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

Abstract

Measurements of the temperature and angular dependencies of the upper critical field ${H}_{c2}$ of a stoichiometric single crystal LiFeAs in pulsed magnetic fields up to 50 T were performed using a tunnel diode resonator. Complete ${H}_{c2}^{\ensuremath{\parallel}c}$$(T)$ and ${H}_{c2}^{\ensuremath{\perp}c}$$(T)$ functions with ${H}_{c2}^{\ensuremath{\parallel}c}$$(0)=17\ifmmode\pm\else\textpm\fi{}1$ T, ${H}_{c2}^{\ensuremath{\perp}c}$$(0)=26\ifmmode\pm\else\textpm\fi{}1$ T, and the anisotropy parameter ${\ensuremath{\gamma}}_{H}(T)\ensuremath{\equiv}{H}_{c2}^{\ensuremath{\perp}c}/{H}_{c2}^{\ensuremath{\parallel}c}$ decreasing from 2.5 at ${T}_{c}$ to 1.5 at $T\ensuremath{\ll}{T}_{c}$ were obtained. The results for both orientations are in excellent agreement with a theory of ${H}_{c2}$ for two-band ${s}^{\ifmmode\pm\else\textpm\fi{}}$ pairing in the clean limit. We show that ${H}_{c2}^{\ensuremath{\parallel}c}$$(T)$ is mostly limited by the orbital pair breaking, whereas the shape of ${H}_{c2}^{\ensuremath{\perp}c}$$(T)$ indicates strong paramagnetic Pauli limiting and the inhomogeneous Fulde-Ferrel-Larkin-Ovchinnikov state below ${T}_{F}~5$ K.