H-T-J phase diagram and pinning mechanism in Ba0.63K0.37BiO3-y single crystals

The H-T-J phase diagram of superconducting Ba1-xKxBiO3-y (x = 0.37, 0.41, 0 .5) single crystals has been studied by means of SQUID and cantilever force magnetometry. Strong upward curvature of the H-c2(T) dependence confirms a t least intermediate strength of the electron-phonon coupling in this 3D-su perconductor family. The normalized critical current deduced from a cantile ver force response is well described by collective pinning theory in the te mperature range 0.01T(c) < T < 0.8 T-c .In contrast to the samples with x = 0.37 and 0.41, where the crossover between deltal- and deltaT(c)-pinning c auses the peak effect, for x = 0.5, the unified pinning mechanism related t o the spatial fluctuations of the carrier mean free path does not allow pro nounced anomalous magnetization over the whole temperature range studied. ( C) 2001 Elsevier Science B.V. All rights reserved.