Global critical temperature in inhomogeneous superconductors induced by multifractality

TL;DR

This paper analytically investigates how multifractality in disordered superconductors influences the spatial distribution of the order parameter and the global critical temperature, revealing conditions for enhancement.

Contribution

It provides an analytical framework for understanding the spatial distribution and critical temperature in inhomogeneous superconductors near an Anderson transition.

Findings

Order parameter distribution is log-normal.

Critical temperature can be enhanced in weakly coupled superconductors.

Enhancement persists despite moderate phase fluctuations.

Abstract

There is growing evidence, from experiments and numerical simulations, that a key feature of sufficiently disordered superconductors is the spatial inhomogeneity of the order parameter. However not much is known analytically about the details of its spatial distribution or the associated global critical temperature that signals the breaking of long-range order. Here we address this problem for disordered systems around an Anderson transition characterized by multifractal one-body eigenstates. In the limit of weak multifractality and for weakly coupled superconductors we compute the superconducting order parameter analytically, including its energy dependence and statistical distribution in space. The spatial distribution of the order parameter is found to be always log-normal. The global critical temperature, computed by percolation techniques and neglecting phase fluctuations, is enhanced with respect to the clean limit only for very weakly coupled superconductors. Some enhancement still persists even in the presence of moderate phase fluctuations crudely modelled by increasing the percolation threshold. Our results are also consistent with experiments, where enhancement of the critical temperature is observed in Al thin films, a very weakly coupled metallic superconductor, but not in more strongly coupled materials.