Axionic $p + is$ superconductivity from two pairing channels

TL;DR

This paper predicts an axionic superconducting state arising from a simple BCS model with competing singlet and triplet channels, characterized by spontaneous breaking of inversion and time-reversal symmetries without spin-orbit coupling.

Contribution

It introduces a theoretical model showing how dominant triplet pairing can lead to axionic superconductivity with spontaneous symmetry breaking.

Findings

Predicts a two-step specific heat transition as a signature of axionic superconductivity.

Analyzes the feasibility of electron-phonon and ferromagnetic mechanisms producing this state.

Abstract

In unconventional superconductors, inversion and/or time-reversal symmetries may be broken, either extrinsically or spontaneously. Here, we consider a simple BCS model with both attractive singlet and attractive triplet pairing channels. We show that when the triplet instability dominates, the model predicts an axionic superconducting state, in which both inversion and time-reversal symmetries are spontaneously broken by the superconductivity without requiring spin-orbit coupling. This leads to characteristic experimental signatures of spontaneous symmetry breaking in superconductors, such as a two-step transition in the specific heat. We critically analyze whether familiar pairing mechanisms such as the electron-phonon interaction and ferromagnetic spin fluctuations could produce such an axionic state.