Electrostatic- and Parallel Magnetic Field- Tuned Two Dimensional Superconductor-Insulator Transitions

TL;DR

This study investigates how electrostatic doping and parallel magnetic fields tune the superconductor-insulator transition in ultrathin bismuth films, revealing universal scaling behavior but also temperature-dependent deviations in the insulating regime.

Contribution

It demonstrates that electrostatic and magnetic tuning of the transition share a universality class but exhibit different low-temperature behaviors in the insulating phase.

Findings

Critical exponent product nu*z = 0.65-0.7

Critical magnetic field scales as (dn_c - dn)^0.33

Resistance increases unexpectedly at low temperatures in the insulator

Abstract

The 2D superconductor-insulator transition in disordered ultrathin amorphous bismuth films has been tuned both by electrostatic electron doping using the electric field effect and by the application of parallel magnetic fields. Electrostatic doping was carried out in both zero and nonzero magnetic fields, and magnetic tuning was conducted at multiple strengths of electrostatically induced superconductivity. The transitions were analyzed using finite size scaling with critical exponent products nu*z = 0.65-0.7. The parallel critical magnetic field increased with electron transfer as (dn_c-dn)^0.33, where dn is the electron transfer and dn_c is its critical value, and the critical resistance decreased linearly with dn. However at lower temperatures, in the insulating regime, the resistance became larger than expected from extrapolation of its temperature dependence at higher temperatures, and scaling failed. These observations imply that although the electrostatic- and parallel magnetic field- tuned superconductor-insulator transitions would appear to belong to the same universality class and to be delineated by a robust phase boundary that can be crossed either by tuning electron density or magnetic field, in the case of the field-tuned transition at the lowest temperatures, some different type of physical behavior turns on in the insulating regime.