Flux pinning and creep in the vortex-glass phase in Bi₂Sr₂CaCu₂O_8+δ single crystals

Measurements of the relaxation of the non-equilibrium magnetic moment of Bi₂Sr₂CaCu₂O_8+δ single crystals in fields between 0.1 and 12T are used to study the low-temperature vortex state in this extremely anisotropic material. From the data, we obtian (1) the critical current density at T=0, j_c(0); (2) the flux-flow resistivity at j=j_c(0)_; (3) the flux-creep activation barrier U as function of current density j, induction B and Temperature T. From collective pinning theory, it is found that (two-dimensional) point vortices in the CuO₂-layers are prinned individually. The flux-creep activation barrier is well described by two-dimensional collective creep theory at sufficienty high current density. Although low-current U(j)-data are inconclusive as to the nature of the low-temperature vortex state, critical scaling shows that it is most likely a vortex glass, with vanishing linear restivity at zero current density. Above the second-order phase transition at T_G, the same activation barrier is measured as in previously published AC-susceptibility experiments. Deviations from critical scaling at low magnetic fields find an explanation if the vortex-glass transitions is viewed as the phase-decoupling line suggested by Glazman and Koshelev [Phys. Rev. B 43 (1991) 2835], but in the presence of pinning.