Dissipative phases across the superconductor-to-insulator transition

TL;DR

This paper explores the complex phase diagram of amorphous NbSi thin films, revealing two metallic states between superconducting and insulating phases, influenced by disorder and film thickness, challenging traditional views of the superconductor-insulator transition.

Contribution

It identifies two distinct dissipative metallic phases in 2D amorphous NbSi films and links their emergence to inhomogeneous superconductivity destruction, expanding understanding of quantum phase transitions.

Findings

Two metallic phases exist between superconducting and insulating states.

Dissipative states have disorder-dependent resistance behavior.

Superconducting fluctuations promote metallic states.

Abstract

Competing phenomena in low dimensional systems can generate exotic electronic phases, either through symmetry breaking or a non-trivial topology. In two-dimensional (2D) systems, the interplay between superfluidity, disorder and repulsive interactions is especially fruitful in this respect although both the exact nature of the phases and the microscopic processes at play are still open questions. In particular, in 2D, once superconductivity is destroyed by disorder, an insulating ground state is expected to emerge, as a result of a direct superconductor-to-insulator quantum phase transition. In such systems, no metallic state is theoretically expected to survive to the slightest disorder. Here we map out the phase diagram of amorphous NbSi thin films as functions of disorder and film thickness, with two metallic phases in between the superconducting and insulating ones. These two dissipative states, defined by a resistance which extrapolates to a finite value in the zero temperature limit, each bear a specific dependence on disorder. We argue that they originate from an inhomogeneous destruction of superconductivity, even if the system is morphologically homogeneous. Our results suggest that superconducting fluctuations can favor metallic states that would not otherwise exist.