The existence of a new of type of instability of a Bose condensate in a rarefied atomic gas due to momentum exchange with an external resonant spatially nonuniform radiation field is demonstrated theoretically.
A semiclassical theory is proposed to describe the light amplification in a Bose—Einstein condensate of a dilute atomic gas observed in [1]. Atomic states with well-defined momenta are taken as a basis of wave functions to derive the Maxwell—Bloch equations for the model of a dilute gas of atoms interacting with electromagnetic field. The evolution of light intensity and populations of coherent atomic states with different recoil momenta is described by solving these equations. The solutions obtained are used to demonstrate theoretically the effects observed in [1].
A new method of description of Bose systems in external fields is proposed. Equations derived on the basis of this method are the Bose analog of the Bogolyubov–De Gennes equations for Fermi systems with pairing. An exact solution of the equations is obtained for the case when a Bose system is in a field varying linearly with the coordinate. The possibility of propagation of previously unknown surface oscillations in a nonuniform field is predicted. The effect of the nonuniform field on the long-range order in two- and three-dimensional Bose systems is investigated.