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.

No abstract

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.

We have studied the electrical resistivity as a function of temperature in Micro-bridges of YBa2Cu3O7-δ deposited by laser ablation on the SrTiO3 substrate face, around superconducting transition region in different magnetic fields. The activation energy U0 was determined and discussed using the Arrhenius relation. The irreversibility line Birr and the upper critical field Bc2were obtained using 10% and 90% criteria of the normal-state resistivity value from ρ (T) curves. A phase diagram of the studied sample is constructed showing the Tg glass line and a very broad vortex-liquid phase regime.

At a certain level in the history of superconductivity, it became indispensable to appropriate a microscopic theory that accounts for all the properties that manifest the superconducting materials. In the absence of such a description, we dispose another socalled Ginzburg-Landau (GL) phenomenological theory established in 1950, capable of describing all the properties listed up to now. The basics of this theory rest on the theory of second-order phase transitions founded in 1937 by Lev Landau. The advent of GL theory comes to fill the insufficiency in the London brothers' electrodynamic theory to explain the coexistence of a superconducting phase and a normal phase in a type II superconductor. In addition, it suggests for the first time that the superconducting state is more ordered than the normal state. This is an intuitive thermodynamic theory without a microscopic basis. For this reason, Western scientists did not give it importance. However, the Bardeen-Cooper-Schrieffer theory will come to justify it a posteriori due to the work of Gorkov in 1959 and another version of GL's theory appeared and was called the GLAG theory in honor of these founders Ginzburg, Landau, Abrikosov, and Gorkov. In this new version, the order parameter is directly associated with the Cooper pair wave function. The GL theory is presented in several variants in literature and the intuitive ideas on which they are based seem to be sometimes disconcerting. In contrast, this theory remains very rich because it accounts for the superconductor-normal transition, as well as the thermodynamic, electrodynamic, and quantum effects of the superconducting phase.The theories of V. L. Ginzburg and L. D. Landau highlight two characteristic lengths in the physics of superconductivity [1,2] : 1) The magnetic flux penetration length, known as the London length λ, which defines the length over which magnetic induction can vary in a superconducting material. 2) The coherence length ξ, which represents the spatial dimension of a superconducting pair, i.e., the minimum length over which superconductivity can vary until it disappears. Pairs have reduced dimensions in high-temperature superconductors, where ξ is comparable with the characteristic quantities of the crystal lattice.The quantities λ and ξ diverge for temperatures close to the critical temperature T c . This divergence relates to the weakening of the pairing interaction in the face of thermal agitation and leads to the transition from the superconducting state to the normal state. [3] On the contrary, they allow expressing the critical quantities that delimit the domain of existence of the superconducting state under the effect of an external magnetic field.The variety of behaviors of a superconducting material subjected to an external magnetic field B ¼ μH can be summarized