📝 SummarXiv

Abstract

A microscopic theory of Cooper-pair fluctuations (CPFs) in a disordered 2D electron system with spin-orbit scatterings under parallel magnetic field is presented in light of the observation, at low temperatures, of large magnetoresistance (MR) above a crossover field to superconductivity in electron-doped SrTiO$_{3}$/LaAlO$_{3}$ interfaces. It is found that in the zero temperature limit the conventional (diagrammatic) microscopic theory of superconducting (SC) fluctuations yields vanishing fluctuation conductivity just above the superconducting transition. However, further analysis of the results of the microscopic theory reveals that due to the diminishing stiffness of the fluctuation modes in a broad range of momentum space, the density of the CPFs, defined consistently with the time dependent Ginzburg-Landau approach, diverges in the zero temperature limit at any finite magnetic field. This field-induced divergence of the CPFs density, within restricted mesoscopic regions in real space, which is relieved by quantum tunneling and pair breaking out of their mesoscopic enclaves, indicates that the grand canonical ensemble underlying the microscopic theory is unsubstantiated. A dynamical equilibrium between the condensed CPFs in real-space mesoscopic puddles and the rarefying system of unpaired electrons controls the residual normal-state conductivity at magnetic fields above the SC transition. It has been, therefore, concluded that under the diminishing fluctuation paraconductivity upon increasing magnetic field the density of the normal-state electrons is also suppressed (due to charge transfer to the localized CPFs) and so, resulting from electron localization, the overall MR is strongly enhanced.