Fermi surface effect on spontaneous breaking of time-reversal symmetry in unconventional superconducting films

TL;DR

This paper investigates how the shape of the Fermi surface influences the stability and extent of a superconducting phase with broken time-reversal and translation symmetries in thin films, using a tight-binding model.

Contribution

It introduces a mechanism showing the Fermi surface shape significantly affects the phase boundary of unconventional superconducting films with broken symmetries.

Findings

Fermi surface nesting enhances the nonuniform superconducting phase

The phase boundary extends to smaller thicknesses with certain Fermi surface shapes

The effect is demonstrated using a square-lattice tight-binding model

Abstract

We propose a mechanism that helps stabilize a superconducting state with broken time-reversal symmetry, which was predicted to realize in a d-wave superconducting film [A. B. Vorontsov, Phys. Rev. Lett. 102, 177001 (2009)]. In this superconducting phase, the time-reversal symmetry breaking is accompanied by spontaneous breaking of the translation symmetry along the film surface. We examine how the normal-superconducting phase boundary in the thickness-temperature phase diagram of the film is modified depending on the Fermi surface shape. In particular, the nonuniform superconducting phase is found to substantially extend to a smaller thickness region in the phase diagram when the Fermi surface satisfies a nesting condition. We demonstrate this Fermi surface effect using a square-lattice tight-binding model.