Observation of a Transition from BCS to HTSC-like Superconductivity in Ba_(1-x)K_xBiO₃ Single Crystals

A study of temperature dependences of the upper critical field B_{c2}(T) and surface impedance Z(T)=R(T)+iX(T) in Ba_{1-x}K_xBiO_3 single crystals that have transition temperatures in the range 6 < T_c < 32 K (roughly 0.6>x>0.4) reveals a transition from BCS to unusual type of superconductivity. B_{c2}(T) curves corresponding to the crystals that have T_c>20 K have positive curvature (like in some HTSC), and those of the crystals with T_c<15 K fall on the usual Werthamer-Helfand-Hohenberg curve. R(T) and X(T) dependences of the crystals with T_c~30 K and T_c~11 K are respectively linear (like in HTSC) and exponential (BCS) in the temperature range T << T_c. The experimental results are discussed in connection with the extended saddle point model by Abrikosov.