Bose Metal as a Nematic Phase Glass

A widely accepted microscopic theory for the Bose metal has yet to emerge despite mounting experimental evidence of a metallic phase intervening between the extremes of superfluid and insulator for low-temperature bosons. The reasons for this are two-fold: metallic solutions tend to be highly unstable, and stable solutions tend to utilize the unphysical limit of arbitrarily low particle density. We present a first-of-its-kind stable microscopic theory for the Bose metal with constant particle density in the thermodynamic limit. In this theory, repulsive interactions cause spontaneous symmetry breaking in an otherwise massively degenerate single-particle ground state. The key physics behind the stability of the Bose metal lies in the structure of the phase coherence. In contrast to the typical isotropic long-range order of superfluidity, our 3D theory exhibits long-range nematic phase coherence in each 2D plane while suffering from short range decoherence between planes. The resulting current response cannot maintain phase coherence beyond a finite length scale. Our discovery of a stable low-temperature phase that exhibits metallic conductivity elevates the Bose metal to one of the fundamentally intrinsic bosonic phases of matter alongside that of the superfluid and insulator.