Crossover from thermally activated to steady flow in the vortex dynamics of Bi2Sr2CaCu2O8+x thin films

We have measured the resistive transition of molecular beam epitaxy prepare d Bi2Sr2CaCu2O8+x thin films in the presence of perpendicular low magnetic fields H (0 Oe < H < 1100 Oe) for different values of the bias current dens ity J (10(2) A cm(-2) < J 10(5) A cm(-2)). The experimental data show two d istinct dissipative behaviours. In the low-current region (J < 10(3) A cm(- 2)) the electrical resistivity rho is independent on J, changing only with H, while in the high-current region (J > 10(4) A cm(-2)) rho is independent of both the bias current and the magnetic field. The result is considered in terms of thermally activated flux flow-flux creep-steady flux flow cross overs. The role of the statistical distribution of the pinning energy in th e vortex dynamics is discussed.