Small anisotropy of the lower critical field and the s_±- wave two-gap feature in single-crystal LiFeAs

The in- and out-of-plane lower critical fields and magnetic penetration depths of LiFeAs were examined. The anisotropy ratio Γ_Hc ₁(0) was smaller than the expected theoretical value, and increased slightly with increasing temperature from 0.6T_c to T_c. The small degree of anisotropy was numerically confirmed by considering the electron correlation effect. The temperature dependence of the penetration depths followed a power law (∼Tⁿ) below 0.3T_c, with n>3.5 for both λ_ab and λ_c. Based on theoretical studies of iron-based superconductors, these results suggest that the superconductivity of LiFeAs can be represented by an extended s _±-wave due to the weak impurity scattering effect. And the magnitudes of the two gaps were also evaluated by fitting the superfluid density for both the in- and out-of-plane directions to the two-gap model. The estimated values for the two gaps are consistent with the results from angle-resolved photoemission spectroscopy and specific-heat experiments.