Generic nodeless Larkin Ovchinnikov states due to singlet-triplet mixing

TL;DR

This paper demonstrates that singlet-triplet mixing in LO states induces a nodeless phase, stabilizes the FFLO state, and shifts midgap states, with implications for unconventional superconductors like organics.

Contribution

It reveals that odd-parity triplet components always emerge at LO nodes, leading to a nodeless phase and enhanced FFLO stability, explained via Lifshitz invariants.

Findings

Triplet component is large and stabilizes FFLO.

Midgap states are shifted away from zero energy.

Nodeless LO phase due to singlet-triplet mixing.

Abstract

Larkin-Ovchinnikov (LO) states typically have a singlet-gap that vanishes along real-space lines. These real-space nodes lead to Andreev midgap states which can serve as a signature of LO pairing. We show that at these nodes, an odd-parity, spin-triplet component is always induced, leading to a nodeless LO phase. We find the two-dimensional weak coupling, clean limit s-wave phase diagram when this spin-triplet part is included. The triplet component is large and increases the stability of the FFLO phase. We also show that the spin-triplet contribution pushes the midgap states away from zero energy. Finally, we show how our results can be explained phenomenologically though Lifshitz invariants. These invariants provide a simple approach to understand the role of unconventional pairing states, spin-orbit coupling, and inhomogeneous mixed singlet-triplet states that are not due to a FFLO instability. We discuss our results in the context of organic superconductors.