9,214 research outputs found
A New Facility for the Experimental Investigation on Nano Heat Transfer between Gas Molecules and Ceramic Surfaces
Since the last decade, the interest has risen in nanoscaled technological products, which have advantages through their size effect. The size effect also plays a significant role in the area of micro- and nanoscale heat transfers. Many applications were developed using this effect, such as nanostructured porous media, e.g. Aerogels or ceramics. This experimental work is focused on the determination of thermal accommodation coefficients (TAC) on ceramic surfaces considering several influencing factors. TAC is influenced by temperature, kind of gas, kind of wall material, roughness, and contamination with adsorbed gas layers. To determine TAC, a parallel plate apparatus is used, although most of the measurements in the past have been arranged with the hot wire method. Using the parallel plates apparatus is advantageous due to its simpler machining of specimens and to analyze the effect of roughness. The apparatus is build similar to the well-known guarded hot-plate method using an unidirectional heat flux through the gas layer due to a small temperature gradient, which is provided by two different heat foils. The measurements take place between 30 and 100°C, 10−4, and 1 mbar
Bound excitons in time-dependent density-functional-theory: optical and energy-loss spectra
A robust and efficient frequency dependent and non-local exchange-correlation
is derived by imposing time-dependent density-functional
theory (TDDFT) to reproduce the many-body diagrammatic expansion of the
Bethe-Salpeter polarization function. As an illustration, we compute the
optical spectra of LiF, \sio and diamond and the finite momentum transfer
energy-loss spectrum of LiF. The TDDFT results reproduce extremely well the
excitonic effects embodied in the Bethe-Salpeter approach, both for strongly
bound and resonant excitons. We provide a working expression for that
is fast to evaluate and easy to implement.Comment: 4 pages, 2 figures. To appear in Phys. Rev. Let
Ecological study of aquatic midges and some related insects with special reference to feeding habits
Die Schweiz ist ein reiches Land. Sie verfügt über viele Millionäre. Der große Reichtum konzentriert sich auf wenige Familien und Personen. In der Schweiz leben aber auch eine halbe Million der Bevölkerung (7,5 Mio.) in Haushalten von Erwerbstätigen, die weniger als das Existenzminimum verdienen. Über 200‘000 Personen sind auf Sozialhilfe angewiesen. Bei den Vermögen und den verfügbaren Einkommen hat sich in den letzten Jahren die Kluft zwischen den obersten und untersten zehn Prozent verschärft. Die Zunahme der sozialen Ungleichheit erhöht die soziale Brisanz, was mehr zu ergründen ist. Die soziale Differenzierung dokumentiert Prozesse der Globalisierung. Sie reproduziert und spezifiziert alte soziale Ungleichheiten. Wichtig ist, dass die Soziale Arbeit das thematisiert und weiter theoretisiert
A time-dependent approach to electron pumping in open quantum systems
We propose a time-dependent approach to investigate the motion of electrons
in quantum pump device configurations. The occupied one-particle states are
propagated in real time and used to calculate the local electron density and
current. An advantage of the present computational scheme is that the same
computational effort is required to simulate monochromatic, polychromatic and
nonperiodic drivings. Furthermore, initial state dependence and history effects
are naturally accounted for. This approach can also be embedded in the
framework of time-dependent density functional theory to include
electron-electron interactions. In the special case of periodic drivings we
combine the Floquet theory with nonequilibrium Green's functions and obtain a
general expression for the pumped current in terms of inelastic transmission
probabilities. This latter result is used for benchmarking our propagation
scheme in the long-time limit. Finally, we discuss the limitations of
Floquet-based schemes and suggest our approach as a possible way to go beyond
them.Comment: 14 pages, 8 figure
Bootstrap approximation for the exchange-correlation kernel of time-dependent density functional theory
A new parameter-free approximation for the exchange-correlation kernel
of time-dependent density functional theory is proposed. This
kernel is expressed as an algorithm in which the exact Dyson equation for the
response as well as a further approximate condition are solved together
self-consistently leading to a simple parameter-free kernel. We apply this to
