Condensation dynamics in a quantum-quenched Bose gas.

By quenching the strength of interactions in a partially condensed Bose gas, we create a "supersaturated" vapor which has more thermal atoms than it can contain in equilibrium. Subsequently, the number of condensed atoms (N(0)) grows even though the temperature (T) rises and the total atom number decays. We show that the nonequilibrium evolution of the system is isoenergetic and, for small initial N(0), observe a clear separation between T and N(0) dynamics, thus explicitly demonstrating the theoretically expected "two-step" picture of condensate growth. For increasing initial N(0) values, we observe a crossover to classical relaxation dynamics. The size of the observed quench-induced effects can be explained using a simple equation of state for an interacting harmonically trapped atomic gas.

Effects of interactions on the critical temperature of a trapped Bose gas.

We perform high-precision measurements of the condensation temperature of a harmonically trapped atomic Bose gas with widely tunable interactions. For weak interactions we observe a negative shift of the critical temperature in excellent agreement with mean-field theory. However for sufficiently strong interactions we clearly observe an additional positive shift, characteristic of beyond-mean-field critical correlations. We also discuss nonequilibrium effects on the apparent critical temperature for both very weak and very strong interactions.

Condensed fraction of an atomic Bose gas induced by critical correlations.

We study the condensed fraction of a harmonically trapped atomic Bose gas at the critical point predicted by mean-field theory. The nonzero condensed fraction f(0) is induced by critical correlations which increase the transition temperature T(c) above T(c) (MF). Unlike the T(c) shift in a trapped gas, f(0) is sensitive only to the critical behavior in the quasiuniform part of the cloud near the trap center. To leading order in the interaction parameter a/λ(0), where a is the s-wave scattering length and λ(0) the thermal wavelength, we expect a universal scaling f(0) proportionally (a/λ(0))(4). We experimentally verify this scaling using a Feshbach resonance to tune a/λ(0). Further, using the local density approximation, we compare our measurements with the universal result obtained from Monte Carlo simulations for a uniform system, and find excellent quantitative agreement.

Can a Bose gas be saturated?

We scrutinize the concept of saturation of the thermal component in a partially condensed trapped Bose gas. Using a 39K gas with tunable interactions, we demonstrate strong deviation from Einstein's textbook concept of a saturated vapor. However, the saturation picture can be recovered by extrapolation to the strictly noninteracting limit. We provide evidence for the universality of our observations through additional measurements with a different atomic species, 87Rb.