Quasiparticle Heat Transport in Ba${}_{1-x}$K${}_x$Fe${}_2$As${}_2$: Evidence for a k-dependent Superconducting Gap without Nodes

Abstract

The thermal conductivity $\kappa$ of the iron-arsenide superconductor Ba${}_{1-x}$K${}_x$Fe${}_2$As${}_2$ ($T_c\simeq$ 30 K) was measured in single crystals at temperatures down to $T\simeq 50$ mK ($\simeq T_c$/600) and in magnetic fields up to $H=15$ T ($\simeq H_{c2}$/4). A negligible residual linear term in $\kappa /T$ as $T\to 0$ 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.

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Cited by in corpus (1)

• Momentum dependence and nodes of the superconducting gap in iron-pnictides