4,899 research outputs found
Fermionic Molecular Dynamics
A quantum molecular model for fermions is investigated which works with
antisymmetrized many-body states composed of localized single-particle wave
packets. The application to the description of atomic nuclei and collisions
between them shows that the model is capable to address a rich variety of
observed phenomena. Among them are shell effects, cluster structure and
intrinsic deformation in ground states of nuclei as well as fusion, incomplete
fusion, dissipative binary collisions and multifragmentation in reactions
depending on impact parameter and beam energy. Thermodynamic properties studied
with long time simulations proof that the model obeys Fermi-Dirac statistics
and time averaging is equivalent to ensemble averaging. A first order
liquid-gas phase transition is observed at a boiling temperature of for finite nuclei of mass .Comment: 61 pages, several postscript figures, uses 'epsfig.sty'. Report to be
published in Prog. Part. Nucl. Phys. 39. More information available at
http://www.gsi.de/~schnack/fmd.htm
Theory and applications of free-electron vortex states
Both classical and quantum waves can form vortices: with helical phase fronts
and azimuthal current densities. These features determine the intrinsic orbital
angular momentum carried by localized vortex states. In the past 25 years,
optical vortex beams have become an inherent part of modern optics, with many
remarkable achievements and applications. In the past decade, it has been
realized and demonstrated that such vortex beams or wavepackets can also appear
in free electron waves, in particular, in electron microscopy. Interest in
free-electron vortex states quickly spread over different areas of physics:
from basic aspects of quantum mechanics, via applications for fine probing of
matter (including individual atoms), to high-energy particle collision and
radiation processes. Here we provide a comprehensive review of theoretical and
experimental studies in this emerging field of research. We describe the main
properties of electron vortex states, experimental achievements and possible
applications within transmission electron microscopy, as well as the possible
role of vortex electrons in relativistic and high-energy processes. We aim to
provide a balanced description including a pedagogical introduction, solid
theoretical basis, and a wide range of practical details. Special attention is
paid to translate theoretical insights into suggestions for future experiments,
in electron microscopy and beyond, in any situation where free electrons occur.Comment: 87 pages, 34 figure
Reactions at surfaces studied by ab initio dynamics calculations
Due to the development of efficient algorithms and the improvement of
computer power it is now possible to map out potential energy surfaces (PES) of
reactions at surfaces in great detail. This achievement has been accompanied by
an increased effort in the dynamical simulation of processes on surfaces. The
paradigm for simple reactions at surfaces -- the dissociation of hydrogen on
metal surfaces -- can now be treated fully quantum dynamically in the molecular
degrees of freedom from first principles, i.e., without invoking any adjustable
parameters. This relatively new field of ab initio dynamics simulations of
reactions at surfaces will be reviewed. Mainly the dissociation of hydrogen on
clean and adsorbate covered metal surfaces and on semiconductor surfaces will
be discussed. In addition, the ab initio molecular dynamics treatment of
reactions of hydrogen atoms with hydrogen-passivated semiconductor surfaces and
recent achievements in the ab initio description of laser-induced desorption
and further developments will be addressed.Comment: 33 pages, 19 figures, submitted to Surf. Sci. Rep. Other related
publications can be found at http://www.rz-berlin.mpg.de/th/paper.htm
Matrix-valued Quantum Lattice Boltzmann Method
We devise a lattice Boltzmann method (LBM) for a matrix-valued quantum
Boltzmann equation, with the classical Maxwell distribution replaced by
Fermi-Dirac functions. To accommodate the spin density matrix, the distribution
functions become 2 x 2 matrix-valued. From an analytic perspective, the
efficient, commonly used BGK approximation of the collision operator is valid
in the present setting. The numerical scheme could leverage the principles of
LBM for simulating complex spin systems, with applications to spintronics.Comment: 18 page
A Study of molecular cooling via Sisyphus processes
We present a study of Sisyphus cooling of molecules: the scattering of a
single-photon remove a substantial amount of the molecular kinetic energy and
an optical pumping step allow to repeat the process. A review of the produced
cold molecules so far indicates that the method can be implemented for most of
them, making it a promising method able to produce a large sample of molecules
at sub-mK temperature. Considerations of the required experimental parameters,
for instance the laser power and linewidth or the trap anisotropy and
dimensionality, are given. Rate equations, as well as scattering and dipolar
forces, are solved using Kinetic Monte Carlo methods for several lasers and
several levels. For NH molecules, such detailed simulation predicts a 1000-fold
temperature reduction and an increase of the phase space density by a factor of
10^7 . Even in the case of molecules with both low Franck-Condon coefficients
and a non-closed pumping scheme, 60% of trapped molecules can be cooled from
100 mK to sub-mK temperature in few seconds. Additionally, these methods can be
applied to continuously decelerate and cool a molecular bea
Nonlinear aspects of quantum plasma physics
Dense quantum plasmas are ubiquitous in planetary interiors and in compact
astrophysical objects, in semiconductors and micro-mechanical systems, as well
as in the next generation intense laser-solid density plasma interaction
experiments and in quantum x-ray free-electron lasers. In contrast to classical
plasmas, one encounters extremely high plasma number density and low
temperature in quantum plasmas. The latter are composed of electrons, positrons
and holes, which are degenerate. Positrons (holes) have the same (slightly
different) mass as electrons, but opposite charge. The degenerate charged
particles (electrons, positrons, holes) follow the Fermi-Dirac statistics. In
quantum plasmas, there are new forces associated with i) quantum statistical
electron and positron pressures, ii) electron and positron tunneling through
the Bohm potential, and iii) electron and positron angular momentum spin.
Inclusion of these quantum forces provides possibility of very high-frequency
dispersive electrostatic and electromagnetic waves (e.g. in the hard x-ray and
gamma rays regimes) having extremely short wavelengths. In this review paper,
we present theoretical backgrounds for some important nonlinear aspects of
wave-wave and wave-electron interactions in dense quantum plasmas.
Specifically, we shall focus on nonlinear electrostatic electron and ion plasma
waves, novel aspects of 3D quantum electron fluid turbulence, as well as
nonlinearly coupled intense electromagnetic waves and localized plasma wave
structures. Also discussed are the phase space kinetic structures and
mechanisms that can generate quasi-stationary magnetic fields in dense quantum
plasmas. The influence of the external magnetic field and the electron angular
momentum spin on the electromagnetic wave dynamics is discussed.Comment: 42 pages, 20 figures, accepted for publication in Physics-Uspekh
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