# Stable vortices in the anomalous metallic state observed on monoatomic-layer superconductors

## TL;DR

This study explores the origin of metallicity in 2D superconductors by observing stable vortices, suggesting that current-driven vortex motion causes the saturated resistance in the metallic state.

## Contribution

The paper presents the first observation of stable vortices in the anomalous metallic state of 2D superconductors.

## Key findings

- Stable and isolated vortices are observed in the metallic regime of 2D superconductors.
- The saturated resistance in the metallic state is attributed to pinning-free vortex motion induced by applied current.
- Disorder-controlled experiments reveal insights into fluctuation-induced phases in ultrathin superconductors.

## Abstract

The superconductor-insulator transition in two-dimensional (2D) systems has been extensively studied as a typical example of quantum phase transition. Recent investigations of highly conductive 2D systems have revealed an intervening metallic regime, in which the electrical resistivity saturates at the limit of zero temperature. The nature and origin of this metallicity remain debated, partly because of the lack of microscopic understanding. In this study, using scanning tunneling spectroscopy, we investigate the metallic state and other phases observed in crystalline Pb monoatomic-layer superconductors formed on vicinal semiconducting substrates. Our spectroscopic images reveal stable and isolated vortices in the metallic regime, distinct from delocalized or liquidized vortices. These findings suggest that the saturated resistance in the metallic state arises from the pinning-free vortex motion driven by the finite current applied for the transport measurements. Our disorder-controlled microscopic experiments provide new insights into the fluctuation-induced phases of ultrathin crystalline 2D superconductors.

Stable vortices in the anomalous metal of 2D superconductors suggest that metallicity arises from current-driven vortex motion.