Naoyuki Tateiwa

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.

Pressure-induced superconductivity in a ferromagnet UGe2

We confirmed bulk-superconductivity of a ferromagnet UGe2 by the specific heat measurement, together with the measurements of the electrical resistivity and ac susceptibility, in a pressure range from p = 1.0 to 1.5 GPa, where the Curie temperature TC( = 22-36xa0K) is still high, but another characteristic temperature T* is close to zero. In this pressure range, the heavy fermion state is found to be formed at low temperatures.

Magnetic properties of a pressure-induced superconductor UGe2

We performed DC-magnetization and neutron scattering experiments under pressure P for a pressure-induced superconductor UGe 2 . We found that the magnetic moment is enhanced at a characteristic temperature T * in the ferromagnetic state, where T * is smaller than a Curie temperature T C . This enhancement becomes remarkable in the vicinity of P C * =1.20 GPa, where T * becomes 0 K and the superconducting transition temperature T SC shows a maximum. The characteristic temperature T * , which decreases with increasing pressure, also depends on the magnetic field.

A change of the Fermi surface in UGe2 across the critical pressure

We carried out the de Haas-van Alphen (dHvA) experiment under pressure for a ferromagnet UGe2. The dHvA frequency of a main dHvA branch named α, that corresponds to a majority up-spin band 40-hole Fermi surface, decreases monotonously with increasing pressure, but in the pressure range from pc* (1.2 GPa) to pc (1.5 GPa) the dHvA signal disappears completely, where pc and pc* correspond to critical pressures for a Curie temperature TC and the second phase transition temperature T* ( pc we observed new dHvA branches with large cyclotron masses of 19-64m0, which correspond to main Fermi surfaces in the paramagnetic state.

Heavy fermions in cerium and uranium compounds studied by the de Haas–van Alphen experiment

Abstract We carried out the de Haas–van Alphen (dHvA) experiment under pressure to clarify the nature of f-electron compounds of CeRh2Si2, UGe2 and CeCoIn5. With increasing pressure, the Neel temperature T N =36 K in CeRh2Si2 and the Curie temperature T C =52 K in UGe2 decrease, and a quantum critical point corresponding to the extrapolation TN or TC→0 is reached at the critical pressure pc≃1.06 and 1.52xa0GPa, respectively. Experimental results indicate a discontinuous change of the Fermi surface in CeRh2Si2 and UGe2 when the pressure crosses pc. A heavy fermion superconductor CeCoIn5 with a quasi-two dimensional Fermi surface was found to be located in the vicinity of the quantum critical point.

A change of the Fermi surface across the metamagnetic transition under pressure in UGe2

We have performed the de Haas-van Alphen (dHvA) and ac-susceptibility experiments on a ferromagnet UGe2 under pressure for the field along the easy a-axis. The p-H phase diagram was determined, indicating three kinds of phase: paramagnetic and weakly and strongly polarized phases. The detected dHvA frequencies as well as the cyclotron masses are found to be very different between the weakly and strongly polarized states. A change of the Fermi surface occurs when the weakly polarized phase is changed into the strongly polarized phase with increasing magnetic field.

Pressure-induced superconductivity in a ferromagnet, UGe2: resistivity measurements in a magnetic field

Electrical resistivity measurements in a magnetic field are carried out on UGe2 which exhibits pressure-induced superconductivity. The superconductivity is observed from 1.06 to 1.44 GPa. In the temperature and field dependences of the resistivity at P > PC where the ferromagnetic ordering disappears, it is observed that the application of an external field along the a-axis increases the coefficient A of the Fermi-liquid behaviour (∝ AT2) abruptly—corresponding to the metamagnetic transition. The characteristic enhancement of HC2 is reconfirmed for H ∥ a-axis. The upper critical field of HC2 is anisotropic: HC2 (T) exhibits positive curvature for H ∥ b-axis and H ∥ c-axis.

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.

Specific heat of an S=1 quasi-1D bond alternating antiferromagnet in a magnetic field

Abstract We report the results of specific heat measurements in a magnetic field on single crystal samples of the S=1 quasi-1D bond alternating antiferromagnet Ni(C9H24N4)NO2(ClO4). The compound has the singlet ground state with an excitation energy gap and the gap closes by application of magnetic field (Hc). We observed a sharp peak corresponding to the long-range ordering above Hc both parallel and perpendicular to the chain at low temperatures. The temperature-magnetic field phase diagram is discussed in comparison with those of the Haldane system Ni(C5H14N2)2N3(PF6).

Heat capacity of the pressure-induced superconductivity in itinerant ferromagnet UGe2

Recently, co-existence of the ferromagnetism and superconductivity was reported in the high-pressure region (1.0-1.6 GPa) in UGe 2 . We performed the heat capacity measurement on UGe 2 under high pressure. At 1.15 GPa, we found a peak corresponding to the superconducting transition. The superconducting temperature T SC and ΔC/(γT SC ) are 0.6 K and 0.25, respectively. The superconducting transition was also confirmed by the appearance of the Meissner effect in the AC susceptibility. From these results, we confirm a bulk nature of the superconductivity in UGe 2 . The value of C/T (∼95 mJ/molK 2 ) just above T SC at 1.15 GPa is as much as 3 times larger than that at ambient pressure, which indicates a large mass enhancement of quasiparticles under high pressure.