Evidence fors±-wave pairing symmetry in LiFeAs from its low-temperature specific heat

Abstract

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 $\ensuremath{\gamma}(H)$ at $T=0$ K is equally well fitted by both a nodal d-wave gap as well as a sign changing multiband ${s}_{\ifmmode\pm\else\textpm\fi{}}$-wave gap. When impurity effects are taken into account, however, the linear temperature dependence of the electronic specific heat ${C}_{\text{el}}/T$ 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 ${s}_{\ifmmode\pm\else\textpm\fi{}}$-wave SC state with the gap values ${\ensuremath{\Delta}}_{\text{small}}=0.46{\ensuremath{\Delta}}_{\text{large}}$.