Observation of a Transition from BCS to HTSC-like Superconductivity in Ba1-xKxBiO3 Single Crystals
Abstract
A study of temperature dependences of the upper critical field Bc2(T) and surface impedance Z(T)=R(T)+iX(T) in Ba1-xKxBiO3 single crystals that have transition temperatures in the range 6 < Tc < 32 K (roughly 0.6>x>0.4) reveals a transition from BCS to unusual type of superconductivity. Bc2(T) curves corresponding to the crystals that have Tc>20 K have positive curvature (like in some HTSC), and those of the crystals with Tc<15 K fall on the usual Werthamer-Helfand-Hohenberg curve. R(T) and X(T) dependences of the crystals with Tc~30 K and Tc~11 K are respectively linear (like in HTSC) and exponential (BCS) in the temperature range T << Tc. The experimental results are discussed in connection with the extended saddle point model by Abrikosov.
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