Bose metals, from prediction to realization

TL;DR

This paper reviews the theoretical prediction and experimental realization of Bose metals, a 2D phase of Cooper pairs arising from topological quantum effects, confirming their existence in Josephson junction arrays.

Contribution

It provides a comprehensive review of the mechanism behind Bose metals, emphasizing topological mutual statistics interactions and their role in creating a bosonic topological insulator.

Findings

Observation of Bose metals in Josephson junction arrays confirms the prediction.

Bose metals are characterized by topological mutual statistics interactions.

The state is identified as a bosonic topological insulator.

Abstract

Bose metals are metals made of Cooper pairs, which form at very low temperatures in superconducting films and Josephson junction arrays as an intermediate phase between superconductivity and superinsulation. We predicted the existence of this 2D metallic phase of bosons in the mid 90s, showing that they arise due to topological quantum effects. The observation of Bose metals in perfectly regular Josephson junction arrays fully confirms our original prediction and rules out alternative models based on disorder. Here, we review the basic mechanism leading to Bose metals. The key points are that the relevant vortices in granular superconductors are core-less, mobile XY vortices which can tunnel through the system due to quantum phase slips, that there is no charge-phase commutation relation preventing such vortices to be simultaneously out of condensate with charges, and that out-of-condensate charges and vortices are subject to topological mutual statistics interactions, a quantum effect that dominates at low temperatures. These repulsive mutual statistics interactions are sufficient to increase the energy of the Cooper pairs and lift them out of condensate. The result is a topological ground state in which charge conduction along edges and vortex movement across them organize themselves so as to generate the observed metallic saturation at low temperatures. This state is known today as a bosonic topological insulator.