Physical Review Letters
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.
Coddington, I.; Engels, Peter; Schweikhard, Volker; and Cornell, Eric A., "Observation of Tkachenko Oscillations in Rapidly Rotating Bose-Einstein Condensates" (2003). Physics Faculty Contributions. 20.