Negative-U Superconductivity on the Surface of Topological Insulators

TL;DR

This paper investigates how negative-U centers on topological insulator surfaces induce superconductivity, revealing complex phase behaviors including chiral, glass, and topological orders driven by disorder and interactions.

Contribution

It introduces a model for negative-U center induced superconductivity on topological insulator surfaces, highlighting the roles of disorder, phase frustration, and emergent complex phases.

Findings

Superconductivity arises from a balance of pair creation and breaking by NUCs.

Global coherence depends on NUC concentration and Josephson coupling.

Novel phases like chiral, glass, and topological orders can emerge.

Abstract

We study the effects of a finite density of negative-$U$ centers (NUCs) on the surface of a three-dimensional topological insulator. The surface Dirac fermions mediate a power-law interaction among the local Cooper pairs at the NUCs, and the interaction remains long-ranged for weak disorder. Superconductivity can be generated in the presence of a random distribution of NUCs. The NUCs play dual roles as both pair creator and pair breaker, and the competition of the two effects results in non-monotonic dependence of the mean field superconducting transition temperature on the density of NUCs. Global phase coherence is established through coupling the locally superconducting puddles via Josephson coupling. Rare fluctuations play important roles, and a globally superconducting phase can only be achieved at large enough concentration of NUCs. The p-wave component of the superconducting order parameter gives rise to frustration among the superconducting grains, which is captured by a Potts-XY type model. New phases with chiral order, glass order, and possibly topological order can then emerge in the system of superconducting grains.