(searched for: doi:10.4236/wjcmp.2020.102004)
Journal of Superconductivity and Novel Magnetism, Volume 34, pp 1551-1561; doi:10.1007/s10948-021-05852-8
Based on the formalism of the Bethe-Salpeter equation, we present here a new approach to deal with the temperature (T)- and the applied magnetic field (H)-dependent critical current densities (jcs) of superconductors (SCs). Saliently, it employs (i) the premise that the chemical potential μ (T, H) subsumes most of the factors to which the widely varying empirical values of jc (T, H) of an SC are conventionally attributed, (ii) the basic equation that defines jc as a product of the electronic charge, the number density of superconducting electrons (ns) and their critical velocity (vc), (iii) a recently derived equation for ns (T, H), and (iv) the empirical values of jc (T, H)—which it is meant to explain—as inputs into one of its equations which, paradoxical though it may seem, is shown to have the virtue of shedding light on several parameters related to jc (T, H), such as μ, ns, vc, m*, nL, and λm, where m* is the effective mass of an electron, nL the number of occupied Landau levels, and λm the magnetic interaction parameter. Remarkably, the approach is found to lead to two branches of μ-dependent solutions with distinctly different values of the parameters corresponding to the same value of jc (T, H). This is reminiscent of the behavior of the chemical potential in SCs with a low density of charge carriers that was reported by Marel [Physica C 165, 35 (1990)] and described as anomalous. Finally, we show that our approach suggests the exciting possibility of fabricating an SC that has a bespoke value of jc at suitably chosen values of T and H. For the sake of concreteness, we employ here some of the available empirical data on the jc (T, H) values of Bi-2212.
Journal of Superconductivity and Novel Magnetism, Volume 33, pp 3681-3685; doi:10.1007/s10948-020-05639-3
Guided by recent studies that point to the pivotal role played by the Fermi energy (EF) in determining the properties of high-temperature superconductors, we have been following a course where the critical temperature, gap(s), and the temperature T- and the applied magnetic field H-dependent critical current density jc (T, H) are addressed in a unified, EF-incorporated framework. Needed in such an approach is an equation for the number density ns (T, H) which is a constituent of jc (T, H). This equation is derived here by applying the field-theoretic Landau quantization prescription to ns (T, H = 0). As an application of the new equation, we also deal herein with a few empirical values of jc (T, H) of MgB2 and show that the corresponding ns (T, H) values lead to bounds on the effective mass of the charge carriers, the Fermi velocity, and the critical velocity.