466 research outputs found
Ab-initio calculation of optical absorption in semiconductors: A density-matrix description
We show how to describe Coulomb renormalization effects and dielectric
screening in semiconductors and semiconductor nanostructures within a
first-principles density-matrix description. Those dynamic variables and
approximation schemes which are required for a proper description of dielectric
screening are identified. It is shown that within the random-phase
approximation the direct Coulomb interactions become screened, with static
screening being a good approximation, whereas the electron-hole exchange
interactions remain unscreened. Differences and similarities of our results
with those obtained from a corresponding GW approximation and Bethe-Salpeter
equation Green's function analysis are discussed.Comment: 10 pages, to be published in Physical Review
Estrutura interna de testes de cohecimento em FĂsica : um exemplo em mecânica
Research findings are presented concerning the internal structure analysis of a multiple choice test constructed to detect whether or not the student has the Newtonian conception of the interrelationship between force and motion. These findings show that the test has two different but related factors: one subset of items checks students' ability to identify forces in certain physical situations according to the Newtonian view while another subset checks whether or not they relate the resultant force with acceleration. In addition, the analysis also shows that the two subsets have slightly different predicting power concerning student performance in mechanics. This kind of analysis might be very relevant for teacher's work regarding learning evaluation
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
Collective oscillations driven by correlation in the nonlinear optical regime
We present an analytical and numerical study of the coherent exciton
polarization including exciton-exciton correlation. The time evolution after
excitation with ultrashort optical pulses can be divided into a slowly varying
polarization component and novel ultrafast collective modes. The frequency and
damping of the collective modes are determined by the high-frequency properties
of the retarded two-exciton correlation function, which includes Coulomb
effects beyond the mean-field approximation. The overall time evolution depends
on the low-frequency spectral behavior. The collective mode, well separated
from the slower coherent density evolution, manifests itself in the coherent
emission of a resonantly excited excitonic system, as demonstrated numerically.Comment: 4 pages, 4 figures, accepted for publication in Physical Review
Letter
Discrepancies in East Asians' perceived actual and ideal phenotypic facial features
The present study tested for the existence of a phenotypic actual-ideal discrepancy in East Asians’ appraisals of their own faces, in the direction of idealizing a phenotypically “Whiter” face than they perceived themselves to have. The study was conducted in two phases. In the first phase, East Asian participants residing in the U.S. (N = 104; Mage = 18.73) came into the lab to have their photograph taken. They were sent a link to complete the second phase online. Participants were required to recall either their previous day, an experience of racial discrimination, or an experience of racial acceptance. They then selected their actual and ideal face from an array of faces comprising their actual face and eight variants of their face that had been transformed to look phenotypically more “White” or more “East Asian”. A robust actual-ideal discrepancy emerged: participants both idealized a phenotypically “Whiter” face and perceived themselves as having a more phenotypically “East Asian” face than they objectively did. This discrepancy arose irrespective of whether participants were reminded of an incident of racial discrimination or acceptance
Revealing and Controlling Energy Barriers and Valleys at Grain Boundaries in Ultrathin Organic Films
Resonant nature of phonon-induced damping of Rabi oscillations in quantum dots
Optically controlled coherent dynamics of charge (excitonic) degrees of
freedom in a semiconductor quantum dot under the influence of lattice dynamics
(phonons) is discussed theoretically. We show that the dynamics of the lattice
response in the strongly non-linear regime is governed by a semiclassical
resonance between the phonon modes and the optically driven dynamics. We stress
on the importance of the stability of intermediate states for the truly
coherent control.Comment: 4 pages, 2 figures; final version; moderate changes, new titl
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
Probing Interband Coulomb Interactions in Semiconductor Nanocrystals with 2D Double-Quantum Coherence Spectroscopy
Using previously developed exciton scattering model accounting for the
interband, i.e., exciton-biexciton, Coulomb interactions in semiconductor
nanocrystals (NCs), we derive a closed set of equations for 2D double-quantum
coherence signal. The signal depends on the Liouville space pathways which
include both the interband scattering processes and the inter- and intraband
optical transitions. These processes correspond to the formation of different
cross-peaks in the 2D spectra. We further report on our numerical calculations
of the 2D signal using reduced level scheme parameterized for PbSe NCs. Two
different NC excitation regimes considered and unique spectroscopic features
associated with the interband Coulomb interactions are identified.Comment: 11 pages, 5 figure
- …