Probing phase fluctuations in a 2D degenerate Bose gas by free expansion.

We measure the power spectrum of the density distribution of a freely expanding two-dimensional (2D) degenerate Bose gas, where irregular density modulations gradually develop due to initial phase fluctuations in the sample. The spectrum has an oscillatory shape, where the peak positions are found to be independent of temperature and show scaling behavior in the course of expansion. The relative intensity of phase fluctuations is estimated from the normalized spectral peak strength and observed to decrease at lower temperatures, confirming the thermal nature of the phase fluctuations. We investigate the relaxation dynamics of nonequilibrium states using the power spectrum. Free vortices are observed with ring-shaped density ripples in a perturbed sample after a long relaxation time.