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

TL;DR

This study measures thermal conductivity in Ba$_{1-x}$K$_x$Fe$_2$As$_2$ and finds no zero-energy quasiparticles, indicating a nodeless, anisotropic superconducting gap that differs from d-wave symmetry and high-$T_c$ cuprates.

Contribution

It provides evidence for a k-dependent, nodeless superconducting gap in Ba$_{1-x}$K$_x$Fe$_2$As$_2$, constraining the symmetry of the order parameter and ruling out d-wave pairing.

Findings

No residual linear term in $/T$ as $T o 0$

Excludes line and point nodes in the gap

Magnetic field induces residual linear term, indicating small gap regions

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.