THEORY OF MIXED-VALENCE - METALS OR SMALL GAP INSULATORS

Mixed valence systems exhibit an interesting variety of low temperature electronic phenomena, in particular, a remarkable division of the known mixed valence compounds into metallic and insulating groups, exemplified respectively by CePd3 and SmB6. We show that for Sm compounds it is appropriate to use a two-band Hubbard Hamiltonian comprised of a narrow f band with a large Coulomb interaction U, a wide d band with negligible interactions, and f-d hybridization, Coulomb, and exchange interactions. Using the Green's function decoupling methods of Hubbard, we show that intersite correlations profoundly affect states near the Fermi energy and may lead to a small insulating gap if the two bands contain an even integral number of electrons per rare earth ion, as in stoichiometric SmB6. Magnetic phase transitions are discussed in terms of exchange vs. hybridization energies, and a first order transition from a paramagnetic insulator to a ferro- or ferrimagnetic metal is predicted within the mixed valence regime.