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 excitation. 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 below the critical density nc, an exponential polarization decay is obtained, which is characterized by a dephasing rate Γ = 1 T2 . This dephasing rate due to phonon scattering shows a pronounced exciton-density dependence in the vicinity of the phase transition. It is well described by the power law Γ ∝ (n− nc) 2 that can be understood by linearization of the equations around the equilibrium solution. Above the critical density we get a non-exponential relaxation to the final condensate value p with |p(t)| − |p| ∝ 1 t that holds for all densities. Furthermore we include the full self-consistent Hartree-Fock-Bogoliubov (HFB) terms due to the excitonexciton interaction and the kinetics of the anomalous functions Fk = 〈aka−k〉. The collision terms are analyzed and an approximation is used which is consistent with the existence of BEC. The inclusion of the coherent x-x interaction does not change the dephasing laws. The anomalous function Fk exhibits a clear threshold behaviour at the critical density. PACS. 71.35.Lk Collective effects( Bose effects and excitonic phase transition) – 71.35.-y Excitons 2 O.M. Schmitt et al.: Dephasing of excitons in the vicinity of nc