Vortex Lattices in Strongly Confined Quantum Droplets

TL;DR

This paper investigates rapidly rotating quantum droplets confined strongly, revealing vortex lattice formation with unique density and size behaviors, differing from traditional Bose-Einstein condensates, and suggesting new avenues for exploring strongly correlated quantum states.

Contribution

It demonstrates the formation of vortex lattices in strongly confined rotating quantum droplets and analyzes their density profiles and vortex core sizes, highlighting differences from single-component BECs.

Findings

Vortex lattices form in strongly confined quantum droplets similar to BECs.

Vortex core size varies significantly at finite density, matching analytical estimates.

Droplets show minimal size change near trapping frequency, enabling lower filling factors.

Abstract

Bose mixture quantum droplets display a fascinating stability that relies on quantum fluctuations to prevent collapse driven by mean-field effects. Most droplet research focuses on untrapped or weakly trapped scenarios, where the droplets exhibit a liquid-like flat density profile. When weakly trapped droplets rotate, they usually respond through center-of-mass motion or splitting instability. Here, we study rapidly rotating droplets in the strong external confinement limit where the external potential prevents splitting and the center-of-mass excitation. We find that quantum droplets form a triangular vortex lattice as in single-component repulsive Bose-Einstein condensates (BEC), but the overall density follows the analytical Thomas-Fermi profile obtained from a cubic equation. We observe three significant differences between rapidly rotating droplets and repulsive BECs. First, the vortex core size changes markedly at finite density, visible in numerically obtained density profiles. We analytically estimate the vortex core sizes from the droplets' coherence length and find good agreement with the numerical results. Second, the change in the density profile gives a slight but observable distortion to the lattice, which agrees with the distortion expected due to nonuniform superfluid density. Lastly, unlike a repulsive BEC, which expands substantially as the rotation frequency approaches the trapping frequency, rapidly rotating droplets show only a fractional change in their size. We argue that this last point can be used to create clouds with lower filling factors, which may facilitate reaching the elusive strongly correlated regime.