1,238 research outputs found
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
Adsorption and desorption of hydrogen at nonpolar GaN(1-100) surfaces: Kinetics and impact on surface vibrational and electronic properties
The adsorption of hydrogen at nonpolar GaN(1-100) surfaces and its impact on
the electronic and vibrational properties is investigated using surface
electron spectroscopy in combination with density functional theory (DFT)
calculations. For the surface mediated dissociation of H2 and the subsequent
adsorption of H, an energy barrier of 0.55 eV has to be overcome. The
calculated kinetic surface phase diagram indicates that the reaction is
kinetically hindered at low pressures and low temperatures. At higher
temperatures ab-initio thermodynamics show, that the H-free surface is
energetically favored. To validate these theoretical predictions experiments at
room temperature and under ultrahigh vacuum conditions were performed. They
reveal that molecular hydrogen does not dissociatively adsorb at the GaN(1-100)
surface. Only activated atomic hydrogen atoms attach to the surface. At
temperatures above 820 K, the attached hydrogen gets desorbed. The adsorbed
hydrogen atoms saturate the dangling bonds of the gallium and nitrogen surface
atoms and result in an inversion of the Ga-N surface dimer buckling. The
signatures of the Ga-H and N-H vibrational modes on the H-covered surface have
experimentally been identified and are in good agreement with the DFT
calculations of the surface phonon modes. Both theory and experiment show that
H adsorption results in a removal of occupied and unoccupied intragap electron
states of the clean GaN(1-100) surface and a reduction of the surface upward
band bending by 0.4 eV. The latter mechanism largely reduces surface electron
depletion
Nonmonotonic Decay of Nonequilibrium Polariton Condensate in Direct-Gap Semiconductors
Time evolution of a nonequilibrium polariton condensate has been studied in
the framework of a microscopic approach. It has been shown that due to
polariton-polariton scattering a significant condensate depletion takes place
in a comparatively short time interval. The condensate decay occurs in the form
of multiple echo signals. Distribution-function dynamics of noncondensate
polaritons have been investigated. It has been shown that at the initial stage
of evolution the distribution function has the form of a bell. Then
oscillations arise in the contour of the distribution function, which further
transform into small chaotic ripples. The appearance of a short-wavelength wing
of the distribution function has been demonstrated. We have pointed out the
enhancement and then partial extinction of the sharp extra peak arising within
the time interval characterized by small values of polariton condensate density
and its relatively slow changes.Comment: 20 pages, LaTeX 2.09; in press in PR
A Cantor set of tori with monodromy near a focus-focus singularity
We write down an asymptotic expression for action coordinates in an
integrable Hamiltonian system with a focus-focus equilibrium. From the
singularity in the actions we deduce that the Arnol'd determinant grows
infinitely large near the pinched torus. Moreover, we prove that it is possible
to globally parametrise the Liouville tori by their frequencies. If one
perturbs this integrable system, then the KAM tori form a Whitney smooth
family: they can be smoothly interpolated by a torus bundle that is
diffeomorphic to the bundle of Liouville tori of the unperturbed integrable
system. As is well-known, this bundle of Liouville tori is not trivial. Our
result implies that the KAM tori have monodromy. In semi-classical quantum
mechanics, quantisation rules select sequences of KAM tori that correspond to
quantum levels. Hence a global labeling of quantum levels by two quantum
numbers is not possible.Comment: 11 pages, 2 figure
Pump Built-in Hamiltonian Method for Pump-Probe Spectroscopy
We propose a new method of calculating nonlinear optical responses of
interacting electronic systems. In this method, the total Hamiltonian (system +
system-pump interaction) is transformed into a different form that (apparently)
does not have a system-pump interaction. The transformed Hamiltonian, which we
call the pump built-in Hamiltonian, has parameters that depend on the strength
of the pump beam. Using the pump built-in Hamiltonian, we can calculate
nonlinear responses (responses to probe beams as a function of the pump beam)
by applying the {\em linear} response theory. We demonstrate the basic idea of
this new method by applying it to a one-dimensional, two-band model, in the
case the pump excitation is virtual (coherent excitation). We find that the
