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{\bf Exciton-Phonon interaction in semiconductors with intermediate
polar coupling} \\[5mm]

P. Gartner and F. Jahnke \\
{\it
Institute for Theoretical Physics, University of Bremen \\
28334 Bremen, Germany} \\[5mm]

W. Sch"afer \\
{\it
John von Neumann--Institute for Computing Forschungszentrum J"ulich \\
52425 J"ulich, Germany} \\[5mm]

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Theoretical investigations of quantum kinetics for electron-phonon
interaction [1,2] have attracted considerable interest in connection
with recent femtosecond four-wave mixing experiments [3]. Theoretical
models based on two-time calculations show deviations from results of
the generalized Kadanoff-Baym ansatz (GKBA) for LO-phonon interaction
in polar semiconductors with intermediate coupling like ZnSe and CdTe.

Previous investigations are based on electron-phonon interaction
described in self-consistent random-phase approximation (RPA). On 
the other hand, in materials where the exciton binding energy is
comparable with the LO-phonon energy, carrier-phonon interaction near
the band edge is modified by excitonic effects [4]. The most prominent
consequences are phonon satellites of the exciton absorption
spectra. Furthermore, excitonic renormalizations of the initial and
final scattering states drastically modify the scattering rates. 

To consider excitonic effects on the carrier-phonon interaction, we
use kinetic equations where carrier-phonon self-energies contain
Coulomb-vertex contributions which lead to ladder-type diagrams. For
the evaluation of the self-energies we compare results of the GKBA
with those of an alternative ansatz. Both methods are tested against
the two-time calculation. The alternative ansatz is based on a formal
solution of the kinetic equation for two-time propagators where in
addition to polaronic renormalizations also static Coulomb-ladder
contributions are included. Results are presented for II-VI bulk and
quantum-well structures under various excitation conditions. \\[-2mm]

[1] M. Hartmann and W. Sch"afer, phys. stat. sol. (b) 173, 165 (1992)

[2] P. Gartner, L. Banyai, and H. Haug, Phys. Rev. B 60, 14234 (1999)

[3] D. Steinbach et al., Phys. Rev. B 60, 12079 (1999)

[4] W. Sch"afer and M. Wegener, Semiconductor Optics and Transport
    Phenomena, Springer (2002)

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