Tadashi Fukuhara

Magnetic and electrical properties of U-Ge intermetallic compounds

We have clarified the magnetic and electrical properties of U 7 Ge, U 5 Ge 3 , U 3 Ge 4 and UGe 2 , measuring the electrical resistivity, thermoelectric power, Hall coefficient, magnetic susceptibility, magnetization and specific heat. U 7 Ge and U 5 Ge 3 , which are Pauli paramagnetic compounds, become superconductive below 1.40 K and 0.99 K, respectively. The latter compounds U 3 Ge 4 and UGe 2 indicate ferromagnetism. Anisotropic properties of UGe 2 are well reflected in the electrical resistivity, magnetic susceptibility and magnetization.

Pressure-induced metallic resistivity of PF6− doped poly(3-methylthiophene)

Abstract Electrical resistivity of poly(3-methylthiophene) doped with PF6− has been studied under pressure up to 13 kbar using a self-clamped beryllium-copper pressure cell. Application of the pressure induces a metallic temperature dependence in resistivity at low temperatures. Below about 4 K, T 1 2 dependence in electrical conductivity was observed, which can be explained taking the electron-electron interaction in a disordered system into account. We show that the extended heterogeneous model by Kaiser and Graham gives a good fit to the temperature dependence of the conductivity at low temperatures in the whole range of the pressure applied.

Metamagnetic Behavior of the Heavy-Fermion Compound CeNi2Ge2.

We have studied the temperature dependence of the electrical resistivity, the magnetic susceptibility and the high-field magnetization of CeNi 2 Ge 2 . Quadratic temperature dependence of the resistivity is observed below 0.7 K. The susceptibility exhibits a broad maximum at 28 K. Magnetization shows metamagnetic behavior at H M ∼42 T as in CeRu 2 Si 2 . These results indicate that CeNi 2 Ge 2 is a new heavy-fermion compound showing the metamagnetic behavior in the nonmagnetic ground state.

Specific heat study of non-fermi liquid behavior in CeNi2Ge2 : Anomalous peak in quasi-particle density-of-states

To investigate the non-Fermi liquid (NFL) behavior in a nonalloyed system CeNi 2 Ge 2 , we have measured the temperature and field dependences of the specific heat C on a CeNi 2 Ge 2 single crystal. The distinctive temperature dependence of C / T (\(\sim\alpha-\beta\sqrt{T}\)) is destroyed in almost the same manner for both field directions of B // c -axis and B // a -axis. The overall behavior of C ( T , B ) and the low-temperature upturn in χ( T ) can be reproduced, assuming an anomalous peak of the quasi-particle-band density-of-states (DOS) at the Fermi energy possessing \(\sqrt{\epsilon}\) energy dependence. Absence of residual entropy around T =0 K in B ∼0 T has been confirmed by the magnetocaloric effect measurements, which are consistent with the present model. The present model can also be applied to the NFL behavior in CeCu 5.9 Au 0.1 using a ln(e)-dependent peak in the DOS. Possible origins of the peak in the DOS are discussed.

From heavy fermion antiferromagnetism to localized ferromagnetism: competition of two ground states in CeRu2Ge2 on cooling

Resistivity, specific heat and neutron scattering measurements performed on single crystals of CeRu2Ge2 are reported. The intrinsic behaviour of this compound is to exhibit a double transition from a paramagnetic to an antiferromagnetic and then a ferromagnetic state on lowering the temperature with TN = 8.3 K and respectively TC = 7.5 K. For the first time the antiferromagnetic order is characterized by neutron scattering. The propagation vector of the sine-wave modulated structure is k = (0.31,0,0) as in the Si doped compounds. This phase is found to be easily weakened by disorder, but an antiferromagnetic moment as large as 1.5 µB develops between TN and TC for the best crystal.

Superconductivity, upper critical field and normal state resistivity in CeNi2Ge2 under pressure

We have performed transport and susceptibility measurements on single-crystal CeNi2 Ge2 under high pressure and magnetic field. Under pressure a 30% decrease of the resistivity is found below 300 mK which could be due to the onset of superconductivity. Assuming this we have established the phase diagram of the superconducting phase under pressure and magnetic field. The analysis of H c 2 and the slope at Tc in comparison to a previously published report of superconductivity at ambient pressure might indicate a change in the order parameter with pressure. The normal state resistivity at ambient pressure does not show a quadratic temperature dependence. Under pressure a quadratic dependence is found and we show that the parameters of the resistivity and the susceptibility obey a simple scaling law. This scaling breaks down however when approaching the magnetic instability.

Magnetic correlations in single-crystalline CeNi2Ge2

The antiferromagnetic correlations in the non-Fermi-liquid compound CeNi2Ge2 have been measured on single crystals using inelastic neutron scattering. The spin fluctuations with a characteristic energy of 4 meV are centred at the incommensurate wave vector (0.23, 0.23, 0.5), which is similar to where magnetic order develops in the related compound CeCu2Ge2. The magnetic correlations are anisotropic, with a quasi-two-dimensional extension along the [110] direction, implying that the moments are mostly correlated in the [110] planes. With increasing temperature, the correlated signal decreases, and above T* ≈ 30 K the magnetic scattering appears to be wave-vector independent.

Two magnetic transitions in Ce3Ir4Sn13

The specific heats of Ce3Ir4Sn13 have been measured between 30 mK and 70 K in magnetic fields up to 4 T. We observed the two main specific heat peaks at 0.6 K and at 2.1 K, which is resolved well into two successive peaks. A magnetic field versus temperature phase diagram is presented.

Superconductivity in intermetallic compound La3Co

The electrical resistivity, the magnetization, and the specific heat were measured. The residual resistivity ρ0 and the residual resistivity ratio ρR.T./ρ0 are 31 μΩ cm and 7.1, respectively. It was found that La3Co shows superconductivity below a transition temperature Tc of 4.4 K. The upper critical field Hc2 and lower critical field Hc1 at 0 K are estimated to be about 21 and 1 kOe, respectively, and using the relation of Hc1/Hc2≂ξ2/λ2, the ratio ξ/λ is estimated to be 0.2. The energy gap 2Δ and the Fermi energy are estimated to be 13×10−4 and 0.14 eV, respectively.

Magnetic Phase Diagram of Ce(Ni1-xPdx)2Ge2.

We have measured the temperature dependence of the magnetic susceptibility, the electrical resistivity, the specific heat and the Hall coefficient on the pseudo-binary series Ce(Ni 1- x Pd x ) 2 Ge 2 (1.0≥ x ≥0.0). The Neel temperature decreases monotonously from 5.1 K for CePd 2 Ge 2 to 2.8 K for Ce(Ni 0.8 Pd 0.2 ) 2 Ge 2 with decreasing Pd concentration. For x ≤0.1, the ground state is paramagnetic.