Superconducting gap with sign reversal between hole pockets in heavily hole-doped Ba1-xKxFe2As2

TL;DR

This study investigates the evolution of the superconducting gap structure in heavily hole-doped Ba$_{1-x}$K$_x$Fe$_2$As$_2$, revealing a transition from fully gapped to nodal superconductivity and suggesting a change in the sign of the order parameter between hole pockets.

Contribution

It provides experimental evidence for a doping-induced crossover in the superconducting gap symmetry and sign structure in Fe-pnictides without electron pockets.

Findings

Fully gapped superconductivity at x=0.76

Nodal superconductivity at higher hole doping

Non-monotonic doping dependence of low-energy excitations

Abstract

To gain insight into the unconventional superconductivity of Fe-pnictides with no electron pockets, we measure the thermal conductivity $\kappa$ and penetration depth $\lambda$ in the heavily hole-doped regime of Ba$_{1-x}$K$_x$Fe$_2$As$_2$. The residual thermal conductivity $(\kappa/T)_{T \rightarrow 0\,{\rm K}}$ and $T$-dependence of $\lambda$ consistently indicate the fully gapped superconductivity at $x=0.76$ and the (line) nodal superconductivity at higher hole concentrations. The magnitudes of $\frac{\kappa}{T}\cdot T_c|_{T \rightarrow 0\,{\rm K}}$ and $\frac{d\lambda}{d(T/T_c)}$ at low temperatures, both of which are determined by the properties of the low-energy excitations, exhibit a highly unusual non-monotonic x-dependence. These results indicate a dramatic change of the nodal characteristics in a narrow doping range, suggesting a doping crossover of the gap function between the s-wave states with and without sign reversal between $\Gamma$-centered hole pockets.