906 research outputs found
Optimized Effective Potential Method in Current-Spin Density Functional Theory
Current-spin density functional theory (CSDFT) provides a framework to
describe interacting many-electron systems in a magnetic field which couples to
both spin- and orbital-degrees of freedom. Unlike in usual (spin-) density
functional theory, approximations to the exchange-correlation energy based on
the model of the uniform electron gas face problems in practical applications.
In this work, explicitly orbital-dependent functionals are used and a
generalization of the Optimized Effective Potential (OEP) method to the CSDFT
framework is presented. A simplifying approximation to the resulting integral
equations for the exchange-correlation potentials is suggested. A detailed
analysis of these equations is carried out for the case of open-shell atoms and
numerical results are given using the exact-exchange energy functional. For
zero external magnetic field, a small systematic lowering of the total energy
for current-carrying states is observed due to the inclusion of the current in
the Kohn-Sham scheme. For states without current, CSDFT results coincide with
those of spin density functional theory.Comment: 11 pages, 3 figure
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
Poly-MTO, {(CH_3)_{0.92} Re O_3}_\infty, a Conducting Two-Dimensional Organometallic Oxide
Polymeric methyltrioxorhenium, {(CH_{3})_{0.92}ReO_{3}}_{\infty} (poly-MTO),
is the first member of a new class of organometallic hybrids which adopts the
structural pattern and physical properties of classical perovskites in two
dimensions (2D). We demonstrate how the electronic structure of poly-MTO can be
tailored by intercalation of organic donor molecules, such as
tetrathiafulvalene (TTF) or bis-(ethylendithio)-tetrathiafulvalene (BEDT-TTF),
and by the inorganic acceptor SbF. Integration of donor molecules leads to
a more insulating behavior of poly-MTO, whereas SbF insertion does not
cause any significant change in the resistivity. The resistivity data of pure
poly-MTO is remarkably well described by a two-dimensional electron system.
Below 38 K an unusual resistivity behavior, similar to that found in doped
cuprates, is observed: The resistivity initially increases approximately as
ln) before it changes into a dependence below 2 K.
As an explanation we suggest a crossover from purely two-dimensional
charge-carrier diffusion within the \{ReO\} planes at high
temperatures to three-dimensional diffusion at low temperatures in a
disorder-enhanced electron-electron interaction scenario (Altshuler-Aronov
correction). Furthermore, a linear positive magnetoresistance was found in the
insulating regime, which is caused by spatial localization of itinerant
electrons at some of the Re atoms, which formally adopt a electronic
configuration. X-ray diffraction, IR- and ESR-studies, temperature dependent
magnetization and specific heat measurements in various magnetic fields suggest
that the electronic structure of poly-MTO can safely be approximated by a
purely 2D conductor.Comment: 15 pages, 16 figures, 2 table
CLASS B0827+525: `Dark lens' or binary radio-loud quasar?
We present radio, optical, near-infrared and spectroscopic observations of
the source B0827+525. We consider this source as the best candidate from the
Cosmic Lens All-Sky Survey (CLASS) for a `dark lens' system or binary
radio-loud quasar. The system consists of two radio components with somewhat
different spectral indices, separated by 2.815 arcsec. VLBA observations show
that each component has substructure on a scale of a few mas. A deep K-band
exposure with the W.M.Keck-II Telescope reveals emission near both radio
components. The K-band emission of the weaker radio component appears extended,
whereas the emission from the brighter radio component is consistent with a
point source. Hubble Space Telescope F160W-band observations with the NICMOS
instrument confirms this. A redshift of 2.064 is found for the brighter
component, using the LRIS instrument on the W.M.Keck-II Telescope. The
probability that B0827+525 consists of two unrelated compact flat-spectrum
radio sources is ~3%, although the presence of similar substructure in both
component might reduce this.
We discuss two scenarios to explain this system: (i) CLASS B0827+525 is a
`dark lens' system or (ii) B0827+525 is a binary radio-loud quasar. B0827+525
has met all criteria that thus far have in 100% of the cases confirmed a source
as an indisputable gravitational lens system. Despite this, no lens galaxy has
been detected with m_F160W<=23 mag. Hence, we might have found the first binary
radio-loud quasar. At this moment, however, we feel that the `dark lens'
hypothesis cannot yet be fully excluded.Comment: 9 pages, 6 figures; Accepted for publication in Astronomy &
Astrophysics; Full-res. images 1 and 3 can be obtained from L.V.E.