the calculation of optical spectra for various small bandgap (Ge, Si, GaAs,
AlN, TiO, SiC), large bandgap (C, LiF, Ar, Ne) and magnetic (NiO)
insulators. The calculated spectra are in very good agreement with experiment
for this diverse set of materials, highlighting the universal applicability of
the new kernel.Comment: 4 figures 5 page
Region of hadron-quark mixed phase in hybrid stars
Hadron--quark mixed phase is expected in a wide region of the inner structure
of hybrid stars. However, we show that the hadron--quark mixed phase should be
restricted to a narrower region to because of the charge screening effect. The
narrow region of the mixed phase seems to explain physical phenomena of neutron
stars such as the strong magnetic field and glitch phenomena, and it would give
a new cooling curve for the neutron star.Comment: to be published in Physical Review
Exchange-correlation orbital functionals in current-density-functional theory: Application to a quantum dot in magnetic fields
The description of interacting many-electron systems in external magnetic
fields is considered in the framework of the optimized effective potential
method extended to current-spin-density functional theory. As a case study, a
two-dimensional quantum dot in external magnetic fields is investigated.
Excellent agreement with quantum Monte Carlo results is obtained when
self-interaction corrected correlation energies from the standard local
spin-density approximation are added to exact-exchange results. Full
self-consistency within the complete current-spin-density-functional framework
is found to be of minor importance.Comment: 5 pages, 2 figures, submitted to PR
Time-dependent quantum transport: A practical scheme using density functional theory
We present a computationally tractable scheme of time-dependent transport
phenomena within open-boundary time-dependent density-functional-theory. Within
this approach all the response properties of a system are determined from the
time-propagation of the set of ``occupied'' Kohn-Sham orbitals under the
influence of the external bias. This central idea is combined with an
open-boundary description of the geometry of the system that is divided into
three regions: left/right leads and the device region (``real simulation
region''). We have derived a general scheme to extract the set of initial
states in the device region that will be propagated in time with proper
transparent boundary-condition at the device/lead interface. This is possible
due to a new modified Crank-Nicholson algorithm that allows an efficient
time-propagation of open quantum systems. We illustrate the method in
one-dimensional model systems as a first step towards a full first-principles
implementation. In particular we show how a stationary current develops in the
system independent of the transient-current history upon application of the
bias. The present work is ideally suited to study ac transport and
photon-induced charge-injection. Although the implementation has been done
assuming clamped ions, we discuss how it can be extended to include dissipation
due to electron-phonon coupling through the combined simulation of the
electron-ion dynamics as well as electron-electron correlations.Comment: 14 pages, 9 figures, one of which consist of two separate file
Ab-initio angle and energy resolved photoelectron spectroscopy with time-dependent density-functional theory
We present a time-dependent density-functional method able to describe the
photoelectron spectrum of atoms and molecules when excited by laser pulses.
This computationally feasible scheme is based on a geometrical partitioning
that efficiently gives access to photoelectron spectroscopy in time-dependent
density-functional calculations. By using a geometrical approach, we provide a
simple description of momentum-resolved photoe- mission including multi-photon
effects. The approach is validated by comparison with results in the literature
and exact calculations. Furthermore, we present numerical photoelectron angular
distributions for randomly oriented nitrogen molecules in a short near infrared
intense laser pulse and helium-(I) angular spectra for aligned carbon monoxide
and benzene.Comment: Accepted for publication on Phys. Rev.
Shot Noise in Digital Holography
We discuss on noise in heterodyne holography in an off-axis configuration. We
show that, for a weak signal, the noise is dominated by the shot noise on the
reference beam. This noise corresponds to an equivalent noise on the signal
beam of one photoelectron per pixel, for the whole sequence of images used to
build the digital hologram
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