Crystallization of Bosonic Quantum Hall States

TL;DR

This paper demonstrates the spontaneous formation of crystalline structures in a bosonic quantum Hall system driven purely by interactions, revealing a transition from a condensate to a droplet array with vortex streets.

Contribution

It provides the first experimental observation of interaction-driven crystallization in bosonic quantum gases under synthetic magnetic fields.

Findings

Observation of magneto-roton condensation leading to crystallization.

Identification of a hydrodynamic Kelvin-Helmholtz instability in the system.

Formation of a stable droplet array with vortex streets.

Abstract

The dominance of interactions over kinetic energy lies at the heart of strongly correlated quantum matter, from fractional quantum Hall liquids, to atoms in optical lattices and twisted bilayer graphene. Crystalline phases often compete with correlated quantum liquids, and transitions between them occur when the energy cost of forming a density wave approaches zero. A prime example occurs for electrons in high magnetic fields, where the instability of quantum Hall liquids towards a Wigner crystal is heralded by a roton-like softening of density modulations at the magnetic length. Remarkably, interacting bosons in a gauge field are also expected to form analogous liquid and crystalline states. However, combining interactions with strong synthetic magnetic fields has been a challenge for experiments on bosonic quantum gases. Here, we study the purely interaction-driven dynamics of a Landau gauge Bose-Einstein condensate in and near the lowest Landau level (LLL). We observe a spontaneous crystallization driven by condensation of magneto-rotons, excitations visible as density modulations at the magnetic length. Increasing the cloud density smoothly connects this behaviour to a quantum version of the Kelvin-Helmholtz hydrodynamic instability, driven by the sheared internal flow profile of the rapidly rotating condensate. At long times the condensate self-organizes into a persistent array of droplets, separated by vortex streets, which are stabilized by a balance of interactions and effective magnetic forces.