Observation of a transition from BCS to HTSC-like superconductivity in Ba1-xKxBiO3 single crystals

The temperature dependences of the upper critical field B c2(T) and surface impedance Z(T) = R(T) + iX(T) have been measured in Ba1 − x KxBiO3 single crystals with transition temperatures 6 ≤ T c ≤ 32 K (0.6 > x > 0.4). A transition from the BCS to an unusual type of superconductivity has been revealed: B c2(T) curves of the crystals with T c > 20 K have positive curvature (as in some HTSCs), and those of the crystals with T c < 15 K described by the usual Werthamer-Helfand-Hohenberg (WHH) formula. The R(T) and X(T) dependences of the crystals with T c ≈ 32 K and T c ≈ 11 K in the temperature range T ≪ T c are linear (as in HTSCs) and exponential (BCS), respectively. The experimental results are discussed using the extended saddle point model by Abrikosov.