4,728 research outputs found
Many-body Green's function theory for electron-phonon interactions: ground state properties of the Holstein dimer
We study ground-state properties of a two-site, two-electron Holstein model
describing two molecules coupled indirectly via electron-phonon interaction by
using both exact diagonalization and self-consistent diagrammatic many-body
perturbation theory. The Hartree and self-consistent Born approximations used
in the present work are studied at different levels of self-consistency. The
governing equations are shown to exhibit multiple solutions when the
electron-phonon interaction is sufficiently strong whereas at smaller
interactions only a single solution is found. The additional solutions at
larger electron-phonon couplings correspond to symmetry-broken states with
inhomogeneous electron densities. A comparison to exact results indicates that
this symmetry breaking is strongly correlated with the formation of a bipolaron
state in which the two electrons prefer to reside on the same molecule. The
results further show that the Hartree and partially self-consistent Born
solutions obtained by enforcing symmetry do not compare well with exact
energetics, while the fully self-consistent Born approximation improves the
qualitative and quantitative agreement with exact results in the same symmetric
case. This together with a presented natural occupation number analysis
supports the conclusion that the fully self-consistent approximation describes
partially the bipolaron crossover. These results contribute to better
understanding how these approximations cope with the strong localizing effect
of the electron-phonon interaction.Comment: 9 figures, corrected typo
Comment on ``Spin Dependent Hopping and Colossal Negative Magnetoresistance in Epitaxial Films in Fields up to 50 T''
Recently Wagner et al. [Phys. Rev. Lett. Vol. 81, P. 3980 (1998)] proposed
that Mott's original model be modified to incorporate a hopping barrier which
depends on the misorientation between the spins of electrons at the initial and
the final states in an elementary process. They further claimed that using the
model they can explain the observed scaling behavior--
negative-magnetoresistivity scaling proportional to the Brillouin function
in the ferromagnetic state and to in the paramagnetic
state. In this comment we argue that the modification needed for Mott's
original model is different from that proposed by Wagner et al. and further
show that our picture will successfully explain the observed scaling in the two
regimes.Comment: 1 pag
Time-dependent natural orbitals and occupation numbers
We report equations of motion for the occupation numbers of natural spin
orbitals and show that adiabatic extensions of common functionals employed in
ground-state reduced-density-matrix-functional theory have the shortcoming of
leading always to occupation numbers which are independent of time. We
illustrate the exact time-dependence of the natural spin orbitals and
occupation numbers for the case of electron-ion scattering and for atoms in
strong laser fields. In the latter case, we observe strong variations of the
occupation numbers in time.Comment: 5 pages, 5 figure
Quantum coherence and carriers mobility in organic semiconductors
We present a model of charge transport in organic molecular semiconductors
based on the effects of lattice fluctuations on the quantum coherence of the
electronic state of the charge carrier. Thermal intermolecular phonons and
librations tend to localize pure coherent states and to assist the motion of
less coherent ones. Decoherence is thus the primary mechanism by which
conduction occurs. It is driven by the coupling of the carrier to the molecular
lattice through polarization and transfer integral fluctuations as described by
the hamiltonian of Gosar and Choi. Localization effects in the quantum coherent
regime are modeled via the Anderson hamiltonian with correlated diagonal and
non-diagonal disorder leading to the determination of the carrier localization
length. This length defines the coherent extension of the ground state and
determines, in turn, the diffusion range in the incoherent regime and thus the
mobility. The transfer integral disorder of Troisi and Orlandi can also be
incorporated. This model, based on the idea of decoherence, allowed us to
predict the value and temperature dependence of the carrier mobility in
prototypical organic semiconductors that are in qualitative accord with
experiments
Fullerene graphs have exponentially many perfect matchings
A fullerene graph is a planar cubic 3-connected graph with only pentagonal
and hexagonal faces. We show that fullerene graphs have exponentially many
perfect matchings.Comment: 7 pages, 3 figure
Back-to-back correlations of high p_T hadrons in relativistic heavy ion collisions
We investigate the suppression factor and the azimuthal correlation function
for high hadrons in central Au+Au collisions at GeV
by using a dynamical model in which hydrodynamics is combined with explicitly
traveling jets. We study the effects of parton energy loss in a hot medium,
intrinsic of partons in a nucleus, and broadening of jets on
the back-to-back correlations of high hadrons. Parton energy loss is
found to be a dominant effect on the reduction of the away-side peaks in the
correlation function.Comment: 4 pages, 4 figures; version to appear in Phys. Rev. Let
Excitations in time-dependent density-functional theory
An approximate solution to the time-dependent density functional theory
(TDDFT) response equations for finite systems is developed, yielding
corrections to the single-pole approximation. These explain why allowed
Kohn-Sham transition frequencies and oscillator strengths are usually good
approximations to the true values, and why sometimes they are not. The
approximation yields simple expressions for G\"orling-Levy perturbation theory
results, and a method for estimating expectation values of the unknown
exchange-correlation kernel.Comment: 4 pages, 1 tabl
Photoionization and transient Wannier-Stark ladder in silicon: First-principles simulations versus Keldysh theory
Nonlinear photoionization of dielectrics and semiconductors is widely treated in the framework of the Keldysh theory whose validity is limited to photon energies that are small compared to the band gap and relatively low laser intensities. The time-dependent density functional theory (TDDFT) simulations, which are free of these limitations, enable one to gain insight into nonequilibrium dynamics of the electronic structure. Here we apply TDDFT to investigate the photoionization of silicon crystal by ultrashort laser pulses in a wide range of laser wavelengths and intensities and compare the results with predictions of the Keldysh theory. Photoionization rates derived from the simulations considerably exceed the data obtained with the Keldysh theory within the validity range of the latter. Possible reasons for the discrepancy are discussed and we provide fundamental data on the photoionization rates beyond the limits of the Keldysh theory. By investigating the features of the Stark shift as a function of photon energy and laser field strength, a manifestation of the transient Wannier-Stark ladder states is revealed, which become blurred with increasing laser field strength. Finally, it is shown that the TDDFT simulations can potentially provide reliable data on the electron damping time that is of high importance for large-scale modeling
Municipal Real Estate deployed as a capital asset
Abstract The aim of this study is to introduce the principles of Corporate Real Estate Management (CREM) within the municipal organization. It is presumed that, looking at the private sector’s accomplishments in this field, these principles will allow a city council to deploy their real estate in a more strategic manner. In this study a model has been created for implementing so-called Municipal Real Estate Management (MREM) within the municipal organization on the desired level and adjusting the municipal real estate portfolio accordingly. This introduction of MREM will contribute to offering adequate accommodation to the different consumers, which heightens customer satisfaction. At the same time, it will add to the company result, balancing costs and benefits and delivering added value through a better control of risks and creating economies of scale
- …