Observation of Phase Defects of a quasi-2D BEC in a Single Dipole Trap

At finite temperature, in a homogeneous two dimensional bosonic gas, long range order is destroyed by thermal fluctuations. For interacting atoms, below a critical temperature, the gas is a superfluid with phase defects consisting of pairs of vortex-antivortex; above this temperature, when the pairs are unbound, the gas enters a normal phase. The nature of the superfluid transition in a non-homogeneous trapped gas has been a topic of some debate. We are studying sodium atoms in a single quasi-2D optical trap. This trap is formed by an elliptical red detuned laser beam and has an aspect ratio of 40. We image the single ``pancake'' from both sides and observe density fluctuations with time of flight as well as phase fluctuations using Bragg interferometry techniques. Preliminary results provide evidence of the formation of vortex-antivortex pairs in this system.