Ultralow-Temperature Properties of Conduction Electrons and Nuclear Spins Interacting with the Singlet Ground State

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In this research project we have studied following two themes.1. Ultralow temperature property of systems consisting of conduction electrons and nuclear spins in the crystalline field singlet ground state (CFSGS).(1) We have studied the effect of the Coulomb interaction on the superconductivity mediated by the virtual excitation of the CFSGS. The superconducting transition temperature is lowered somewhat, but the effect is not pronounced.(2) We have derived the Coqblin-Schrieffer type s-f exchange interaction for (4f)ィイD12ィエD1 configuration with strong Hund and LS couplings. On the basis of it the RKKY interaction is derived for nuclear spins in the CFSGS system. It is of tensorial form and brings sinusoidal or helical nuclear spin structures.(3) Nuclear magnetism of PrCu6 is studied theoretically. Experimental study by Hata, one of the investigator of this project, found an evidence for a cannted weak ferromagnet, i. e., antiferromagnetic along the b axis and ferromagnetic along the c-axis together with a hysteresis phenomenon. We payed attention to the monoclinic crystal structure in which the inversion symmetry does not hold, and then derived a Dzyaloshinsky-Moriya type RKKY interaction for nuclear spins. The theory can explain the magnetic properties found in the experiment.2. Nuclear spin relaxation at ultralow temperatures.(4) We have solved the anomaly found experimentally in Rh near T=0, i. e., the nuclear spin relaxation is two-times slower at negative T than at positive T. We have derived a formula for relaxation effective at ultralow temperatures and applied the high-T expansion method to it to explain the experiment.