Rapidly Rotating Bose-Einstein Condensates in and near the Lowest Landau Level

We create rapidly rotating Bose-Einstein condensates in the lowest Landau level, by spinning up the condensates to rotation rates $Omega>99%$ of the centrifugal limit for a harmonically trapped gas, while reducing the number of atoms. As a consequence, the chemical potential drops below the cyclotron energy $2hbarOmega$. While in this mean-field quantum Hall regime we still observe an ordered vortex lattice, its elastic shear strength is strongly reduced, as evidenced by the observed very low frequency of Tkachenko modes. Furthermore, the gas approaches the quasi-two-dimensional limit. The associated cross-over from interacting- to ideal-gas behavior along the rotation axis results in a shift of the axial breathing mode frequency.