Ōnuki Yoshichika

Enhancement of Superconducting Transition Temperature in CeCu 2Ge 2 under High Pressures

Enhancement of superconducting transition temperature T c is confirmed at around 16 GPa in the pressure-induced superconductor CeCu2Ge2. The superconducting H c - T c phase diagrams at the low-T c and high-T c pressure regions indicates that the heavy quasi particles form the singlet Cooper pairs. At both low-T c and high-T c pressure regions, Fermi liquid behavior of ρ ∼ AT 2 is observed only at low temperatures below 1 K in the normal state induced by the external field above H c2 . At 21 GPa where the superconductivity disappears down to 50 mK, the temperature region following ρ ∼ AT 2 increases drastically up to 40 K and the coefficient A decreases rapidly. These results indicate that the strongly correlated electrons are responsible essentially for the appearance of superconductivity in this system.

Characteristic Heavy Fermion Properties in YbCu2Si2 and YbT2Zn20 (T: Co, Rh, Ir)

We studied the Fermi surface properties of YbCu 2 Si 2 and YbCu 2 Ge 2 with the tetragonal structure by measuring the de Haas–van Alphen (dHvA) oscillations, together with the energy band calculations. It was clarified that 4 f electrons contribute to the Fermi surface of a valence fluctuating compound YbCu 2 Si 2 , but not to that of a divalent compound YbCu 2 Ge 2 . Namely, the Fermi surface of YbCu 2 Si 2 contains the 4 f -components considerably and the corresponding cyclotron effective mass m c * of the main Fermi surface is large, m c * =30–40 m 0 ( m 0 : rest mass of an electron). We also studied the heavy fermion properties of YbT 2 Zn 20 (T: Co, Rh, Ir) with the cubic caged structure. The metamagnetic behavior or an abrupt nonlinear increase of magnetization was observed at 6 kOe in YbCo 2 Zn 20 , 63 kOe in YbRh 2 Zn 20 , and 97 kOe in YbIr 2 Zn 20 for H ∥ , which was measured at 60 mK in YbCo 2 Zn 20 and 1.3 K in YbRh 2 Zn 20 and YbIr 2 Zn 20 . The metamagnetic field is found to be a good pa...

Specific Heat on the Non-centrosymmetric Superconductor LaPt3Si

We have measured specific heat of poly and single crystalline LaPt 3 Si samples under various magnetic fields. For zero magnetic field, we observed a distinct peak and jump in the specific heat at the superconducting transition temperature T c ∼0.64 and 0.61 K for the poly and single crystals, respectively. Temperature dependences of the specific heat of both samples below T c can be well-described by a exponential equation for a conventional superconductor at zero magnetic field. The jump at T c of single crystal is still distinct under magnetic fields. On the other hand, the peak of the poly crystal becomes broad above 40 G. The broad additional specific heat tail appears at higher temperatures than the T c expected from the results of the single crystal. We show, for the first time, the temperature dependence of superconducting critical magnetic field H c ( T ) obtained from the specific heat measurement. Small anisotropy of H c ( T ) was observed for the single crystal, with H c (0) = 66 and 61 G for ...

de Haas–van Alphen Study of the Pr-based Filled Skutterudite PrOs4P12

We have succeeded in observing the de Haas–van Alphen effect in PrOs 4 P 12 which shows no anomaly down to 50 mK in contrast to PrFe 4 P 12 and PrRu 4 P 12 . The Fermi surface (FS) topology is reasonably explained by the energy band structure calculation based on a full potential linearized augmented-plane-wave method within the local-density-approximation. Whereas the localized nature of 4 f -electrons, we have confirmed a highly enhanced cyclotron effective mass up to 18 m 0 in PrOs 4 P 12 , which is enhanced 3.8 times compared to that of LaOs 4 P 12 . No nesting property with q =(1,0,0) in the FS of PrOs 4 P 12 has been confirmed, which suggests both the 4 f -electrons contribution and the FS nesting property play the role for the phase transitions in PrFe 4 P 12 and PrRu 4 P 12 .

