Anisotropic upper critical field and a possible Fulde-Ferrel-Larkin-Ovchinnikov state in a stoichiometric pnictide superconductor LiFeAs

TL;DR

This study measures the anisotropic upper critical fields of LiFeAs, revealing orbital and paramagnetic limiting effects, and suggests the possible existence of an FFLO state below 5 K in this superconductor.

Contribution

It provides detailed measurements of $H_{c2}$ in LiFeAs and proposes the potential realization of an FFLO state in a stoichiometric pnictide superconductor.

Findings

$H_{c2}$ anisotropy decreases from 2.5 to 1.5 as temperature lowers.

$H_{c2}$ for both orientations aligns with two-band $s^$ pairing theory.

Evidence suggests an FFLO state below approximately 5 K.

Abstract

Measurements of the temperature and angular dependencies of the upper critical fields $H_{c2}$ of a stoichiometric single crystal LiFeAs in pulsed magnetic fields up to 50 T were performed using a tunnel diode resonator. Full $H^{\parallel c}_{c2}(T)$ and $H^{\bot c}_{c2}(T)$ curves with $H^{\parallel c}_{c2}(0)=17 \pm 1$ T, $H^{\bot c}_{c2}(0)=26 \pm 1$ T, and the anisotropy parameter ${\gamma_H}(T) \equiv H_{c2}^{\bot c}/H_{c2}^{\parallel c}$ decreasing from $\approx 2.5$ at $T_c$ to 1.5 at $T\ll T_c$ were observed. The results for both orientations are in excellent agreement with a theory of $H_{c2}$ for two-band $s^\pm$ pairing in the clean limit. We show that $H^{\parallel c}_{c2}(T)$ is mostly limited by the orbital pairbreaking, whereas the shape of $H^{\bot c}_{c2}(T)$ indicates strong paramagnetic Pauli limiting and the inhomogeneous Fulde-Ferrel-Larkin-Ovchinnikov (FFLO)state below $T_F \sim 5$ K.