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

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(parallel to c)(T) and H-c2(perpendicular to c)(T) functions with H-c2(parallel to c)(0) = 17 +/- 1 T, H-c2(perpendicular to c) (0) = 26 +/- 1 T, and the anisotropy parameter gamma(H)(T) equivalent to H-c2(perpendicular to c) / H-c2(parallel to c) decreasing from 2.5 at T-c to 1.5 at T << T-c were obtained. The results for both orientations are in excellent agreement with a theory of H-c2 for two-band s(+/-) pairing in the clean limit. We show that H-c2(parallel to c)(T) is mostly limited by the orbital pair breaking, whereas the shape of H-c2(perpendicular to c)(T) indicates strong paramagnetic Pauli limiting and the inhomogeneous Fulde-Ferrel-Larkin-Ovchinnikov state below T-F similar to 5 K.