Single and two-particle energy gaps across the disorder-driven superconductor-insulator transition

TL;DR

This study uses advanced quantum Monte Carlo simulations to explore the superconductor-insulator transition, revealing persistent energy gaps, a pseudogap above critical disorder, and contrasting behaviors of single and two-particle gaps.

Contribution

It introduces a comprehensive simulation approach that captures inhomogeneous amplitude and phase fluctuations, providing new microscopic insights into the SIT and making testable predictions.

Findings

Energy gap persists across the transition.

Coherence peaks only in the superconducting phase.

Two-particle gap vanishes at the transition despite a persistent single-particle gap.

Abstract

The competition between superconductivity and localization raises profound questions in condensed matter physics. In spite of decades of research, the mechanism of the superconductor-insulator transition (SIT) and the nature of the insulator are not understood. We use quantum Monte Carlo simulations that treat, on an equal footing, inhomogeneous amplitude variations and phase fluctuations, a major advance over previous theories. We gain new microscopic insights and make testable predictions for local spectroscopic probes. The energy gap in the density of states survives across the transition, but coherence peaks exist only in the superconductor. A characteristic pseudogap persists above the critical disorder and critical temperature, in contrast to conventional theories. Surprisingly, the insulator has a two-particle gap scale that vanishes at the SIT, despite a robust single-particle gap.