On the existence of a Bose Metal at T=0

TL;DR

This paper provides a microscopic theoretical framework for the existence of a two-dimensional Bose metal phase at zero temperature, emphasizing quantum phase fluctuations and the role of two order parameters.

Contribution

It introduces a model showing how quantum frustration leads to a disordered phase of both phase and charge degrees of freedom, supporting the Bose metal concept.

Findings

Bose metal arises from disordered phase and charge degrees of freedom.

Model equivalent to two coupled XY models in (2+1)-d.

Consistency with low-T_c thin film experiments.

Abstract

This paper aims to justify, at a microscopic level, the existence of a two-dimensional Bose metal, i.e. a metallic phase made out of Cooper pairs at T=0. To this end, we consider the physics of quantum phase fluctuations in (granular) superconductors in the absence of disorder and emphasise the role of two order parameters in the problem, viz. phase order and charge order. We focus on the 2-d Bose Hubbard model in the limit of very large fillings, i.e. a 2-d array of Josephson junctions. We find that the algebra of phase fluctuations is that of the Euclidean group $E_{2}$ in this limit, and show that the model is equivalent to two coupled XY models in (2+1)-d, one corresponding to the phase degrees of freedom, and the other the charge degrees of freedom. The Bose metal, then, is the phase in which both these degrees of freedom are disordered(as a result of quantum frustration). We analyse the model in terms of its topological excitations and suggest that there is a strong indication that this state represents a surface of critical points, akin to the gapless spin liquid states. We find a remarkable consistency of this scenario with certain low-T_c thin film experiments.