Theory of Heavy Fermions and Valence Fluctuations

Book excerpt: This volume contains the proceedings of the Eighth Taniguchi Interna tional Symposium on the Theory of Condensed Matter, which was held at Shima Kanko Hotel in Shima, Japan, 10-13 April 1985. The topic of the Symposium was Valence Fluctuation and Heavy Fermion Systems, one of the most fundamental problems in present-day condensed matter physics. The dilute Kondo problem, which is one of the most typical and unique many-body problems in condensed matter physics, developed recently into the dense Kondo and the coherent Kondo lattice problems in the 4f elec tron systems. It is accepted now that a large degeneracy in f-electron systems makes this latter situation possible by enhancing the single-site Kondo state relative to the inter-site magnetic interactions. Now, anoma lous behavior in f-electron systems show rich variety and are called valence fluctuation phenomena as a whole. They have, however, a common fea ture. In the lowest temperature region, they show either heavy Fermion like character or a narrow gap formation at the Fermi energy. Discovery of superconductivity in the heavy Fermion systems is attracting more in terest. Anyway, the valence fluctuating states are thought to be of fun damental importance to bridge the gap between the localized magnetic states and the delocalized nownagnetic states.