Enhancing Superconductivity by Disorder

We study two mechanisms for enhancing the superconducting transition temperature Tc by nonmagnetic disorder in both conventional (sign-preserving gaps) and unconventional (sign-changing gaps) superconductors (SC). In the first scenario, relevant to multi-band systems in the dilute impurity limit of both conventional and unconventional SC, 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 the dense impurity limit where random disorder-generated local density of states modulations cause a boosted Tc for conventional SC with short coherence lengths. We analyze the basic physics of both mechanisms within simplified models, and discuss the relevance to existing materials.