Quasiparticle Heat Transport in Ba₁₋ₓKₓFe₂As₂: Evidence for a k-dependent Superconducting Gap without Nodes

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The thermal conductivity κ of the iron-arsenide superconductor Ba₁₋ₓKₓFe₂As₂ (Tc ≃ 30 K) was measured in single crystals at temperatures down to T ≃ 50 mK (≃ Tc/600) and in magnetic fields up to H = 15 T (≃ Hc₂/4). 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-Tc cuprates. However, the fact that a magnetic field much smaller than Hc₂ 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.