Surface modes of ultracold atomic clouds with very large number of vortices

TL;DR

This paper investigates the unique surface excitations of vortex liquids in rapidly rotating ultracold bosonic systems, revealing their chiral nature, observable signatures, and implications for stability and characterization.

Contribution

It provides the first detailed analysis of surface modes in vortex liquids, highlighting their linear dispersion, chirality, and observable effects distinct from quantum Hall systems.

Findings

Surface modes are linearly dispersing and chiral.

Surface wave signals depend on vortex liquid type.

Surface modes influence the density profile and stability.

Abstract

We study the surface modes of some of the vortex liquids recently found by means of exact diagonalizations in systems of rapidly rotating bosons. In contrast to the surface modes of Bose condensates, we find that the surface waves have a frequency linear in the excitation angular momentum, $\hbar l > 0$. Furthermore, in analogy with the edge waves of electronic quantum Hall states, these excitations are {\it chiral}, that is, they can be excited only for values of $l$ that increase the total angular momentum of the vortex liquid. However, differently from the quantum Hall phenomena for electrons, we also find other excitations that are approximately degenerate in the laboratory frame with the surface modes, and which decrease the total angular momentum by $l$ quanta. The surface modes of the Laughlin, as well as other scalar and vector boson states are analyzed, and their {\it observable} properties characterized. We argue that measurement of the response of a vortex liquid to a weak time-dependent potential that imparts angular momentum to the system should provide valuable information to characterize the vortex liquid. In particular, the intensity of the signal of the surface waves in the dynamic structure factor has been studied and found to depend on the type of vortex liquid. We point out that the existence of surface modes has observable consequences on the density profile of the Laughlin state. These features are due to the strongly correlated behavior of atoms in the vortex liquids. We point out that these correlations should be responsible for a remarkable stability of some vortex liquids with respect to three-body losses.