Evidence for collective δT_c pinning in superconducting (K, Ba)BiO₃ single crystals

The nature of pinning has been investigated in the three-dimensional high-T_c superconductor (K, Ba)BiO₃. We show that the magnetic-field dependence of the critical current deduced from pulsed-field magnetization measurements can be well described by the collective-pinning theory on a large temperature range 0.06 < T/T_c < 0.62. This permits us to construct a nonequilibrium H-T phase diagram including both the single-vortex and vortex bundle regimes. Furthermore, the temperature dependence of the collective-pinning length L_c is in excellent agreement with a model of flux pinning induced by spatial fluctuations of the critical temperature (δT_c pinning model).