Magnetoresistance investigation in the mixed state of a high temperature superconductor

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.