Superconductivity at the three-dimensional Anderson metal-insulator transition

We study a disordered weakly coupled superconductor around the Anderson transition by solving numerically the Bogoliubov--de Gennes (BdG) equations in a three-dimensional lattice of size up to $20\ifmmode\times\else\texttimes\fi{}20\ifmmode\times\else\texttimes\fi{}20$ in the presence of a random potential. The spatial average of the order parameter is moderately enhanced as disorder approaches the transition but decreases sharply in the insulating region. The spatial distribution of the order parameter is sensitive to the disorder strength: for intermediate disorders below the transition, we already observe a highly asymmetric distribution with an exponential tail. Around the transition, it is well described by a log-normal distribution and a parabolic singularity spectrum. These features are typical of a multifractal measure. We determine quantitatively the critical disorder at which the insulator transition occurs by an analysis of level statistics in the spectral region that contributes to the formation of the order parameter. Interestingly, spectral correlations at the transition are similar to those found in noninteracting disordered systems at the Anderson transition. A percolation analysis suggests that the loss of phase coherence may occur around the critical disorder.