Observation of a transition from BCS to HTSC-like superconductivity in Ba_{1 − x}K_xBiO₃ 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-xKxBiO3 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(7) dependences of the crystals with T-c approximate to 32 K and T-c approximate to 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.