s- and d-wave superconductivity in a two-band model

TL;DR

This paper investigates superconductivity in a two-band model with strong correlations, analyzing phase diagrams, quantum critical points, and the stability of s- and d-wave pairing symmetries, with implications for high-temperature superconductors.

Contribution

It introduces a detailed two-band model incorporating strong Coulomb correlations and explores the effects of band mixing, pressure, and doping on superconducting phases and quantum criticality.

Findings

Superconducting quantum critical points can be induced by increasing band mixing.

Transitions from superconducting to normal states can lead to metallic, correlated metallic, or insulating phases.

The stability of s- and d-wave pairing varies across parameter space, and the BCS-BEC crossover is analyzed.

Abstract

Superconductivity in strongly correlated systems is a remarkable phenomenon that attracts a huge interest. The study of this problem is relevant for materials as the high Tc oxides, pnictides and heavy fermions. In this work we study a realistic model that includes the relevant physics of superconductivity in the presence of strong Coulomb correlations. We consider a two-band model, since most of these correlated systems have electrons from at least two different atomic orbitals coexisting at their Fermi surface. The Coulomb repulsion is taken into account through a local repulsive interaction. Pairing is considered among quasi- particles in neighbouring sites and we allow for different symmetries of the order parameter. In order to deal with the strong local correlations, we use the well known slave boson approach that has proved very successful for this problem. Here we are interested in obtaining the zero temperature properties of the model, specifically its phase diagram and the existence and nature of superconducting quantum critical points. We show that these can arise by increasing the mixing between the two bands. Since this can be controlled by external pressure or doping, our results have a direct relation with experiments. We show that the superconductor-to-normal transition can be either to a metal, a correlated metal or to an insulator. Also we compare the relative stability of s and d-wave paired states for different regions of parameter space and investigate the BCS- BEC crossover in the two-band lattice model as function of the strength of the pairing interaction.