HIGHT-T_c SUPERCONDUCTORS AS ALMOST-LOCALIZED SYSTEMS : RESONANCE AND FLUCTUATIONS

Abstract
Many experimental results seem to relate the superconducting oxides to the phenomenon of Mott localization in strongly correlated systems. We will give a brief review of the different theories developed until then to treat the Mott localization in the Hubbard model : variational Ansatz of Gutzwiller or RVB Ansatz, leading to fundamentally different ground states. In this context, we propose a new approach which consists to take account of the quantum fluctuations around the mean-field solution provided by the Gutzwiller approximation. We show how to understand the ground state in terms of a resonance (split by the Mott gap in the almost-localized regime) in complete analogy with the Abrikosov-Suhl resonance of the Kondo problem - heavy fermions. Superconductivity appears as induced by the constrained intersite correlations developed by quantum fluctuations around this resonant state.