Collisionless dynamics of the condensate predicted in the random phase approximation

Abstract

From the microscopic theory, we derive a number conserving quantum kinetic equation, valid for a dilute Bose gas at any temperature, in which the binary collisions between the quasi-particles are mediated by phonon-like excitations (called ``condenson''). This different approach starts from the many-body Hamiltonian of a Boson gas and uses, in an appropriate way, the generalized random phase approximation. As a result, the collision term of the kinetic equation contains higher order contributions in the expansion in the interaction parameter. This different expansion shows up that a scattering involves the emission and the absorption of a phonon-like excitation. The major interest of this particular mechanism is that, in a regime where the condensate is stable, the collision process between condensed and non condensed particles is totally blocked due to a total annihilation of the mutual interaction potential induced by the condensate itself. As a consequence, the condensate is not constrained to relax and can be superfluid. Furthermore, a Boltzmann-like H-theorem for the entropy exists for this equation and allows to distinguish between dissipative and non dissipative phenomena (like vortices). We also illustrate the analogy between this approach and the kinetic theory for a plasma, in which the excitations correspond precisely to a plasmon. Finally, we show the equivalence of this theory with the non-number conserving Bogoliubov theory at zero temperature.