Sublattice model of atomic scale pairing inhomogeneity in a superconductor

TL;DR

This paper introduces a simplified bipartite lattice model to analytically explore how atomic-scale pairing inhomogeneity affects superconducting properties, revealing that such inhomogeneity can enhance critical temperature and produce stable gapless states.

Contribution

It provides an analytic study of a bipartite lattice superconductor model with inhomogeneous pairing, highlighting effects on transition temperature and superfluid density.

Findings

Inhomogeneity can increase the critical temperature.

A stable gapless superconducting state exists for alternating sign couplings.

Superfluid density remains robust except in the gapless phase.

Abstract

We study a toy model for a superconductor on a bipartite lattice, where intrinsic pairing inhomogeneity is produced by two different coupling constants on the sublattices. The simplicity of the model allows for analytic solutions and tests of the consequences of atomic-scale variations in pairing interactions which have been considered recently in the cuprates. We present results for the transition temperature, density of states, and thermodynamics of the system over a phase diagram in the plane of two pairing coupling constants. For coupling constants of alternating sign, a gapless superconducting state is stable. Inhomogeneity is generally found to enhance the critical temperature, and at the same time the superfluid density is remarkably robust: at T=0, it is suppressed only in the gapless phase.