Observation of Tkachenko Oscillations in Rapidly Rotating Bose-Einstein Condensates

TL;DR

This paper reports the direct imaging and measurement of Tkachenko oscillations in vortex lattices of rapidly rotating Bose-Einstein condensates, revealing low-frequency waves and their dependence on rotation rate.

Contribution

It provides the first direct observation of Tkachenko waves in BEC vortex lattices and compares experimental results with hydrodynamic theory at high rotation rates.

Findings

Tkachenko oscillations observed in BEC vortex lattices.

Measured frequencies up to 98% of centrifugal limit.

Discrepancies with theory increase at higher rotation rates.

Abstract

We directly image Tkachenko waves in a vortex lattice in a dilute-gas Bose-Einstein condensate. The low (sub-Hz) resonant frequencies are a consequence of the small but nonvanishing elastic shear modulus of the vortex-filled superfluid. The frequencies are measured for rotation rates as high as 98% of the centrifugal limit for the harmonically confined gas. Agreement with a hydrodynamic theory worsens with increasing rotation rate, perhaps due to the increasing fraction of the volume displaced by the vortex cores. We also observe two low-lying m=0 longitudinal modes at about 20 times higher frequency.