Measuring Cosmological Parameters with the JVAS and CLASS Gravitational Lens Surveys
The JVAS (Jodrell Bank-VLA Astrometric Survey) and CLASS (Cosmic Lens All-Sky
Survey) are well-defined surveys containing about ten thousand flat-spectrum
radio sources. For many reasons, flat-spectrum radio sources are particularly
well-suited as a population from which one can obtain unbiased samples of
gravitational lenses. These are by far the largest gravitational (macro)lens
surveys, and particular attention was paid to constructing a cleanly-defined
sample for the survey itself and for the underlying luminosity function. Here
we present the constraints on cosmological parameters, particularly the
cosmological constant, derived from JVAS and combine them with constraints from
optical gravitational lens surveys, `direct' measurements of ,
and the age of the universe, and constraints derived from CMB
anisotropies, before putting this final result into the context of the latest
results from other, independent cosmological tests.Comment: LaTeX, 9 pages, 6 PostScript figures, uses texas.sty. To appear in
the Proceedings of the 19th Texas Symposium on Relativistic Astrophysics and
Cosmology (CD-ROM). Paper version available on request. Actual poster (A0 and
A4 versions) available from
http://multivac.jb.man.ac.uk:8000/helbig/research/publications/info/
texas98.htm
A New Quadruple Gravitational Lens System: CLASS B0128+437
High resolution MERLIN observations of a newly-discovered four-image
gravitational lens system, B0128+437, are presented. The system was found after
a careful re-analysis of the entire CLASS dataset. The MERLIN observations
resolve four components in a characteristic quadruple-image configuration; the
maximum image separation is 542 mas and the total flux density is 48 mJy at 5
GHz. A best-fit lens model with a singular isothermal ellipsoid results in
large errors in the image positions. A significantly improved fit is obtained
after the addition of a shear component, suggesting that the lensing system is
more complex and may consist of multiple deflectors. The integrated radio
spectrum of the background source indicates that it is a GigaHertz-Peaked
Spectrum (GPS) source. It may therefore be possible to resolve structure within
the radio images with deep VLBI observations and thus better constrain the
lensing mass distribution.Comment: Accepted for publication in MNRAS. 4 pages, 3 included PostScript
figure
Potential-density pairs for axisymmetric galaxies: the influence of scalar fields
We present a formulation for potential-density pairs to describe axisymmetric
galaxies in the Newtonian limit of scalar-tensor theories of gravity. The
scalar field is described by a modified Helmholtz equation with a source that
is coupled to the standard Poisson equation of Newtonian gravity. The net
gravitational force is given by two contributions: the standard Newtonian
potential plus a term stemming from massive scalar fields. General solutions
have been found for axisymmetric systems and the multipole expansion of the
Yukawa potential is given. In particular, we have computed potential-density
pairs of galactic disks for an exponential profile and their rotation curves.Comment: 8 pages, no figures, corrected version to the one that will appear in
Gen. Relativ. Gravit., where a small typo in eq. (13) is presen
(Micro)evolutionary changes and the evolutionary potential of bird migration
Seasonal migration is the yearly long-distance movement of individuals between their breeding and wintering grounds. Individuals from nearly every animal group exhibit this behavior, but probably the most iconic migration is carried out by birds, from the classic V-shape formation of geese on migration to the amazing nonstop long-distance flights undertaken by Arctic Terns Sterna paradisaea. In this chapter, we discuss how seasonal migration has shaped the field of evolution. First, this behavior is known to turn on and off quite rapidly, but controversy remains concerning where this behavior first evolved geographically and whether the ancestral state was sedentary or migratory (Fig. 7.1d, e). We review recent work using new analytical techniques to provide insight into this topic. Second, it is widely accepted that there is a large genetic basis to this trait, especially in groups like songbirds that migrate alone and at night precluding any opportunity for learning. Key hypotheses on this topic include shared genetic variation used by different populations to migrate and only few genes being involved in its control. We summarize recent work using new techniques for both phenotype and genotype characterization to evaluate and challenge these hypotheses. Finally, one topic that has received less attention is the role these differences in migratory phenotype could play in the process of speciation. Specifically, many populations breed next to one another but take drastically different routes on migration (Fig. 7.2). This difference could play an important role in reducing gene flow between populations, but our inability to track most birds on migration has so far precluded evaluations of this hypothesis. The advent of new tracking techniques means we can track many more birds with increasing accuracy on migration, and this work has provided important insight into migration's role in speciation that we will review here
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