Doping evolution of the quasiparticle excitations in heavily hole-doped Ba1-xKxFe2As2: A possible superconducting gap with sign-reversal between hole pockets

To gain insight into the unconventional superconductivity of Fe pnictides with no electron pockets, we measure the thermal conductivity. and penetration depth. in the heavily hole- doped regime of Ba1- x K x Fe2As2. The residual thermal conductivity (./ T) T. 0 K and T dependence of. consistently indicate the fully gapped superconductivity at x = 0.76 and the ( line) nodal superconductivity at higher hole concentrations. The magnitude of. T Tc| T. 0 K and the slope of.( T) at low temperatures, both of which are determined by the properties of the low- energy excitations, exhibit a highly unusual nonmonotonic x dependence. These results indicate a dramatic change of the nodal characteristics in a narrow doping range. We suggest that the observed x dependence is naturally explained by a doping crossover of the gap function between the s- wave states with and without sign reversal between - centered hole pockets.