700 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
Electron momentum distribution of a single mobile hole in the t-J model
We investigate the electron momentum distribution function (EMDF) for the
two-dimensional t-J model. The results are based on the self-consistent Born
approximation (SCBA) for the self-energy and the wave function. In the Ising
limit of the model we give the results in a closed form, in the Heisenberg
limit the results are obtained numerically. An anomalous momentum dependence of
EMDF is found and the anomaly is in the lowest order in number of magnons
expressed analitycally. We interpret the anomaly as a fingerprint of an
emerging large Fermi surface coexisting with hole pockets.Comment: M2S - submitted to Physica
Spectral Properties of the Attractive Hubbard Model
Deviations from Fermi liquid behavior are well documented in the normal state
of the cuprate superconductors, and some of these differences are possibly
related to pre-formed pairs appearing at temperatures above T_c. In order to
test these ideas we have investigated the attractive Hubbard model within a
self-consistent, conserving ladder approximation. In this version of the
theory, no feature is present which can be related to the pseudo gap found in
the high-T_c materials. Further, the interactions between two-particle bound
states change the physics of the superconducting instability in a profound
fashion, and lead to a completely different phenomenology that one predicts
based on the non-self-consistent version of the same theory.Comment: 4 pages including 2 figures, to appear in the proceedings of the
SNS'9
Evolution of Hole and Spin Dynamics in High Temperature Superconductors within the Small Hole Density Limit of the t-J Model
The evolution of hole and spin dynamics in high temperature superconductors
is studied within the self-consistent noncrossing approximation of the t-J
model in the small hole density limit. As the doping concentration is
increased, long-range electron correlations disappear rapidly and the
quasiparticle energy band becomes considerably narrow. At a small hole density
long-range antiferromagnetic order is destroyed leading to the inadequacy of
spin wave basis approximation near small wave vectors. Spin excitations near
the antiferromagnetic zone boundary are strongly renormalized and damped but
they are still well described within spin wave basis approximation.Comment: 4 pages, 4 figures, revetex, accepted for publication in Solid State
Communicatio
On the interpretation of wave function overlaps in quantum dots
The spontaneous emission rate of excitons strongly confined in quantum dots
is proportional to the overlap integral of electron and hole envelope wave
functions. A common and intuitive interpretation of this result is that the
spontaneous emission rate is proportional to the probability that the electron
and the hole are located at the same point or region in space, i.e. they must
coincide spatially to recombine. Here we show that this interpretation is not
correct even loosely speaking. By general mathematical considerations we
compare the envelope wave function overlap, the exchange overlap integral, and
the probability of electrons and holes coinciding and find that the frequency
dependence of the envelope wave function overlap integral is very different
from that expected from the common interpretation. We show that these
theoretical considerations lead to predictions for measurements. We compare our
qualitative predictions with recent measurements of the wave function overlap
and find good agreement.Comment: 4 pages, 3 figure
Prerequisite for superconductivity: appropriate spin-charge correlations
This work investigates the relation between superconductivity and
correlations. A simple calculation shows that the appropriate spin-charge
correlation is the key role to any superconductivity, and this calculation is
consistent with the analyses of unusual properties of superconductors. (Note:
the Tc of this model is not given clearly in this work, but we have advanced
this mechanism to a t-x model which includes various superconductivities and
magnetisms (please see arXiv:0707.3660 and following works).)Comment: 7page
Femtosecond study of the interplay between excitons, trions, and carriers in (Cd,Mn)Te quantum wells
We present an absorption study of the neutral and positively charged exciton
(trion) under the influence of a femtosecond, circularly polarized, resonant
pump pulse. Three populations are involved: free holes, excitons, and trions,
all exhibiting transient spin polarization. In particular, a polarization of
the hole gas is created by the formation of trions. The evolution of these
populations is studied, including the spin flip and trion formation processes.
The contributions of several mechanisms to intensity changes are evaluated,
including phase space filling and spin-dependent screening. We propose a new
explanation of the oscillator strength stealing phenomena observed in p-doped
quantum wells, based on the screening of neutral excitons by charge carriers.
We have also found that binding heavy holes into charged excitons excludes them
from the interaction with the rest of the system, so that oscillator strength
stealing is partially blockedComment: 4 pages, 4 figure
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