exponent of the Fermi edge singularity varies with the pump intensity.Comment: 6 page
Ultrafast Coulomb-induced dynamics of 2D magnetoexcitons
We study theoretically the ultrafast nonlinear optical response of quantum
well excitons in a perpendicular magnetic field. We show that for
magnetoexcitons confined to the lowest Landau levels, the third-order
four-wave-mixing (FWM) polarization is dominated by the exciton-exciton
interaction effects. For repulsive interactions, we identify two regimes in the
time-evolution of the optical polarization characterized by exponential and
{\em power law} decay of the FWM signal. We describe these regimes by deriving
an analytical solution for the memory kernel of the two-exciton wave-function
in strong magnetic field. For strong exciton-exciton interactions, the decay of
the FWM signal is governed by an antibound resonance with an
interaction-dependent decay rate. For weak interactions, the continuum of
exciton-exciton scattering states leads to a long tail of the time-integrated
FWM signal for negative time delays, which is described by the product of a
power law and a logarithmic factor. By combining this analytic solution with
numerical calculations, we study the crossover between the exponential and
non-exponential regimes as a function of magnetic field. For attractive
exciton-exciton interaction, we show that the time-evolution of the FWM signal
is dominated by the biexcitonic effects.Comment: 41 pages with 11 fig
Pseudogap phase formation in the crossover from Bose-Einstein condensation to BCS superconductivity
A phase diagram for a 2D metal with variable carrier density has been
derived. It consists of a normal phase, where the order parameter is absent; a
so-called ``abnormal normal'' phase where this parameter is also absent but the
mean number of composite bosons (bound pairs) exceeds the mean number of free
fermions; a pseudogap phase where the absolute value of the order parameter
gradually increases but its phase is a random value, and finally a
superconducting (here Berezinskii-Kosterlitz-Thouless) phase. The
characteristic transition temperatures between these phases are found. The
chemical potential and paramagnetic susceptibility behavior as functions of the
fermion density and the temperature are also studied. An attempt is made to
qualitatively compare the resulting phase diagram with the features of
underdoped high- superconducting compounds above their critical
temperature.Comment: 26 pages, revtex, 5 EMTeX figures; more discussion and references
added; to be published in JET
Theory of exciton-exciton correlation in nonlinear optical response
We present a systematic theory of Coulomb interaction effects in the
nonlinear optical processes in semiconductors using a perturbation series in
the exciting laser field. The third-order dynamical response consists of
phase-space filling correction, mean-field exciton-exciton interaction, and
two-exciton correlation effects expressed as a force-force correlation
function. The theory provides a unified description of effects of bound and
unbound biexcitons, including memory-effects beyond the Markovian
approximation. Approximations for the correlation function are presented.Comment: RevTex, 35 pages, 10 PostScript figs, shorter version submitted to
Physical Review
Optical absorption in semiconductor quantum dots: Nonlocal effects
The optical absorption of a single spherical semiconductor quantum dot in an
electrical field is studied taking into account the nonlocal coupling between
the field of the light and the polarizability of the semiconductor. These
nonlocal effects lead to a small size anf field dependent shift and broadening
of the excitonic resonance which may be of interest in future high precision
experiments.Comment: 6 pages, 4 figure
Effective Hamiltonian for Excitons with Spin Degrees of Freedom
Starting from the conventional electron-hole Hamiltonian , we
derive an effective Hamiltonian for excitons with
spin degrees of freedom. The Hamiltonian describes optical processes close to
the exciton resonance for the case of weak excitation. We show that
straightforward bosonization of does not give the correct form
of , which we obtain by a projection onto the subspace
spanned by the excitons. The resulting relaxation and renormalization
terms generate an interaction between excitons with opposite spin. Moreover,
exciton-exciton repulsive interaction is greatly reduced by the
renormalization. The agreement of the present theory with the experiment
supports the validity of the description of a fermionic system by bosonic
fields in two dimensions.Comment: 12 pages, no figures, RevTe
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