Evidence for +-s-wave pairing symmetry in LiFeAs: specific heat study

We report specific heat capacity measurements on a LiFeAs single crystal at temperatures down to 400 mK and magnetic fields up to 9 Tesla. A small specific heat jump at Tc 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 sub-linear magnetic field dependence of the Sommerfeld coefficient \gamma(H) at T=0 K is equally well fitted by both a nodal d-wave gap as well as a sign changing multiband \pm 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 \pm s-wave SC state with the gap values \Delta_{small}=0.46 \Delta_{large}.