438 research outputs found
Many-body Correlation Effects in the Ultrafast Nonlinear Optical Response of Confined Fermi Seas
The dynamics of electrons and atoms interacting with intense and ultrashort
optical pulses presents an important problem in physics that cuts across
different materials such as semiconductors and metals. The currently available
laser pulses, as short as 5 fs, provide a time resolution shorter than the
dephasing and relaxation times in many materials. This allows for a systematic
study of many-body effects using nonlinear optical spectroscopy. In this review
article, we discuss the role of Coulomb correlations in the ultrafast dynamics
of modulation-doped quantum wells and metal nanoparticles. We focus in
particular on the manifestations of non-Markovian memory effects induced by
strong electron-hole and electron-plasmon correlations.Comment: 107 pages including 15 figures. Review article to appear in Surf.
Sci. Report
Spin dynamics in nonlinear optical spectroscopy of Fermi sea systems
We discuss the role of many-body spin correlations in nonlinear optical
response of a Fermi sea system with a deep impurity level. Due to the Hubbard
repulsion between electrons at the impurity, the optical transitions between
the impurity level and the Fermi sea states lead to an optically-induced Kondo
effect. In particular, the third-order nonlinear optical susceptibility
logarithmiclly diverges at the absorption threshold. The shape of the
pump-probe spectrum is governed by the light-induced Kondo temperature, which
can be tuned by varying the intensity and frequency of the pump optical field.
In the Kondo limit, corresponding to off-resonant pump excitation, the
nonlinear absorption spectrum exhibits a narrow peak below the linear
absorption onset.Comment: 7 pages inluding 2 figures. Invited paper for SPIE's Optoelectronics
2001 Conferenc
Non--Heisenberg Spin Dynamics of Double-Exchange Ferromagnets with Coulomb Repulsion
With a variational three--body calculation we study the role of the interplay
between the onsite Coulomb, Hund's rule, and superexchange interactions on the
spinwave excitation spectrum of itinerant ferromagnets. We show that
correlations between a Fermi sea electron--hole pair and a magnon result in a
very pronounced zone boundary softening and strong deviations from the
Heisenberg spinwave dispersion. We show that this spin dynamics depends
sensitively on the Coulomb and exchange interactions and discuss its possible
relevance to experiments in the manganites.Comment: 4 pages, 4 figures, published in Physical Review B as rapid
communication
Three--body Correlation Effects on the Spin Dynamics of Double--Exchange Ferromagnets
We present a variational calculation of the spin wave excitation spectrum of
double--exchange ferromagnets in different dimensions. Our theory recovers the
Random Phase approximation and 1/S expansion results as limiting cases and can
be used to study the intermediate exchange coupling and electron concentration
regime relevant to the manganites. In particular, we treat exactly the long
range three--body correlations between a Fermi sea electron--hole pair and a
magnon excitation and show that they strongly affect the spin dynamics in the
parameter range relevant to experiments in the manganites. The manifestations
of these correlations are many-fold. We demonstrate that they significantly
change the ferromagnetic phase boundary. In addition to a decrease in the
magnon stiffness, we obtain an instability of the ferromagnetic state against
spin wave excitations close to the Brillouin zone boundary.Within a range of
intermediate concentrations, we find a strong softening of the spin wave
dispersion as compared to the Heisenberg ferromagnet with the same stiffness,
which changes into hardening for other concentrations. We discuss the relevance
of these results to experiments in colossal magnetoresistance ferromagnets.Comment: 14 pages, 11 figures, published in Phys. Rev. B (1 figure added,
references added
Canonical Transformation Approach to the Ultrafast Non-linear Optical Dynamics of Semiconductors
We develop a theory describing the effects of many-particle Coulomb
correlations on the coherent ultrafast nonlinear optical response of
semiconductors and metals. Our approach is based on a mapping of the nonlinear
optical response of the ``bare'' system onto the linear response of a
``dressed'' system. The latter is characterized by effective time-dependent
optical transition matrix elements, electron/hole dispersions, and interaction
potentials, which in undoped semiconductors are determined by the
single-exciton and two-exciton Green functions in the absence of optical
fields. This mapping is achieved by eliminating the optically-induced charge
fluctuations from the Hamiltonian using a Van Vleck canonical transformation.
It takes into account all many-body contributions up to a given order in the
optical fields as well as important Coulomb-induced quantum dynamics to all
orders in the optical field. Our approach allows us to distinguish between
optical nonlinearities of different origins and provides a physically-intuitive
interpretation of their manifestations in ultrafast coherent nonlinear optical
spectroscopy.Comment: 24 page
Ultrafast light-induced magnetization dynamics in ferromagnetic semiconductors
We develop a theory of the magnetization dynamics triggered by ultrafast
optical excitation of ferromagnetic semiconductors. We describe the effects of
the strong carrier spin relaxation on the nonlinear optical response by using
the Lindblad semigroup method. We demonstrate magnetization control during
femtosecond timescales via the interplay between circularly polarized optical
excitation, hole-spin damping, polarization dephasing, and the Mn-hole spin
interactions. Our results show a light-induced magnetization precession and
relaxation for the duration of the optical pulse.Comment: 4 pages, 2 figure
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