Sep 10, 2002
Correlation Effects in the Coherent Optical Response of Semiconductors
Dr. Daniel Chemla, Berkeley

http://www.lbl.gov/msd/chemla/
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Over the last decade a wealth of novel information on Coulomb correlation
and electronic dynamics in semiconductors has been obtained in regimes
where traditional assumptions fail. The usual description of solids assumes
a quasi-stationary limit in which the Random Phase Approximation is valid.
However, at low density and on a time-scale short compared to the time
between quasi-particle collisions, not enough events happen over the time
span of an experiment for a quasi-particle to interact witha substantial
fraction of its neighbors. Thus it become possible to observed deviations
from mean-field theory, a regime where the scattering fluctuations induce
large fluctuations of the mean-field order parameters and high order
correlations become dominant. Another regime where traditional assumptions
fail is that of strongly correlated dynamics where it is no more justified
to treat the interactions between quasi-particles as scattering events
local in space and time, a key assumption of the Boltzmann theory. Recent
coherent time resolved spectroscopy studies with time resolution much
shorter than quasi-particle scattering time scales have revealed features
related to genuine 4-particle and 6-particle correlations and memory
effects in electron-electron interaction and electron-phonon interaction in
semiconductors. In this conference I give a comprehensive and balanced
account of recent advances, both experimental and theoretical, in our
understanding of dynamical Coulomb correlations in semiconductors. I shall
try to focus on the most important physics and, as much as possible, give
an intuitive picture of the new phenomena that have been observed.
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