Overactivated transport in the localized phase of the superconductor-insulator transition

TL;DR

This study investigates the insulating phase near the superconductor-insulator transition in 2D NbSi films, revealing overactivated transport behavior linked to superconducting gaps and disorder, supported by experiments and simulations.

Contribution

It provides new experimental and numerical evidence of overactivated transport in the localized phase near the SIT, connecting activation energy increases to superconducting gaps and disorder effects.

Findings

Overactivated behavior observed near the SIT at low temperatures.

Activation energy increases are related to the superconducting gap.

Localization length varies exponentially with film thickness.

Abstract

Beyond a critical disorder, two-dimensional (2D) superconductors become insulating. In this Superconductor-Insulator Transition (SIT), the nature of the insulator is still controversial. Here, we present an extensive experimental study on insulating Nb_{x}Si_{1-x} close to the SIT, as well as corresponding numerical simulations of the electrical conductivity. At low temperatures, we show that electronic transport is activated and dominated by charging energies. The sample thickness variation results in a large spread of activation temperatures, fine-tuned via disorder. We show numerically and experimentally that the localization length varies exponentially with thickness. At the lowest temperatures, overactivated behavior is observed in the vicinity of the SIT and the increase in the activation energy can be attributed to the superconducting gap. We derive a relation between the increase in activation energy and the temperature below which overactivated behavior is observed. This relation is verified by many different quasi-2D systems.