Evidence of Andreev bound states as a hallmark of the FFLO phase in $\kappa$-(BEDT-TTF)$_2$Cu(NCS)$_2$

TL;DR

This paper provides microscopic evidence for the FFLO superconducting phase in $$-(BEDT-TTF)$_2$Cu(NCS)$_2$ by detecting Andreev bound states via NMR measurements, confirming a long-standing theoretical prediction.

Contribution

The study presents the first direct microscopic evidence of Andreev bound states as a hallmark of the FFLO phase in a real material.

Findings

Detection of spin-polarized quasiparticles indicating Andreev bound states.

Confirmation of spatially inhomogeneous superconducting state.

Evidence supporting the existence of the FFLO phase in the material.

Abstract

Superconductivity is a quantum phenomena arising, in its simplest form, from pairing of fermions with opposite spin into a state with zero net momentum. Whether superconductivity can occur in fermionic systems with unequal number of two species distinguished by spin, atomic hyperfine states, flavor, presents an important open question in condensed matter, cold atoms, and quantum chromodynamics, physics. In the former case the imbalance between spin-up and spin-down electrons forming the Cooper pairs is indyced by the magnetic field. Nearly fifty years ago Fulde, Ferrell, Larkin and Ovchinnikov (FFLO) proposed that such imbalanced system can lead to exotic superconductivity in which pairs acquire finite momentum. The finite pair momentum leads to spatially inhomogeneous state consisting of of a periodic alternation of "normal" and "superconducting" regions. Here, we report nuclear magnetic resonance (NMR) measurements providing microscopic evidence for the existence of this new superconducting state through the observation of spin-polarized quasiparticles forming so-called Andreev bound states.