Fulde-Ferrell-Larkin-Ovchinnikov States in Two-Band Superconductors

TL;DR

This paper investigates the phase diagram of FFLO states in two-band superconductors, revealing a complex structure with multiple phases, transitions, and a devil's staircase pattern in the modulation vectors.

Contribution

It demonstrates the division of FFLO phases into two distinct phases due to competing modulation scales and uncovers a devil's staircase structure of subphases with rational modulation vectors.

Findings

FFLO phase splits into two phases with a first-order transition.

Presence of a devil's staircase structure in modulation vectors.

Critical magnetic field for FFLO stabilization is lower than in single-band superconductors.

Abstract

We examine the possible phase diagram in an $H$-$T$ plane for Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) states in a two-band Pauli-limiting superconductor. We here demonstrate that, as a result of the competition of two different modulation length scales, the FFLO phase is divided into two phases by the first-order transition: the $Q_1$- and $Q_2$-FFLO phases at the higher and lower fields. The $Q_2$-FFLO phase is further divided by successive first order transitions into an infinite family of FFLO subphases with rational modulation vectors, forming a {\it devil's staircase structure} for the field dependences of the modulation vector and paramagnetic moment. The critical magnetic field above which the FFLO is stabilized is lower than that in a single-band superconductor. However, the tricritical Lifshitz point $L$ at $T_{\rm L}$ is invariant under two-band parameter changes.