Magnetic Penetration Depth and Coherence Length in a Single‐Crystal YBa₂Cu₃O_7−δ

Abstract

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

Excess Conductivity Analysis of an YBCO Foam Strut and Its Microstructure

“…ξ c (0) diminishes with increasing the magnetic field. This behavior is consistent with that ξ c (0) follows a proportional relationship with T c which goes against the general theory of superconductivity [48] (where ξ c (0) ∼ 1/T c ). Similar behavior has been recently reported by Subhasis Shit et al in single grain GdBa 2 Cu 3 O 7−δ superconductor [49].…”

Excess Conductivity Analysis of an YBCO Foam Strut and Its Microstructure

“…As can be observed from Figure 8, both E J and ξ c (0) diminished with increasing the applied magnetic field. This behavior is consistent with ξ c (0) following a proportional relationship with T c which goes against the general theory of superconductivity [52] (where ξ c (0) ∼ 1/T c ). In our case, when the magnetic field increases, the coupling strength in the product diminishes, meaning that ξ c (0) drops as the magnetic field rises.…”