Takao Nakama

Reentrant superconductivity of CeRu2

Magnetization curves [MC] and temperature dependence of the AC susceptibility [TS] were measured in the superconducting phase of CeRu 2 with T c =6.55 K in zero field. For fields higher than 0.8 T in [TS], the superconducting susceptibility behaves normally up to the lowest transition temperature T cL where it decreases suddenly; the field-induced reentrant superconductivity occurs at T cL and breaks very sharply to the normal state at T cH (the highest transition temperature). In [MC], an anomaly appears when the magnetization reaches to the extrapolated magnetization of the normal state at H R (reentrant field). Magnetization drops suddenly to negative in the field-increasing process for temperatures below 3.9 K, and it only increases for temperatures above 4.0 K. Reentrant superconductivity occurs between, H R and H c2 . The reentrant phase has positive magnetization in the background.

Thermal and elastic properties of superconducting CeRu2

Abstract The specific heat, magnetocaloric effect and elastic modulus of CeRu 2 have been measured in magnetic fields in order to clarify the unusual superconducting state. The Fulde-Ferrell state is discussed as a possible origin of multiple superconducting phases in CeRu 2 .

Heavy Fermion State Based on the Kondo Effect in EuNi2P2

EuNi2P2 is known as a heavy fermion compound with an electronic specific heat coefficient γ=100 mJ/(K2 \(\cdot\)mol). We grew single crystals and studied their electronic and magnetic properties by measuring the electrical resistivity, magnetic susceptibility, high-field magnetization, specific heat, and thermal expansion. The present heavy fermion state is clarified to be based on the Kondo effect as in CeRu2Si2, revealing an intensive shrinkage of the volume below about 100 K in the temperature dependence of thermal expansion. The temperature dependences of the \(4f\)-electron contribution to the volume thermal expansion Δ V/V)4f and the average Eu valence are found to show good scaling in EuNi2P2.

Electrical properties of polycrystalline SrSi2

The electrical properties of polycrystalline SrSi2 were examined by electrical resistivity measurements at temperatures ranging from 2 to 760 K and Hall coefficient measurements at temperatures ranging from 10 to 300 K. These measurements revealed that SrSi2 is a narrow-gap semiconductor with an energy gap of 0.035 eV whose dominant carriers are holes.

Evidence of multiple superconducting phases in CeRu2

Magnetoresistance, thermopower, Hall resistivity, ac susceptibility and magnetization have been measured for CeRu 2 in the normal and superconducting phases. In the normal state, there is a magnetic anomaly at 50 K, which is presumably due to development of antiferromagnetic order. The H-T superconducting phase diagram obtained from ac susceptibility indicates the possibility of multiple superconducting phases.

Transport Properties of Heusler Compounds Fe3-xVxAl

The electrical resistivity ρ and thermopower S of Heusler compounds Fe 3- x V x Al have been measured at the temperature range from 1.5 to 300 K at magnetic fields up to 15 T. The measurements of ρ and S in the high temperature range up to 800 K have been performed. ρ and S show a semiconducting behavior at high temperatures, and the obtained experimental energy gap increases almost linearly with increasing V concentration x . S is positive for x <1.0 and negative for x ≥1.0 at all measured temperatures, indicating a drastic change of major carriers at x =1.0. The electron scattering due to the magnetic fluctuations of Fe clusters has a dominant effect on the behavior of ρ of the magnetic compounds at low temperatures. On the other hand, S is not sensitive to the magnetic state and the external magnetic field, which is related to the energy dependence of magnetic scattering at the Fermi level.

