Heavy Fermion Aspects of Strong Electron-Phonon Coupling Compound

Abstract

Heavy fermion aspects of strong electron-phonon coupling compounds are discussed on the basis of the idea that two-level systems (TLS) play a role of the localized spins in the dense Kondo system. Using the Vladar and Zawadowski’s results that, at low temperatures, a single TLS interacting with electrons can be transformed to spin-1/2 Kondo system, it is shown that a heavy fermion band is formed through the coherence among virtual bound states around each TLS. The effective interaction between thus obtained heavy fermions consists of on-site repulsion and intersite attraction; so that the Cooper pair becomes inevitably anisotropic. It is shown that the local-phonon model can be transformed to TLS when the adiabatic potential for an ion becomes double-well. For this, it is essential to take into account the term quadratic in ionic displacement in the local-phonon model.