Half-Vortex Unbinding and Ising Transition in Constrained Superfluids

TL;DR

This paper investigates the phase transitions in constrained superfluids within a 2D Bose-Hubbard model, revealing a novel half-vortex unbinding transition and an Ising-type transition between atomic and pair superfluids.

Contribution

It identifies a new half-vortex unbinding transition and characterizes the Ising transition between atomic and pair superfluids in a constrained 2D Bose-Hubbard model.

Findings

Half-vortex unbinding leads to a BKT transition with an unusual helicity modulus jump.

A continuous Ising transition separates atomic and pair superfluids with specific critical exponents.

Parity fluctuations in winding numbers distinguish the two superfluid phases.

Abstract

We analyze the thermodynamics of the atomic and (nematic) pair superfluids appearing in the attractive two-dimensional Bose-Hubbard model with a three-body hard-core constraint that has been derived as an effective model for cold atoms subject to strong three-body losses in optical lattices. We show that the thermal disintegration of the pair superfluidity is governed by the proliferation of fractional half-vortices leading to a Berezinskii-Kosterlitz-Thousless transition with unusual jump in the helicity modulus. In addition to the (conventional) Berezinskii-Kosterlitz-Thousless transition out of the atomic superfluid, we furthermore identify a direct thermal phase transition separating the pair and the atomic superfluid phases, and show that this transition is continuous with critical scaling exponents consistent with those of the two-dimensional Ising universality class. We exhibit a direct connection between the partial loss of quasi long-range order at the Ising transition between the two superfluids and the parity selection in the atomic winding number fluctuations that distinguish the atomic from the pair superfluid.