Emergent anisotropy in the Fulde-Ferrell-Larkin-Ovchinnikov state

TL;DR

This paper reports the experimental detection of anisotropic acoustic responses in the FFLO superconducting state, revealing its spatial modulation and momentum structure, which advances understanding of exotic superconductivity.

Contribution

First experimental verification of anisotropy in the FFLO state, linking spatial modulation to the Fermi surface nesting vector.

Findings

Anisotropic acoustic responses depend on sound propagation direction.

FFLO state exhibits a center-of-mass momentum aligned with the Fermi surface nesting vector.

Reveals spatial modulation and anisotropy in the superconducting state.

Abstract

Exotic superconductivity is formed by unconventional electron pairing and exhibits various unique properties that cannot be explained by the basic theory. The Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state is known as an exotic superconducting state in that the electron pairs have a finite center-of-mass momentum leading to a spatially modulated pattern of superconductivity. The spatial modulation endows the FFLO state with emergent anisotropy. However, the anisotropy has never been experimentally verified despite numerous efforts over the years. Here, we report detection of anisotropic acoustic responses depending on the sound propagation direction appearing above the Pauli limit. This anisotropy reveals that the two-dimensional FFLO state has a center-of-mass momentum parallel to the nesting vector on the Fermi surface. The present findings will facilitate our understanding of not only superconductivity in solids but also exotic pairings of various particles.