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We investigate the role played by nonequilibrium dynamical screening in thethermalization of carriers in a simplified two-component two-band model of asemiconductor. The main feature of our approach is the theoretically soundtreatment of collisions. We abandon Fermi's Golden rule in favor of anonequilibrium field theoretic formalism as the former is applicable only inthe long-time regime. We also introduce the concept of nonequilibrium dynamicalscreening. The dephasing of excitonic quantum beats as a result ofcarrier-carrier scattering is brought out. At low densities it is found thatthe dephasing times due to carrier-carrier scattering is in picoseconds and notfemtoseconds, in agreement with experiments. The polarization dephasing ratesare computed as a function of the excited carrier density and it is found thatthe dephasing rate for carrier-carrier scattering is proportional to thecarrier density at ultralow densities. The scaling relation is sublinear athigher densities, which enables a comparison with experiment.Comment: Revised version with additional refs. 12 pages, figs. available upon  request; Submitted to Phys. Rev.

Role of nonequilibrium dynamical screening in carrier thermalization