Vortex-glass transition and fishtail effect in the cubic (K,Ba)BiO3 superconductor

We present the H-T phase diagram of the cubic (K,Ba)BiO3 high T-c superconductor (T-c similar to 30 K). We show that the vortex solid melts into a liquid phase for T > T-g(H) through a second order phase transition which can be well described by the vortex-glass transition scaling laws down to the lowest temperatures. Moreover, as usually observed in other high T-c materials, the 'apparent' critical current deduced from the width of the magnetization loops present a non monotonic behavior (so-called 'fishtail effect'). The position of the peak is time-independent at low temperature but disappears for long waiting times above similar to T-c/2. This may be related to a sharp increase of the relaxation rate in the vicinity of the vortex-glass transition line. (C) 1999 Elsevier Science B.V. All rights reserved.