Electron-hole liquid model for high-Tc superconductors: Metal-insulator transition and doping behavior

Abstract

We propose a theoretical model for high-Tc superconductivity based on the existence of two types of mobile charge carriers: electrons and hole bosons. The bosons are assumed to be mobile in CuO2 planes and the electrons in separate parts of the crystal in order to prevent annihilation. We simulate this situation with a uniform mixture of electrons and bosons by adding a short-range repulsion between them. The model predicts the existence of a linear electronic sound mode which is analogous to the first sound in He mixtures. The electron component contributes a linear term and the sound mode a cubic term in the electronic specific heat. The transition at Tc is interpreted as a λ transition whereas the pairs are formed at a higher temperature TBCS. For small hole concentrations the charge carriers form an excitonic bound state of heavy-fermion type which is immobile and hence provides an explanation for metal-insulator transition. The model predicts also the doping behavior in agreement with experiment including the pressure dependence of Tc. The specific-heat linear term is predicted to increase with pressure for hole-doped and to decrease for electron-doped superconductors. The predictions are valid for all types of high-Tc compounds and also for the new electron superconductors. The value of normalized slope of specific heat discontinuity at Tc can exceed the maximum values obtainable by the BCS and Eliashberg theories. Some features of NMR experiments can be understood.

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