# Bosonic topological insulator intermediate state in the superconductor-insulator transition

## TL;DR

This paper proposes a gauge theory describing the anomalous metallic phase in 2D superconductor-insulator transitions as a bosonic topological insulator with unique edge modes and mutual statistics effects, supported by experimental evidence.

## Contribution

It introduces a gauge theory framework for the anomalous metal as a bosonic topological insulator and links it to experimental observations in niobium titanium nitride devices.

## Key findings

- Identification of the anomalous metal as a bosonic topological insulator.
- Experimental signatures of bosonic topological insulator behavior in resistance measurements.
- Observation of quantum BKT criticality in phase transitions.

## Abstract

A low-temperature intervening metallic regime arising in the two-dimensional superconductor-insulator transition challenges our understanding of electronic fluids. Here we develop a gauge theory revealing that this emergent anomalous metal is a bosonic topological insulator where bulk transport is suppressed by mutual statistics interactions between out-of-condensate Cooper pairs and vortices and the longitudinal conductivity is mediated by symmetry-protected gapless edge modes. We explore the magnetic-field-driven superconductor-insulator transition in a niobium titanium nitride device and find marked signatures of a bosonic topological insulator behavior of the intervening regime with the saturating resistance. The observed superconductor-anomalous metal and insulator-anomalous metal dual phase transitions exhibit quantum Berezinskii-Kosterlitz-Thouless criticality in accord with the gauge theory.