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 ΓHc1(0) was smaller than the expected theoretical value, and increased slightly with increasing temperature from 0.6Tc to Tc. 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 0.3Tc, with n>3.5 for both λab and λc. 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.