin–orbit interactions of Ni-3d, Sb-5p, and S-3p electrons in NiSbS are smaller in magnitude than those of Pd-4d, Bi-6p, and Se-4p electrons in PdBiSe, respectively.

Magnetization Study on the History-Dependent Peak Effect in the Superconducting Mixed State of CeRu2.

A huge peak of irreversible magnetization in CeRu 2 single crystal, together with the equilibrium magnetization, was investigated by means of a static magnetization measurement using a Faraday force magnetometer. The residual resistance ratio of the used crystal exceeds 250, indicating that it is a clean superconductor. Contrary to the previous reports, there is no pinning-free region in the magnetization curves below the upper critical field H c2 . The detailed minor-hysteresis-loop measurement has revealed that the peak structure in the increasing-field magnetization curve around the onset field of the peak strongly depends on previous field-history, resulting in an apparent asymmetry of the peak structure. No trace of paramagnetic suppression of superconductivity in the vortex-penetrated state is observed from the superconducting equilibrium magnetization curves. The peak position, although highly anisotropic, can be interrelated to the peak strength, regardless of the applied field direction. It is li...

Pressure-Induced Valence Transition and Characteristic Electronic States in EuRh2Si2

EuRh2Si2 is well known to show a valence transition at a pressure of about 1 GPa from a Eu-divalent (antiferromagnetic) state to a nearly Eu-trivalent (paramagnetic) state, which was clarified using polycrystalline samples. We have succeeded in growing single crystals of EuRh2Si2 by the Bridgman method and studied their electronic properties measuring the electrical resistivity under pressure. EuRh2Si2 indicates a first-order valence transition in the pressure range from 1 to 2 GPa, distinguished by a sharp transition and a prominent hysteresis in the temperature dependence of the electrical resistivity. A critical end point in the valence transition is estimated as \(P_{\text{CEP}} \simeq 2.05\) GPa and \(T_{\text{CEP}} \simeq 170\) K. In the pressure range from 2 to 3 GPa, the electrical resistivity is found to exhibit a characteristic behavior for a moderately heavy-fermion compound such as EuIr2Si2. At pressures higher than 3 GPa, the resistivity reveals a normal metallic behavior in a nearly trivalen...

Magnetic and Electrical Properties in CePtSi3 without Inversion Symmetry in the Crystal Structure

We succeeded in growing a single crystal of CePtSi 3 with the tetragonal BaNiSn 3 -type crystal structure by the Sn-flux method. CePtSi 3 is found to be an antiferromagnet with successive two trans...

Electrical Resistivity of (TMTTF)2PF6 under High Pressure

We investigated the resistivity of quasi 1D organic conductor (TMTTF) 2 PF 6 under high pressure ( P ≦10 GPa) using a cubic anvil pressure system, which can generate a high quality hydrostatic pressure. We observed a superconducting state of (TMTTF) 2 PF 6 under 4 ≦ P ≦5 GPa. Zero resistivity was observed under 4 ≦ P ≦4.5 GPa for the first time. We will discuss the pressure-induced superconductivity with a P – T phase diagram of (TMTTF) 2 PF 6 .

Verification of Anisotropic s-Wave Superconducting Gap Structure in CeRu2 from Low-Temperature Field-Angle-Resolved Specific Heat Measurements

The field-angle-resolved specific heat C(T,H,φ) of the \(f\)-electron superconductor CeRu2 (\(T_{\text{c}}=6.3\) K) has been measured at low temperatures down to 90 mK on two single crystals of slightly different qualities. We reveal that the C(φ) oscillation in a rotating magnetic field, originating from the gap anisotropy, diminishes at low temperatures below the characteristic field \(H^{\ast}\), as expected for an anisotropic gap without nodes. We also observe the suppression of \(H^{\ast}\) by decreasing the gap anisotropy ratio Δmin/Δmax, a behavior that has been predicted from a microscopic theory for anisotropic \(s\)-wave superconductors. The present technique is established as a powerful tool for investigating minimum-gap structures as well as nodal structures.The field-angle-resolved specific heat C(T,H, φ) of the f -electron superconductor CeRu 2 (Tc = 6.3 K) has been measured at low temperatures down to 90 mK on two sing le crystals of slightly di fferent qualities. We reveal that the C(φ) oscillation in a rotating magnetic field, originating from the gap anisotropy, diminishes at low temperatures below the characteristic fie ld H∗, as expected for an anisotropic gap without nodes. We also observe the suppression of H∗ by decreasing the gap anisotropy ratio ∆min/∆max, a behavior that has been predicted from a microscopic theory f or anisotropics-wave superconductors. The present technique is established as a powerful tool for inve stigating minimum-gap structures as well as nodal structures.

Calorimetry Study of the Phase Diagrams of EuNi2Ge2 and Eu2Ni3Ge5 under Pressure

We report here the phase diagrams of EuNi2Ge2 and Eu2Ni3Ge5 studied by ac calorimetry under pressure using a diamond anvil cell. We follow the antiferromagnetic transition for EuNi2Ge2 up to 1.5 GPa. The sudden disappearance of magnetic order at around 2 GPa is confirmed, consistent with the probable occurrence of a first-order valence transition near that pressure. The ac calorimetry results on Eu2Ni3Ge5 clearly show two antiferromagnetic transitions, and suggest that magnetic order persists up to higher pressure than previously expected. At high pressure, where heavy-fermion behavior has been reported, the Neel temperature is decreasing, and magnetic order is expected to disappear at an extrapolated pressure of 12–14 GPa. A semi quantitative analysis of the pressure dependence of the specific heat does not show any large changes, but is compatible with a moderate enhancement of γ. The phase diagrams of Yb and Ce heavy fermion systems are compared and discussed with our system.

Transport and Magnetic Properties of EuAl4 and EuGa4

We succeeded in growing a single crystal of the Eu-divalent compound EuAl4 with the BaAl4-type tetragonal structure by the Al self-flux method and measured the electrical resistivity, magnetic susceptibility, magnetization, specific heat, and thermoelectric power. EuAl4 orders antiferromagnetically below TN1 = 15.4 K, with three successive antiferromagnetic transitions at TN2 = 13.2 K, TN3 = 12.2 K, and TN4 = 10.0 K. The latter two transitions are of the first-order. The corresponding magnetization curve indicates three successive metamagnetic transitions with hystereses and saturates above 16 kOe. We observed a plausible characteristic feature of the charge density wave (CDW) below TCDW = 140 K. We also studied an effect of pressure on the electronic state by measuring the electrical resistivity and thermoelectric power. TCDW is found to decrease with increasing pressure at a rate of dTCDW/dP = −54.7 K/GPa and becomes zero at about 2.5 GPa. The present antiferromagnetic ordering is, however, found to be ...