Heavy fermion state and quantum criticality

摘要

The heavy fermion state in the f-electron systems is due to competition between the RKKY interaction and the Kondo effect. The typical compound is CeCu_6. To understand the electronic state, we studied the Fermi surface properties via the de Haas-van Alphen (dHvA) experiments and energy band calculations for CeSn_3, CeRu_2Si_2, UPt_3, and nowadays, transuranium compounds of NpGe_3 and PuIn_3, together with YbCu_2Si_2. Pressure is also an important technique to control the electronic state. For example, the Neel temperature T_N decreases with increasing pressure P and becomes zero at the critical pressure P_c:T_N→ for P→P_c. The typical compound is an antiferromagnet CeRhIn5, which we studied from the dHvA experiment under pressure. A change of the 4f-electronic state from localized to itinerant is realized at P_c≈2.4 GPa, revealing the first-order phase transition, together with a divergent tendency of the cyclotron mass at P_c. It is stressed that appearance of superconductivity in CeRhIn_5 is closely related to the heavy-fermion state. It is also noted that the parity-mixed novel superconducting state might be realized in a pressure-induced superconductor CeIrSi_3 without inversion symmetry in the crystal structure.