We report on thermally assisted flux flow in YBa2Cu3O7-delta superconductor. The resistivity measurements rho(T,B) for many values of the magnetic field up to 14T in two directions with a dc weak transport current density were investigated in order to determine the activation energy and then understand the vortex dynamic phenomena and therefore deduce the vortex phase diagram. The apparent activation energy U0(T) is calculated using an Arrhenius relation. The measured results of the resistivity were adjusted to the modified thermally assisted flux flow model accounting for the temperature-field dependence of the activation energy U(T,B). The obtained values from the thermally assisted activation energy, exhibit a behavior similar to the one showed with the Arrhenius model, albeit larger than the apparent activation energy. The vortex glass model was used to obtain the vortex-glass transition temperature from linear fitting of [dLn(Rho)/dT]^-1 plots. In the course of this work thanks to the resistivity measurements we could locate the upper critical magnetic field Hc2(T), the irreversibility line Hirr(T) and the crossover field Hcrossover(T) . These three parameters allowed us to establish a phase diagram of the studied material where limits of each vortex phase are sketched in order to optimize its applicability as a practical high temperature superconductor used for diverse purposes.

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In this work, we analyze the behavior of the magnetoresistance R (H, T) of a high temperature superconductor YBa 2 Cu 3 O 7−δ thin film optimally doped. Measurements of the magnetoresistance were carried out in the mixed state for a magnetic field up to 14 T applied parallel to c − axis then parallel to ab − plane with a dc transport current of 100 and 500 nA, 0.1 and 0.3 mA perpendicular to the magnetic field direction in both cases. The obtained results show that the magnetoresistance is in a good agreement with the thermally assisted flux creep. However, the magnetoresistance in the flux flow regime cannot be described by the Barden-Stephen model which conjectures another origin of these results. In this sense, the quantum fluctuations of the order parameter is a good alternative to understanding the shape of the magnetoresistance. The comparison of the magnetoresistance of the two directions shows that [see formula in PDF] is larger than [see formula in PDF] with anisotropy factor [see formula in PDF] depending on temperature and magnetic field. The irreversibility line H irr  (T) as well as H C2 (T) are determined and the first one is fitted with the phenomenological model H irr  (T) = H irr  (0) (1 − T/T o )  α where H irr  (0), T o and α are parameters obtained from the fit. The investigation of the scaling law allows us to prove that a crossover from 3D to 2D behavior is a feature of our sample which justifies the existence of the decoherence effect.

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