The effect of Coulomb interactions on the disorder driven superconductor-insulator transition: THz versus tunneling spectroscopy

TL;DR

This study compares tunneling and THz spectroscopy to understand how Coulomb interactions influence the superconductor-insulator transition in disordered materials, revealing their role in the evolution of the energy gap.

Contribution

It provides new insights into the impact of Coulomb interactions on the superconductor-insulator transition by contrasting screened and unscreened spectroscopic measurements.

Findings

Coulomb interactions significantly affect the energy gap across the SIT.

Screening of Coulomb interactions alters the observed superconducting properties.

The nature of the phase transition is influenced by electronic interactions.

Abstract

The interplay between disorder and superconductivity has intrigued physicists for decades. Of particular interest is the influence of disorder on the superconducting energy gap $\Delta$. In the absence of Coulomb interactions between electrons, disorder leads to emergent granularity of the local order parameter resulting in a pseudogap at temperatures above the critical temperature $T_c$, as well as a finite gap $\Delta$ on the insulating side of the disorder-driven superconductor-insulator transition (SIT). At the same time, disorder also enhances the Coulomb interactions, which subsequently may influence $\Delta$ in a manner that is still not fully understood. Here we investigate the evolution of the energy gap through the SIT by two different experimental methods: tunneling spectroscopy, in which a metallic electrode is placed close to the studied sample thus screening the Coulomb interactions, and terahertz (THz) spectroscopy, which probes the unscreened sample. The comparison between the two methods illustrates the role played by electronic interactions in determining the nature of the phases across the SIT and sheds light on the mechanisms involved in the destruction of superconductivity.