Isotropic three-dimensional gap in the iron arsenide superconductor LiFeAs from directional heat transport measurements

The thermal conductivity kappa of the iron-arsenide superconductor LiFeAs (T-c similar or equal to 18 K) was measured in single crystals at temperatures down to T similar or equal to 50 mK and in magnetic fields up to H = 17 T, very close to the upper critical field H-c2 similar or equal to 18 T. For both directions of the heat current, parallel and perpendicular to the tetragonal c axis, a negligible residual linear term kappa/T is found as T -> 0, showing that there are no zero-energy quasiparticles in the superconducting state. The increase in kappa with magnetic field is the same for both current directions and it follows the dependence expected for an isotropic superconducting gap. These findings show that the superconducting gap in LiFeAs is isotropic in 3D, without nodes or deep minima anywhere on the Fermi surface. We discuss how this behavior of the thermal conductivity may be reconciled with the multiband character of superconductivity in LiFeAs inferred from other measurements. Comparison with other iron-pnictide superconductors suggests that a nodeless isotropic gap is a common feature at optimal doping (maximal T-c).