Vortex phases in modulation-free (Bi,Pb)-2212 crystals

We have investigated the vortex phase diagrams of modulation-free Bi1.7Pb0.4Sr2Ca0.9Cu2O8 crystals and Pb-free Bi2Sr2CaCu2O8+delta crystals. In (Bi,Pb)-2212 crystals, the strongly reduced anisotropy as well as the presence of a significant amount of disorder induce important differences compared to Bi-2212 crystals. We have probed the activation energies of the vortices by means of magnetic relaxation measurements, both above and below the second-magnetization peak. In the vortex phase below the peak, a power law dependence U alpha (1/j)(mu) is found, with an exponent mu = 0.54 which compares favorably with the Bragg Glass prediction. In the disordered phase situated above the second-magnetization peak line, the barriers are found to slowly diverge as U alpha In(1/j). This suggests the existence of a glassy state in this region of the phase diagram. We have carefully measured the onset of the irreversibility in the disordered phase in both crystals. Above the 2D-3D crossover field H,,., the resulting irreversibility lines are shown to be qualitatively described using a Lindemann approach in a 2D fluctuation regime. (C) 2004 Elsevier B.V. All rights reserved.