A Classical-Field Approach for Bose Gases

Fei-Jie Huang, Qi-Hui Chen, Y. Fu +3 more

We study the effects of vector Bose gases, which are described by massive vector Bose fields, on fermionic superfluids in the low-energy region. It is demonstrated that the vector Bose gases can give rise to the Meissner effect, the flux quantization, and the Josephson effect, which are similar to those of the electromagnetic field in fermionic superfluids. However, unlike the electromagnetic field where the Meissner effect is unrelated to mass and the quantized flux is a constant, the Meissner effect of the vector Bose gases can be related to mass via a classical kinetic energy, and the value of the quantized flux is tunable. Our analyses also show that, although the origin model violates gauge invariance, the vector Bose gases in fact are gauge-invariant Maxwell-Chern-Simons systems under a Gauss constraint. It is also proposed that these effects can be observed by using the spin-1 cold atomic Bose gases in experiments.

Toshihiro Sato

Alice Sinatra, Carlos Lobo, Yvan Castin

We propose a method to study the time evolution of Bose-Einstein condensed gases perturbed from an initial thermal equilibrium, based on the Wigner representation of the N-body density operator. We show how to generate a collection of random classical fields sampling the initial Wigner distribution in the number conserving Bogoliubov approximation. The fields are then evolved with the time dependent Gross-Pitaevskii equation. We illustrate the method with the damping of a collective excitation of a one-dimensional Bose gas.

C. J. Pethick, H. Smith

C. J. Pethick, H. Smith

Bess Fang, T. Jacqmin, Tarik Berrada +1 more

We investigate the momentum distribution of weakly interacting 1D Bose gases at thermal equilibrium both experimentally and theoretically. Momentum distribution of single 1D Bose gases is measured using a focusing technique, whose resolution we improve via a levitation scheme. The momentum distribution compares very well with quantum Monte Carlo calculations for the Lieb-Liniger model at finite temperature, allowing for an accurate thermometry of the gas that agrees with (and improves upon) the thermometry based on {\it in situ} density fluctuation measurements. The quasi-condensation crossover is investigated via two different experimental parameter sets, corresponding to the two different sides of the crossover. Classical field theory is expected to correctly describe the quasi-condensation crossover of weakly interacting gases. We derive the condition of validity of the classical field theory, and find that, in typical experiments, interactions are too strong for this theory to be accurate. This is confirmed by a comparison between the classical field predictions and the numerically exact quantum Monte Carlo calculations.