Ai Nakamura

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.

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\...

Split Fermi Surface Properties based on the Relativistic Effect in Superconductor PdBiSe with the Cubic Chiral Crystal Structure

We grew single crystals of PdBiSe with the ullmannite-type cubic chiral structure and carried out de Haas–van Alphen (dHvA) experiments to clarify the Fermi surface properties. The Fermi surfaces are found to split into two different Fermi surfaces, reflecting the non-centrosymmetric crystal structure. A splitting energy between two nearly spherical Fermi surfaces named α and α′ is determined as 1050–1260 K. These Fermi surfaces are identified to be due the band-149 and -150 electron Fermi surfaces centered at the Γ point from the results of full-potential linearized augmented plane wave (FLAPW) energy band calculations under consideration of a mass correction in the spin–orbit interaction for Bi-6p electrons based on the relativistic effect. The theoretical splitting energy between these Fermi surfaces is 1080–1150 K, which is in good agreement with the experimental value.

First-Order Antiferromagnetic Transition and Fermi Surfaces in Semimetal EuSn3

We grew high-quality single crystals of the antiferromagnet EuSn3 with the AuCu3-type cubic crystal structure by the Sn self-flux method and measured the electrical resistivity, magnetic susceptibility, high-field magnetization, specific heat, thermal expansion, and de Haas–van Alphen (dHvA) effect, in order to study the magnetic and Fermi surface properties. We observed steplike changes in the electrical resistivity and magnetic susceptibility, and a sharp peak of the specific heat and thermal expansion coefficient at a Neel temperature TN = 36.4 K. The first-order nature of the antiferromagnetic transition was ascertained by the observation of thermal hysteresis as well as of latent heat at TN. The present antiferromagnetic transition is found to be not a typical second-order phase transition but a first-order one. From the results of dHvA experiment, we clarified that the Fermi surface is very similar to that of the divalent compound YbSn3, mainly consisting of a nearly spherical hole Fermi surface and...

Effect of pressure on thermopower of EuNi2Ge2

EuNi2Ge2 is antiferromagnetic below TN ≍ 30 K with an effective moment µeff ≍ 7.7 μB, indicating the 4f7 electron configuration (Eu2+) in the ground state. In order to investigate the electronic state of EuNi2Ge2, we have simultaneously measured thermopower S and electrical resistivity ρ at the temperature range between 2 K and 300 K and under pressures up to 3.5 GPa. In the pressure region of P 2.3 GPa, ρ increases with increasing temperature, and shows an anomaly in the form of a kink at the Neel temperature TN. S(T) also reveals a kink at TN. Both ρ(T) and S(T) indicate a small pressure dependence at the low pressure range. However, ρ(T) and S(T) curves in the low temperature region suddenly change their features at P ≍ 2.3 GPa, where the magnetic ordering disappears. ρ linearly decreases with decreasing temperature, and shows a sudden drop at the valence transition temperature Tv ≍ 30 K. S(T) reveals a drastic increase at Tv, changing its sign from negative to positive around 35 K, and takes maximum at T ≍ 7 K. The thermal hysteresis was clearly observed in both ρ(T) and S(T) curves around Tv.

De Haas–van Alphen Effect and Fermi Surface Properties in Nearly Ferromagnet SrCo2P2

We grew high-quality single crystals of SrCo2P2 with the ThCr2Si2-type tetragonal structure, and clarified the Fermi surface properties by carrying out de Haas–van Alphen (dHvA) experiments and energy band calculations. SrCo2P2 is known to be a nearly ferromagnetic compound, where the magnetic susceptibility follows the Curie–Weiss law above 200 K, with the effective magnetic moment μeff = 1.72 μB/Co, but becomes almost constant below about 100 K. The electronic specific heat coefficient γ is thus relatively large, being γ = 40 mJ/(K2·mol). Detected dHvA branches possess the corresponding cyclotron effective masses \(m_{\text{c}}^{*}\), ranging from 0.87 to 7.2 m0 (m0: rest mass of an electron). The angular dependences of the dHvA frequencies are well explained by the results of full-potential linearized augmented plane wave (FLAPW) energy band calculations, revealing a multiply-connected band 25th hole Fermi surface and a compensated nearly cylindrical band 26th electron Fermi surface. It is thus conclud...

Chiral-Structure-Driven Split Fermi Surface Properties in TaSi2, NbSi2, and VSi2

We carried out de Haas–van Alphen (dHvA) experiments for TaSi2, NbSi2, and VSi2 with the hexagonal chiral structure, and compared them with the results of energy band calculations. The Fermi surface is found to be split into two different Fermi surfaces, reflecting the non-centrosymmetric crystal structure. A magnitude of the antisymmetric spin–orbit interaction or a splitting energy between the two Fermi surfaces is determined to be 493 K for dHvA branch α (α′) and 564 K for branch β (β′), where these dHvA branches correspond to main Fermi surfaces. The present splitting values are compared with 209 and 244 K in NbSi2, and 19 and 39 K in VSi2, respectively. The splitting energy is found to be larger in the Ta-5d conduction electrons than those with the Nb-4d and V-3d conduction electrons.

Contribution of J-Multiplet Levels to the Physical Properties of EuPd3 with the Trivalent Electronic State

EuPd3 is one of the few Eu intermetallic compounds with the trivalent electronic state of the Eu ion. We succeeded in growing single crystals of EuPd3 by the Bridgeman method, and studied the electronic, thermal, and magnetic properties by measuring the electrical resistivity, specific heat, thermal expansion, magnetic susceptibility, and magnetization. The observed temperature dependences of the magnetic susceptibility in the temperature range between 2 and 800 K and of specific heat up to 150 K are analyzed by considering the contribution of the J-multiplet levels 7FJ with J = 0–6. Since the ground-state J-multiplet 7F0 is a singlet for the trivalent Eu ion, the magnetic susceptibility is due mainly to the mixing of the excited J-multiplet levels to the ground state by applying a magnetic field. The observed magnetic susceptibility can be explained by considering the mixing of the J-multiplet levels with J = 0–4. In addition, the specific heat data are well analyzed using the same J-multiplet levels wit...