Enhancing superconductivity by disorder

We study two mechanisms for enhancing the superconducting mean-field transition temperature Tc by nonmagnetic disorder in both conventional (sign-preserving gaps) and unconventional (sign-changing gaps) superconductors (SCs). In the first scenario, relevant to multiband systems of both conventional and unconventional SCs, we demonstrate how favorable density-of-states enhancements driven by resonant states in off-Fermi-level bands lead to significant enhancements of Tc in the condensate formed by the near-Fermi-level bands. The second scenario focuses on systems close to localization where random disorder-generated local density-of-states modulations cause a boosted Tc even for conventional single-band SCs. We analyze the basic physics of both mechanisms within simplified models, and we discuss the relevance to existing materials.