A microscopic two-band model for the electron-hole asymmetry in high-T c superconductors and reentering behavior

Abstract

To our knowledge there is no rigorously analyzed microscopic model explaining the electron-hole asymmetry of the critical temperature seen in high-Tc cuprate superconductors – at least no model not breaking artificially this symmetry. We present here a microscopic two-band model based on the structure of energetic levels of holes in CuO2 conducting layers of cuprates. In particular, our Hamiltonian does not contain ad hoc terms implying – explicitly – different masses for electrons and holes. We prove that two energetically near-lying interacting bands can explain the electron-hole asymmetry. Indeed, we rigorously analyze the phase diagram of the model and show that the critical temperatures for fermion densities below half-filling can manifest a very different behavior as compared to the case of densities above half-filling. This fact results from the inter-band interaction and intra-band Coulomb repulsion in interplay with thermal fluctuations between two energetic levels. So, if the energy difference between bands is too big (as compared to the energy scale defined by the critical temperatures of superconductivity) then the asymmetry disappears. Moreover, the critical temperature turns out to be a non-monotonic function of the fermion density and the phase diagram of our model shows “superconducting domes” as in high-Tc cuprate superconductors. This explains why the maximal critical temperature is attained at donor densities away from the maximal one. Outside the superconducting phase and for fermion densities near half-filling the thermodynamics governed by our Hamiltonian corresponds, as in real high-Tc materials, to a Mott-insulating phase. The nature of the inter-band interaction can be electrostatic (screened Coulomb interaction), magnetic (for instance, some Heisenberg-type one-site spin–spin interaction), or a mixture of both. If the inter-band interaction is predominately magnetic then – additionally to the electron-hole asymmetry – we observe a reentering behavior meaning that the superconducting phase can only occur in a finite interval of temperatures. This phenomenon is rather rare, but has also been observed in the so-called magnetic superconductors. The mathematical results here are direct consequences of [J.-B. Bru and W. de Siqueira Pedra, Rev. Math. Phys. 22, 233 (2010)] which is reviewed in the introduction.