Zoran Hadzibabic
Laboratoire Kastler Brossel
Ecole Normale Superieure, Paris, France
Tuesday, April 4, 2006
2:45 pm in 52 SPL
Condensates in Flatland
Abstract: I will give a short introduction to the physics of trapped two-dimensional (2D) atomic gases, and present some recent experimental results obtained with quantum degenerate 2D clouds of rubidium atoms.
Physics of a Bose gas in 2D is quite different from the usual 3D situation. In a homogeneous 2D fluid of identical bosons long-range order is always destroyed by long wavelength thermal fluctuations, but the system can nevertheless become superfluid at a finite critical temperature. This phase transition does not involve any symmetry breaking and in the Berezinskii-Kosterlitz-Thouless (BKT) paradigm it is explained in terms of binding and unbinding of pairs of vortices with opposite circulations. Above the critical temperature, proliferation of unbound vortices is expected. In a trapped atomic gas, the harmonic confinement can make the superfluid phase more robust, and the nature of the low temperature state is a topic of some theoretical debate.
Using optical lattice potentials we can create two parallel, independent 2D atomic clouds with similar temperatures and chemical potentials. When the clouds are suddenly released from the trapping potential and allowed to freely expand, they overlap and interfere. This realizes a matter wave heterodyning experiment which gives direct access to several features of the phase distributions in the two planes. Long wavelength phase fluctuations create a smooth and random variation of the interference fringes and free vortices appear as sharp dislocations in the interference pattern. Temperature study of these effects supports the BKT picture of the development of quasi-long-range coherence in these systems.