Observation of Tkachenko Oscillations in Rapidly Rotating Bose-Einstein Condensates

Coddington, I.; Engels, P.; Schweikhard, V.; Cornell, E. A.

Observation of Tkachenko Oscillations in Rapidly Rotating Bose-Einstein Condensates

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Description

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.
5 pages, 4figures