303 research outputs found
Non-equilibrium Green's function approach to inhomogeneous quantum many-body systems using the Generalized Kadanoff Baym Ansatz
In non-equilibrium Green's function calculations the use of the Generalized
Kadanoff-Baym Ansatz (GKBA) allows for a simple approximate reconstruction of
the two-time Green's function from its time-diagonal value. With this a drastic
reduction of the computational needs is achieved in time-dependent
calculations, making longer time propagation possible and more complex systems
accessible. This paper gives credit to the GKBA that was introduced 25 years
ago. After a detailed derivation of the GKBA, we recall its application to
homogeneous systems and show how to extend it to strongly correlated,
inhomogeneous systems. As a proof of concept, we present results for a
2-electron quantum well, where the correct treatment of the correlated electron
dynamics is crucial for the correct description of the equilibrium and dynamic
properties
Quantum Breathing Mode of Interacting Particles in a One-dimensional Harmonic Trap
Extending our previous work, we explore the breathing mode---the [uniform]
radial expansion and contraction of a spatially confined system. We study the
breathing mode across the transition from the ideal quantum to the classical
regime and confirm that it is not independent of the pair interaction strength
(coupling parameter). We present the results of time-dependent Hartree-Fock
simulations for 2 to 20 fermions with Coulomb interaction and show how the
quantum breathing mode depends on the particle number. We validate the accuracy
of our results, comparing them to exact Configuration Interaction results for
up to 8 particles
Partially ionized plasmas in electromagnetic fields
The interaction of partially ionized plasmas with an electromagnetic field is
investigated using quantum statistical methods. A general statistical
expression for the current density of a plasma in an electromagnetic field is
presented and considered in the high field regime. Expressions for the
collisional absorption are derived and discussed. Further, partially ionized
plasmas are considered. Plasma Bloch equations for the description of
bound-free transitions are given and the absorption coefficient as well as rate
coefficients for multiphoton ionization are derived and numerical results are
presented.Comment: 18 pages, 8 figures, accepted for publication in J. Phys.: Conf. Se
Efficient grid-based method in nonequilibrium Green's function calculations. Application to model atoms and molecules
We propose and apply the finite-element discrete variable representation to
express the nonequilibrium Green's function for strongly inhomogeneous quantum
systems. This method is highly favorable against a general basis approach with
regard to numerical complexity, memory resources, and computation time. Its
flexibility also allows for an accurate representation of spatially extended
hamiltonians, and thus opens the way towards a direct solution of the two-time
Schwinger/Keldysh/Kadanoff-Baym equations on spatial grids, including e.g. the
description of highly excited states in atoms. As first benchmarks, we compute
and characterize, in Hartree-Fock and second Born approximation, the ground
states of the He atom, the H molecule and the LiH molecule in one spatial
dimension. Thereby, the ground-state/binding energies, densities and
bond-lengths are compared with the direct solution of the time-dependent
Schr\"odinger equation.Comment: 11 pages, 5 figures, submitted to Physical Review
On the Coulomb-dipole transition in mesoscopic classical and quantum electron-hole bilayers
We study the Coulomb-to-dipole transition which occurs when the separation
of an electron-hole bilayer system is varied with respect to the
characteristic in-layer distances. An analysis of the classical ground state
configurations for harmonically confined clusters with reveals that
the energetically most favorable state can differ from that of two-dimensional
pure dipole or Coulomb systems. Performing a normal mode analysis for the N=19
cluster it is found that the lowest mode frequencies exhibit drastic changes
when is varied. Furthermore, we present quantum-mechanical ground states
for N=6, 10 and 12 spin-polarized electrons and holes. We compute the
single-particle energies and orbitals in self-consistent Hartree-Fock
approximation over a broad range of layer separations and coupling strengths
between the limits of the ideal Fermi gas and the Wigner crystal
Multiconfigurational time-dependent Hartree-Fock calculations for photoionization of one-dimensional Helium
The multiconfigurational time-dependent Hartree-Fock equations are discussed
and solved for a one-dimensional model of the Helium atom. Results for the
ground state energy and two-particle density as well as the absorption spectrum
are presented and compared to direct solutions of the time-dependent
Schroedinger equation.Comment: 10 pages, 3 figures, 1 tabl
Dynamical equilibration of strongly interacting "infinite" parton matter within the parton-hadron-string dynamics transport approach
We study the kinetic and chemical equilibration in "infinite" parton matter
within the parton-hadron-string dynamics off-shell transport approach, which is
based on a dynamical quasiparticle model (DQPM) for partons matched to
reproduce lattice QCD results-including the partonic equation of state-in
thermodynamic equilibrium. The "infinite" parton matter is simulated by a
system of quarks and gluons within a cubic box with periodic boundary
conditions, at different energy densities, initialized slightly out of kinetic
and chemical equilibrium. We investigate the approach of the system to
equilibrium and the time scales for the equilibration of different observables.
We, furthermore, study particle distributions in the strongly interacting
quark-gluon plasma (sQGP) including partonic spectral functions, momentum
distributions, abundances of the different parton species and their
fluctuations (scaled variance, skewness, and kurtosis) in equilibrium. We also
compare the results of the microscopic calculations with the ansatz of the
DQPM. It is found that the results of the transport calculations are in
equilibrium well matched by the DQPM for quarks and antiquarks, while the gluon
spectral function shows a slightly different shape due to the explicit
interaction of partons. The time scales for the relaxation of fluctuation
observables are found to be shorter than those for the average values.
Furthermore, in the local subsystem, a strong change of the fluctuation
observables with the size of the local volume is observed. These fluctuations
no longer correspond to those of the full system and are reduced to Poissonian
distributions when the volume of the local subsystem becomes much smaller than
the total volume.Comment: 21 pages, 20 figure
Berezinskii-Kosterlitz-Thouless transition in two-dimensional dipole systems
The superfluid to normal fluid transition of dipolar bosons in two dimensions
is studied throughout the whole density range using path integral Monte Carlo
simulations and summarized in the phase diagram. While at low densities, we
find good agreement with the universal results depending only on the scattering
length , at moderate and high densities, the transition temperature is
strongly affected by interactions and the elementary excitation spectrum. The
results are expected to be of relevance to dipolar atomic and molecular systems
and indirect excitons in quantum wells
Kadanoff-Baym equations and non-Markovian Boltzmann equation in generalized T-matrix approximation
A recently developed method for incorporating initial binary correlations
into the Kadanoff-Baym equations (KBE) is used to derive a generalized T-matrix
approximation for the self-energies. It is shown that the T-matrix obtains
additional contributions arising from initial correlations. Using these results
and taking the time-diagonal limit of the KBE, a generalized quantum kinetic
equation in binary collision approximation is derived. This equation is a
far-reaching generalization of Boltzmann-type kinetic equations: it
selfconsistently includes memory effects (retardation, off-shell T-matrices) as
well as many-particle effects (damping, in-medium T-Matrices) and
spin-statistics effects (Pauli-blocking).Comment: 9 pages, 7 figures, corrected misprints in eqs. 48-5
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