The gap equations for spin singlet and triplet ferromagnetic superconductors

TL;DR

This paper derives and analyzes gap equations for superconductors coexisting with ferromagnetism, revealing distinct behaviors for singlet and triplet states, including the absence of the Clogston--Chandrasekhar limit in certain triplet states.

Contribution

It presents a comprehensive derivation of gap equations for various superconducting states coexisting with ferromagnetism, highlighting their different responses to exchange splitting.

Findings

Singlet s-wave state reproduces Clogston--Chandrasekhar limit behavior.

OSP triplet states behave similarly to singlet states under exchange splitting.

ESP triplet states show increased critical temperature with exchange splitting.

Abstract

We derive gap equations for superconductivity in coexistence with ferromagnetism. We treat singlet states and triplet states with either equal spin pairing (ESP) or opposite spin pairing (OSP) states, and study the behaviour of these states as a function of exchange splitting. For the s-wave singlet state we find that our gap equations correctly reproduce the Clogston--Chandrasekhar limiting behaviour and the phase diagram of the Baltensperger--Sarma equation (excluding the FFLO region). The singlet superconducting order parameter is shown to be independent of exchange splitting at zero temperature, as is assumed in the derivation of the Clogston--Chandrasekhar limit. P-wave triplet states of the OSP type, behave similarly to the singlet state as a function of exchange splitting. On the other hand, ESP triplet states show a very different behaviour. In particular there is no Clogston--Chandrasekhar limiting and the superconducting critical temperature is actually increased by exchange splitting.