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

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 $(\kappa /T{)}_{T\to 0\mathrm{K}}$ 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 $\frac{\kappa }{T}\cdot T_c{|}_{T\to 0\mathrm{K}}$ 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.

5 pages, 3 figures