Vortices in rotating optical lattices: commensurability, hysteresis, and proximity to the Mott State

TL;DR

This paper explores how rotating optical lattices affect vortex configurations in superfluid Bose gases, revealing novel arrangements, hysteresis effects, and experimental signatures near the Mott insulator transition.

Contribution

It introduces a model showing how lattice-induced density profiles lead to unique vortex arrangements and hysteresis phenomena in rotating Bose gases near the Mott state.

Findings

Vortices can form rings or giant vortices depending on parameters.

Hysteresis occurs due to vortex pinning and energy barriers.

Time-of-flight expansion can reveal predicted vortex structures.

Abstract

Quantized vortices stunningly illustrate the coherent nature of a superfluid Bose condensate of alkali atoms. Introducing an optical lattice depletes this coherence. Consequently, novel vortex physics may emerge in an experiment on a harmonically trapped gas in the presence of a rotating optical lattice. The most dramatic effects would occur in proximity to the Mott state, an interaction dominated insulator with a fixed integer number of particles per site. We model such a rotating gas, showing that the lattice-induced spatial profile of the superfluid density drives a gross rearrangement of vortices. For example, instead of the uniform vortex lattices commonly seen in experiments, we find parameters for which the vortices all sit at a fixed distance from the center of the trap, forming a ring. Similarly, they can coalesce at the center, forming a giant vortex. We find that the properties of this system are hysteretic, even far from the Mott state. We explain this hysteresis in terms of vortex pinning, commensurability between vortex density and pinning site density, and energy barriers against changing the number of vortices. Finally, we model time-of-flight expansion, demonstrating the experimental observability of our predictions.