Crossover Phase Diagram in Heavy Fermion Compound YbCo2Zn20

We studied the electronic state and metamagnetic behavior in the heavy fermion compound YbCo 2 Zn 20 by measuring the specific heat, ac-susceptibility, and electrical resistivity under magnetic fields. The magnetic specific heat in the form of C mag / T increases steeply with decreasing temperature below 10 K and becomes constant with an extremely large value 8 J/(K 2 ·mol) below about 0.2 K, indicating a Fermi-liquid nature in this temperature range. The magnetic entropy increases rapidly below 10 K and reaches R ln 8 around 60 K, suggesting a small crystalline-electric-field (CEF) splitting energy. From the precise electrical resistivity measurements, together with the C ( H )/ T and χ ac ( T ) measurements, it was found that there are two crossover lines at low temperatures in the H – T phase diagram for H ∥ . The Fermi-liquid state evolves at lower temperatures than these crossover lines.

High-Field de Haas–van Alphen Effect in Non-Centrosymmetric CeCoGe3 and LaCoGe3

We report on de Haas–van Alphen effect measurements in the non-centrosymmetric systems CeCoGe 3 and LaCoGe 3 in magnetic field up to 28 Tesla. In both compounds, two new high frequencies were observed in high fields. The frequencies were not detected in previous lower field measurements. The frequencies do not originate from magnetic breakdown, and, therefore, are likely to be intrinsic features of the compounds. In CeCoGe 3 , the corresponding effective masses are strongly enhanced, being of the order of 30 bare electron masses.

Anisotropic residual resistivity under high pressure in UIr

Anisotropic resistivity in UIr has been investigated under high pressure. A huge enhancement of residual resistivity was found under high pressure for the measuring current parallel to the [0 1 0] direction while any anomalies were not observed in the resistivity for [1 0 1 ]. The residual resistivity for [0 1 0] reduced to the original value in the pressure range where superconductivity appears. This anomalous behavior indicates that the ground state in the high-ρ 0 state is not simple ferromagnet and the application of pressure induces the first-order transition to slightly different electronic state.

Josephson Effect of Heavy-Fermion Superconductors Measured in a 3-Axes Magnetometer

We have fabricated a 3-axes magnetometer that can reduce the residual magnetic field below 50 µG for each direction. Josephson critical current I c between a noncentrosymmetric superconductor (CePt 3 Si or LaPt 3 Si) and Al has been investigated inside it to avoid the magnetic-flux trapping during the superconducting transition and found to show a magnetic field H dependence similar to the previous one obtained using one-axis magnetometer; asymmetric patterns with respect to H =0 are commonly observed for CePt 3 Si and LaPt 3 Si. This suggests that the residual-field components perpendicular to H applied for I c ( H ) measurements does not basically affect the I c ( H ) patterns.

Metamagnetic Behaviour in CeCu6

The metamagnetic behaviour, which corresponds to one of the characteristic properties in the heavy fermion compounds, was studied by measuring the electrical resistivity under pressure and magnetic field for a typical heavy fermion compound CeCu 6 with orthorhombic structure. The transverse magnetoresistance at 100 mK indicated a peak structure at H m = 20 kOe for the magnetic field H \varparallel c -axis under ambient pressure. H m is found to increase linearly as a function of pressure and indicates H m = 102 kOe at 1.75 GPa, for example. The quantum critical point is thus likely to exist at a slightly negative pressure value of ∼ 0.21 GPa from extrapolating H m to zero. The ρ 0 and A values in the electrical resistivity ρ= ρ 0 + A T 2 decrease steeply in magnitude with increasing pressure and magnetic field, indicating that the heavy fermion state is destroyed by pressure and magnetic field.

Field-Induced Antiferromagnetic State in a Pressure-Induced Superconductor CeIrSi3

CeIrSi 3 is an antiferromagnet with a Neel temperature T N =5.0 K at ambient pressure. With increasing pressure, T N decreases monotonically and the antiferromagnetic (AF) ordering disappears at pressure P c =2.25 GPa, and shows pressure-induced superconductivity above 1.9 GPa. We have investigated the superconducting and field-induced AF states in CeIrSi 3 by measureing the electrical resistivity and ac-specific heat measurements under pressure and magnetic field. At 2.40 GPa, for example, the AF phase is induced by magnetic field above 6 T. The upper critical field H c2 is suppressed in the field-induced antiferromagneic phase. We discussed the field-induced AF state in CeIrSi 3 , comparing with that in CeRhIn 5 , which possesses a similar pressure phase diagram.