Yoshichika Ōnuki

We review the nature of strongly correlated electronic states in rare earth and actinide compounds, focusing on localized versus itinerant electronic states in CeRhIn₅, quantum critical phenomena in YbIr₂Zn₂₀, residual resistivity in CeCu₆, metamagnetism in heavy fermion compounds, and unconventional superconductivity in CeIrSi₃ without inversion symmetry in the crystal structure, emphasizing that sample quality is essentially important to clarify the characteristic features for the heavy fermion compounds.

Most of the Eu compounds are in the divalent (Eu ) electronic state and order magnetically. On the other hand, the Eu-trivalent (Eu ) compounds exist but are small in number. An energy difference between the Eu and Eu states is, however, not extremely large. Therefore, the valence transition occurs in some Eu compounds. We present the characteristic properties of the Eu compounds from several viewpoints: a canting magnetisation in the Eu -antiferromagnets, the Fermi surface property in the Eu -compounds of EuPd and EuCo Si , the heavy fermion state in EuNi P , the temperature-induced valence transition in EuPd Si , the pressure-induced valence transition in EuRh Si and EuRu P , and the heavy fermion state approaching to the quantum critical point with increasing pressure in Eu Ni Ge .

We succeeded in growing single crystals of EuCo₂Si₂ by the Bridgman method, and carried out the de Haas-van Alphen (dHvA) experiments. EuCo₂Si₂ was previously studied from a viewpoint of the trivalent electronic state on the basis of the magnetic susceptibility and X-ray absorption experiments, whereas most of the other Eu compounds order magnetically, with the divalent electronic state. The detected dHvA branches in the present experiments are found to be explained by the results of the full potential linearized augmented plane wave energy band calculations on the basis of a local density approximation (LDA) for YCo₂Si₂ (LDA) and EuCo₂Si₂ (LDA + U), revealing the trivalent electronic state. The detected cyclotron effective masses are moderately large, ranging from 1.2 to 2.9 m₀.