Doping evolution of the quasiparticle excitations in heavily hole-doped Ba_(1-x)K_xFe_2As_2: A possible superconducting gap with sign-reversal between hole pockets

摘要

To gain insight into the unconventional superconductivity of Fe pnictides with no electron pockets, we measure the thermal conductivity k and penetration depth X in the heavily hole-doped regime of Ba_(1-x)K_xFe_2As_2. The residual thermal conductivity (κ/ T)_T→0 k and T dependence of λ consistently indicate the fully gapped superconductivity at x = 0.76 and the (line) nodal superconductivity at higher hole concentrations. The magnitude of κ/T_T_c|T→0 κ and the slope of λ(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 Γ-centered hole pockets.