Nature of the Quantum Metal in a Two-Dimensional Crystalline Superconductor

TL;DR

This study reveals an intermediate metallic state in a disorder-free 2D superconductor, induced by magnetic field, supporting the Bose metal model with phase fluctuations as its underlying mechanism.

Contribution

It demonstrates the existence of a metallic phase in a pristine 2D superconductor, challenging the traditional view that such systems are only superconducting or insulating.

Findings

Identification of a metallic state in a disorder-free 2D superconductor.

Observation of a power law scaling with magnetic field in the metallic phase.

Support for the Bose metal model involving phase fluctuations.

Abstract

Two-dimensional (2D) materials are not expected to be metals at low temperature due to electron localization. Consistent with this, pioneering studies on thin films reported only superconducting and insulating ground states, with a direct transition between the two as a function of disorder or magnetic field. However, more recent works have revealed the presence of an intermediate metallic state occupying a substantial region of the phase diagram whose nature is intensely debated. Here, we observe such a state in the disorder-free limit of a crystalline 2D superconductor, produced by mechanical co-lamination of NbSe$_2$ in inert atmosphere. Under a small perpendicular magnetic field, we induce a transition from superconductor to the intermediate metallic state. We find a new power law scaling with field in this phase, which is consistent with the Bose metal model where metallic behavior arises from strong phase fluctuations caused by the magnetic field.