Quasiparticle heat transport in single-crystalline Ba_1−xK_xFe₂As₂: Evidence for a k-dependent superconducting gap without nodes

The thermal conductivity κ of the iron-arsenide superconductor Ba_1−xK_xFe₂As₂(T_c≃30 K) was measured in single crystals at temperatures down to T≃50 mK(≃T_c/600) and in magnetic fields up to H=15 T(≃H_c2/4). A negligible residual linear term in κ/T as T→0 shows that there are no zero-energy quasiparticles in the superconducting state. This rules out the existence of line and in-plane point nodes in the superconducting gap, imposing strong constraints on the symmetry of the order parameter. It excludes d-wave symmetry, drawing a clear distinction between these superconductors and the high-T_c cuprates. However, the fact that a magnetic field much smaller than H_c2 can induce a residual linear term indicates that the gap must be very small on part of the Fermi surface, whether from strong anisotropy or band dependence, or both.