Topologically non-trivial gap function and topology-induced time-reversal symmetry breaking in a superconductor with singular dynamical interaction

TL;DR

This paper demonstrates that in certain strongly correlated systems with singular interactions, a topologically nontrivial superconducting gap can be energetically favored, leading to a phase with broken time-reversal symmetry.

Contribution

It introduces a model where a topologically nontrivial gap state becomes energetically favorable due to a finite cutoff in a singular dynamical interaction.

Findings

A topologically nontrivial gap state can be stabilized in a model with singular interactions.

The transition between trivial and nontrivial states involves an intermediate phase with broken time-reversal symmetry.

The model highlights the role of a finite cutoff in selecting topologically distinct superconducting states.

Abstract

In many strongly correlated electron systems, non-Fermi liquid behavior and unconventional superconductivity can be viewed as emerging from an effective 4-fermion interaction with a singular frequency dependence. A pairing instability in such a system is qualitatively different from that in a Fermi liquid and generally gives rise to multiple pairing states with topologically distinct gap functions. However, in the systems studied so far, a topologically trivial solution has the lowest energy. Here we show that a repulsive Hubbard-type interaction with a finite cutoff added to a model with a singular dynamical interaction selects, in some parameter range, the theretofore subleading, topologically nontrivial solution. We consider a minimal model that displays this behavior and show that the transformation between the topologically trivial and nontrivial gap functions necessarily occurs via an intermediate phase with topology-induced breaking of time-reversal symmetry.