Sequential superconductor-Bose insulator-Fermi insulator phase transitions in two-dimensional a-WSi

TL;DR

This study investigates the magnetic-field-driven superconductor-insulator transition in amorphous WSi, revealing a sequential phase transition from superconductor to Bose insulator and then to Fermi insulator, with distinct critical fields and critical exponents.

Contribution

It provides evidence for a sequential phase transition involving multiple critical points, challenging the conventional single critical point scenario in two-dimensional superconductors.

Findings

Localization of Cooper pairs at B_c^1 indicating Bose insulator phase

Persistence of superconducting fluctuations beyond B_c^1

Transition to Fermi insulator at B_c^2 with destroyed Cooper pairs

Abstract

A zero-temperature magnetic-field-driven superconductor to insulator transition (SIT) in quasi-two-dimensional superconductors is expected to occur when the applied magnetic-field crosses a certain critical value. A fundamental question is whether this transition is due to the localization of Cooper pairs or due to the destruction of them. Here we address this question by studying the SIT in amorphous WSi. Transport measurements reveal the localization of Cooper pairs at a quantum critical field B_c^1 (Bose-insulator), with a product of the correlation length and dynamical exponents zv~4/3 near the quantum critical point (QCP). Beyond B_c^1, superconducting fluctuations still persist at finite temperatures. Above a second critical field B_c^2>B_c^1, the Cooper pairs are destroyed and the film becomes a Fermi-insulator. The different phases all merge at a tricritical point at finite temperatures with zv=2/3. Our results suggest a sequential superconductor to Bose insulator to Fermi insulator phase transition, which differs from the conventional scenario involving a single quantum critical point.