8,607 research outputs found
Conserving Approximations in Time-Dependent Density Functional Theory
In the present work we propose a theory for obtaining successively better
approximations to the linear response functions of time-dependent density or
current-density functional theory. The new technique is based on the
variational approach to many-body perturbation theory (MBPT) as developed
during the sixties and later expanded by us in the mid nineties. Due to this
feature the resulting response functions obey a large number of conservation
laws such as particle and momentum conservation and sum rules. The quality of
the obtained results is governed by the physical processes built in through
MBPT but also by the choice of variational expressions. We here present several
conserving response functions of different sophistication to be used in the
calculation of the optical response of solids and nano-scale systems.Comment: 11 pages, 4 figures, revised versio
Crossover from Reptation to Rouse dynamics in the Cage Model
The two-dimensional cage model for polymer motion is discussed with an
emphasis on the effect of sideways motions, which cross the barriers imposed by
the lattice. Using the Density Matrix Method as a solver of the Master
Equation, the renewal time and the diffusion coefficient are calculated as a
function of the strength of the barrier crossings. A strong crossover influence
of the barrier crossings is found and it is analyzed in terms of effective
exponents for a given chain length. The crossover scaling functions and the
crossover scaling exponents are calculated.Comment: RevTeX, 11 PostScript figures include
Crossover behavior for long reptating polymers
We analyze the Rubinstein-Duke model for polymer reptation by means of
density matrix renormalization techniques. We find a crossover behavior for a
series of quantities as function of the polymer length. The crossover length
may become very large if the mobility of end groups is small compared to that
of the internal reptons. Our results offer an explanation to a controversy
between theory, experiments and simulations on the leading and subleading
scaling behavior of the polymer renewal time and diffusion constant.Comment: 4 Pages, RevTeX, and 4 PostScript figures include
|V|: New insight into the circular polarization of radio pulsars
We present a study of single pulses from nine bright northern pulsars to
investigate the behaviour of circular polarisation, V. The observations were
conducted with the Effelsberg 100-m radio telescope at 1.41 GHz and 4.85 GHz
and the Westerbork radio telescope at 352 MHz. For the first time, we present
the average profile of the absolute circular polarisation |V| in the single
pulses. We demonstrate that the average profile of |V| is the distinguishing
feature between pulse components that exhibit low V in the single pulses and
components that exhibit high V of either handedness, despite both cases
resulting in a low mean. We also show that the |V| average profile remains
virtually constant with frequency, which is not generally the case for V,
leading us to the conclusion that |V| is a key quantity in the pulsar emission
problem.Comment: 5 pages, accepted for publication in MNRAS letter
Waiting and Residence Times of Brownian Interface Fluctuations
We report on the residence times of capillary waves above a given height
and on the typical waiting time in between such fluctuations. The measurements
were made on phase separated colloid-polymer systems by laser scanning confocal
microscopy. Due to the Brownian character of the process, the stochastics vary
with the chosen measurement interval . In experiments, the discrete
scanning times are a practical cutoff and we are able to measure the waiting
time as a function of this cutoff. The measurement interval dependence of the
observed waiting and residence times turns out to be solely determined by the
time dependent height-height correlation function . We find excellent
agreement with the theory presented here along with the experiments.Comment: 5 figure
Time correlations in a confined magnetized free-electron gas
The time-dependent pair correlation functions for a degenerate ideal quantum
gas of charged particles in a uniform magnetic field are studied on the basis
of equilibrium statistics. In particular, the influence of a flat hard wall on
the correlations is investigated, both for a perpendicular and a parallel
orientation of the wall with respect to the field. The coherent and incoherent
parts of the time-dependent structure function in position space are determined
from an expansion in terms of the eigenfunctions of the one-particle
Hamiltonian. For the bulk of the system, the intermediate scattering function
and the dynamical structure factor are derived by taking successive Fourier
transforms. In the vicinity of the wall the time-dependent coherent structure
function is found to decay faster than in the bulk. For coinciding positions
near the wall the form of the structure function turns out to be independent of
the orientation of the wall. Numerical results are shown to corroborate these
findings.Comment: 25 pages, 14 figures, to be published in Journal of Physics
Conserving approximations in time-dependent quantum transport: Initial correlations and memory effects
We study time-dependent quantum transport in a correlated model system by
means of time-propagation of the Kadanoff-Baym equations for the nonequilibrium
many-body Green function. We consider an initially contacted equilibrium system
of a correlated central region coupled to tight-binding leads. Subsequently a
time-dependent bias is switched on after which we follow in detail the
time-evolution of the system. Important features of the Kadanoff-Baym approach
are 1) the possibility of studying the ultrafast dynamics of transients and
other time-dependent regimes and 2) the inclusion of exchange and correlation
effects in a conserving approximation scheme. We find that initial correlation
and memory terms due to many-body interactions have a large effect on the
transient currents. Furthermore the value of the steady state current is found
to be strongly dependent on the approximation used to treat the electronic
interactions.Comment: 5 pages, 2 figure
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