Quasiparticle heat transport in single-crystalline Ba1-xKxFe2As2: Evidence for a k-dependent superconducting gap without nodes

The thermal conductivity kappa of the iron-arsenide superconductor Ba1-xKxFe2As2 (T-c similar or equal to 30 K) was measured in single crystals at temperatures down to T not similar or equal to 50 mK (similar or equal to T-c/600) and in magnetic fields up to H = 15 T(similar or equal to H-c2/4). A negligible residual linear term in kappa/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.