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

The famous Fulde-Ferrell-Larkin- Ovchinnikov (FFLO) state is a spatially-modulated superconducting state with a predicted spatial anisotropy, but this anisotropy has never been experimentally verified. Here, the authors present ultrasound evidence for anisotropy of the sound velocity in the FFLO state of a 2D organic superconductor. 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.

Automated summary

In this study, the authors experimentally verified the anisotropy of the FFLO state in a 2D organic superconductor using ultrasound evidence. This exotic superconducting state exhibits spatial modulation but has not been experimentally validated for its anisotropy. The researchers detected anisotropic acoustic responses above the Pauli limit, indicating that the two-dimensional FFLO state has a center-of-mass momentum parallel to the nesting vector on the Fermi surface.