Pressure-Induced Valence Transition and Heavy Fermion State in Eu2Ni3Ge5 and EuRhSi3

We succeeded in growing single crystals of Eu2Ni3Ge5 and EuRhSi3 by the In-flux method, and measured the electrical resistivity under pressures. Both compounds are Eu-divalent antiferromagnets with the Neel temperatures TN = 18.6 and 47.7 K, respectively. With increasing pressure, the resistivity in Eu2Ni3Ge5 indicates a clear kink at Tv = 220 K under 8.0 GPa, for example, which corresponds to a valence transition. The valence of Eu ions deviates from divalence toward trivalence. Correspondingly, antiferromagnetic ordering disappears completely. The low-temperature resistivity is, surprisingly, very similar to the resistivity of a well-known heavy fermion superconductor CeCu2Si2, possessing a two-peak-structure based on the Kondo effect. In fact, the A value of the resistivity ρ = ρ0 + AT2 in the Fermi liquid relation becomes large, A = 0.3 µΩ·cm/K2 at 8.0 GPa. The similar characteristic features are observed in another compound EuRhSi3.

De Haas-van Alphen Effect and Fermi Surface Properties of EuPd3 with the Trivalent Electronic State

EuPd3, which is in the trivalent (Eu3+) electronic state, is a rare compound because most Eu compounds are in the divalent (Eu2+) electronic state and order magnetically. The electronic state of EuPd3 was previously studied by Mossbauer, magnetic susceptibility, and X-ray absorption experiments using polycrystalline samples. We succeeded in growing single crystals of EuPd3 by the Bridgeman method and carried out a de Haas–van Alphen (dHvA) experiment. The dHvA branches detected in EuPd3 are well explained by the results of the full potential linear augmented plane wave (FLAPW) energy band calculation based on a local density approximation (LDA) with additional treatments for EuPd3, revealing the trivalent electronic state. Conduction electrons are mainly Pd-4d electrons. The cyclotron effective masses are determined from the temperature dependence of the dHvA amplitude, ranging from 0.3 to 1.0m0 (m0: rest mass of an electron), which are also consistent with the corresponding band masses.

Fermi Surface and Magnetic Properties of Antiferromagnet EuBi3

EuBi3 with the AuCu3-type cubic structure is known to be a Eu-divalent antiferromagnet with the Neel temperature \(T_{\text{N}}\simeq 7.5\) K. We succeeded in growing a high-quality single crystal by the Bi self-flux method. The magnetization at 1.3 K for the magnetic field along the \(\langle 100\rangle\) direction increases linearly as a function of magnetic field, and saturates at a critical field \(H_{\text{c}}=225\) kOe, reaching a saturated magnetic moment of 7 µB/Eu. \(H_{\text{c}}\) is well explained by the magnetic exchange interaction based on a two-sublattice model, using the simple relation Hc = (kB/3µB)(TN-θp), namely, Hc [kOe]=4.9 (TN-θp) [K], where θp is the paramagnetic Curie temperature θp=-36 K. The present anti ferromagnetic state is found to be stable under pressures up to 8 GPa, where the Neel temperature increases with increasing pressure, being \(T_{\text{N}}=16.5\) K at 8 GPa. From the results of de Haas–van Alphen experiments on EuBi3 and energy band calculations for the non-\(4f\...

Hopping Conductivity in CuIr2S4 Spinel Compound: I. Empirical Model for Electronic Configuration and Mechanism of Metal–Insulator Transition

Two types of spinel compound CuIr 2 S 4 samples were investigated: sample 1 was synthesized under protective Ar atmosphere to avoid contamination by oxygen, whereas sample 2 was exposed to oxygen during preparation procedure. According to the results of scanning electron microscopy, the oxygen concentration in sample 2 is 5 times higher than in sample 1. However, the metal–insulator transition (MIT) temperature is the same for both samples irrespective of oxygen concentration. The conductivity σ in the insulating phase (LIP) is proportional to exp [-( T * / T ) 1/2 ] at T ≤120 K for sample 1 and at T ≤50 K for sample 2, while in the metallic phase the resistivity ρ is proportional to the same function of σ in LIP for both samples. Temperature independent diamagnetic susceptibility and temperature dependence of thermopower suggest that this compound is not a normal activation-type semiconductor. The electronic configuration of CuIr 2 S 4 is estimated to a first approximation from the experimental results. ...