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

V. Humbert,L. Bergé,C. A. Marrache-Kikuchi,M. Ortuño,A. M. Somoza,L. Dumoulin

doi:10.1038/s41467-021-26911-7

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 NbxSi1−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 this originates from the localization length varying exponentially with thickness. At the lowest temperatures, there is an increase in activation energy related to the temperature at which this overactivated regime is observed. This relation, observed in many 2D systems shows that conduction is dominated by single charges that have to overcome the gap when entering superconducting grains.

Highlights

Beyond a critical disorder, two-dimensional (2D) superconductors become insulating
Electrons have to compromise between tunneling to close neighbors at the price of an energy mismatch, or traveling further where the hopping energy difference may be smaller. This process is known as variablerange hopping (VRH)[26] and results in a temperature dependence of the resistance of the form: R 1⁄4 R0 exp
The Superconductor-Insulator Transition (SIT) can be driven either by a reduction of the sample thickness or by a variation in stoichiometry which directly affects the amount of disorder in the system

Summary

Introduction

Two-dimensional (2D) superconductors become insulating. In this Superconductor-Insulator Transition (SIT), the nature of the insulator is still controversial. Intriguing are the reports of very strong insulating behaviors in the immediate vicinity of the SIT, with activated or even overactivated temperature dependence of the resistivity at the lowest experimentally accessible temperatures[15,16,17,18,19,20,21,22,23,24] These findings have prompted fierce debate as to their possible interpretation. Electrons have to compromise between tunneling to close neighbors at the price of an energy mismatch, or traveling further where the hopping energy difference may be smaller This process is known as variablerange hopping (VRH)[26] and results in a temperature dependence of the resistance of the form: T α 0: ð2Þ

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Results