Anomalous dephasing of bosonic excitons interacting with phonons in the vicinity of the Bose-Einstein condensation

The dephasing and relaxation kinetics of bosonic excitons interacting with a thermal bath of acoustic phonons is studied after coherent pulse excitati on. The kinetics of the induced excitonic polarization is calculated within Markovian equations both for subcritical and supercritical excitation with respect to a Bose-Einstein condensation (BEC). For excited densities n bel ow the critical density n,, an exponential polarization decay is obtained, which is characterized by a dephasing rate Gamma = 1/T2. This dephasing rat e due to phonon scattering shows a pronounced exciton-density dependence in the vicinity of the phase transition. It is well described by the power la w Gamma proportional to (n - n(c))(2) that can be understood by linearizati on of the equations around the equilibrium solution. Above the critical den sity we get a non-exponential relaxation to the final condensate value p(0) with \p(t)\ - \p(0)\ proportional to 1/t that holds for all densities. Fur thermore we include the full self-consistent Hartree-Fock-Bogoliubov (HFB) terms due to the exciton-exciton interaction and the kinetics of the anomal ous functions f(k) = (a(k)a-k). The collision terms are analyzed and an app roximation is used which is consistent with the existence of BEG. The inclu sion of the coherent exciton-exciton interaction does not change the dephas ing laws. The anomalous function Fk exhibits a clear threshold behaviour at the critical density.