Strong parity mixing in the FFLO superconductivity in systems with coexisting spin and charge fluctuations

TL;DR

This paper investigates how coexisting charge and spin fluctuations influence the FFLO superconducting state, revealing transitions between pairing types and enhanced parity mixing, with potential relevance to organic superconductors.

Contribution

It demonstrates that charge fluctuations induce consecutive pairing transitions and strong parity mixing in the FFLO state, a novel insight into superconductivity mechanisms.

Findings

Transitions from singlet to FFLO to triplet pairing with increasing magnetic field.

Charge fluctuations significantly enhance parity mixing in the FFLO state.

Potential realization of these phenomena in quasi-one-dimensional organic superconductors.

Abstract

We study the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state of spin fluctuation mediated pairing, and focus on the effect of coexisting charge fluctuations. We find that (i) consecutive transitions from singlet pairing to FFLO and further to $S_z=1$ triplet pairing can generally take place upon increasing the magnetic field when strong charge fluctuations coexist with spin fluctuations, and (ii) the enhancement of the charge fluctuations lead to a significant increase of the parity mixing in the FFLO state, where the triplet/singlet component ratio in the gap function can be close to unity. We propose that such consecutive pairing state transition and strong parity mixing in the FFLO state may take place in a quasi-one-dimensional organic superconductor (TMTSF)$_2X$.