Experimental observation of the 'Tilting Mode' of an array of vortices in a dilute Bose-Einstein Condensate

TL;DR

This paper reports the experimental observation of a collective tilting mode in a vortex array within a Bose-Einstein condensate, revealing insights into vortex dynamics and Kelvin wave excitations.

Contribution

It is the first experimental measurement of the tilting mode of vortex arrays in a BEC, linking vortex tilt dynamics to Kelvin wave excitations and angular momentum.

Findings

Observed precession frequency of vortex lattice

Identified collective tilting mode as Kelvin wave excitation

Measured angular momentum via scissors mode excitation

Abstract

We have measured the precession frequency of a vortex lattice in a Bose-Einstein condensate of 87Rb atoms. The observed mode corresponds to a collective motion in which all the vortices in the array are tilted by a small angle with respect to the z-axis (the symmetry axis of the trapping potential) and synchronously rotate about this axis. This motion corresponds to excitation of a Kelvin wave along the core of each vortex and we have verified that it has the handedness expected for such helical waves, i.e. precession in the opposite sense to the rotational flow around the vortices. The experimental method used to excite this collective mode closely resembles that used to study the scissors mode and excitation of the scissors mode for a condensate containing a vortex array was used to determine the angular momentum of the system. Indeed, the collective tilting of the array that we have observed has previously been referred to as an `anomalous' scissors mode.