Bosonic topological insulators at the superconductor-to-superinsulator transition

We review the topological gauge theory of the superconductor-to-superinsulator transition. The possible intermediate Bose metal phase intervening between these two states is a bosonic topological insulator. We point out that the correct treatment of a bosonic topological insulator requires a normally neglected, additional dimensionless parameter, which arises because of the non-commutativity between the infinite gap limit and phase space reduction. We show that the bosonic topological insulator is a functional first Landau level. The additional parameter drives two Berezinskii–Kosterlitz–Thouless (BKT) quantum transitions to superconducting and superinsulating phases, respectively. The two BKT correlation scales account for the emergent granularity observed around the transition. Finally, we derive the ground state wave function for a system of charges and vortices in the Bose metal phase.