Evidence for s _±-wave pairing symmetry in LiFeAs from its low-temperature specific heat

We report specific heat capacity measurements on a LiFeAs single crystal at temperatures down to 400 mK and magnetic fields up to 9 T. A small specific heat jump at T _c and finite residual density of states at T=0 K in the superconducting (SC) state indicate that there are strong unitary scatterers that lead to states within the SC gap. A sublinear magnetic field dependence of the Sommerfeld coefficient γ(H) at T=0 K is equally well fitted by both a nodal d-wave gap as well as a sign changing multiband s _±-wave gap. When impurity effects are taken into account, however, the linear temperature dependence of the electronic specific heat C _el/T at low temperatures argues against a nodal d-wave superconducting gap. We conclude that the SC state of LiFeAs is most compatible with the multiband s _±-wave SC state with the gap values Δ _small=0. 46Δ _large.