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 at least intermediate strength of the electron-phonon coupling in this 3D-superconductor family. The normalized critical current deduced from a cantilever force response is well described by collective pinning theory in the temperature 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 causes the peak effect, for x = 0.5, the unified pinning mechanism related to the spatial fluctuations of the carrier mean free path does not allow pronounced anomalous magnetization over the whole temperature range studied. (C) 2001 Elsevier Science B.V. All rights reserved.