14,609 research outputs found
An Enhanced Perturbational Study on Spectral Properties of the Anderson Model
The infinite- single impurity Anderson model for rare earth alloys is
examined with a new set of self-consistent coupled integral equations, which
can be embedded in the large expansion scheme ( is the local spin
degeneracy). The finite temperature impurity density of states (DOS) and the
spin-fluctuation spectra are calculated exactly up to the order . The
presented conserving approximation goes well beyond the -approximation
({\em NCA}) and maintains local Fermi-liquid properties down to very low
temperatures. The position of the low lying Abrikosov-Suhl resonance (ASR) in
the impurity DOS is in accordance with Friedel's sum rule. For its shift
toward the chemical potential, compared to the {\em NCA}, can be traced back to
the influence of the vertex corrections. The width and height of the ASR is
governed by the universal low temperature energy scale . Temperature and
degeneracy -dependence of the static magnetic susceptibility is found in
excellent agreement with the Bethe-Ansatz results. Threshold exponents of the
local propagators are discussed. Resonant level regime () and intermediate
valence regime () of the model are thoroughly
investigated as a critical test of the quality of the approximation. Some
applications to the Anderson lattice model are pointed out.Comment: 19 pages, ReVTeX, no figures. 17 Postscript figures available on the
WWW at http://spy.fkp.physik.th-darmstadt.de/~frithjof
Where is the fuzz? Undetected Lyman alpha nebulae around QSOs at z~2.3
We observed a small sample of 5 radio-quiet QSOs with integral field
spectroscopy to search for possible extended emission in the Ly line.
We subtracted the QSO point sources using a simple PSF self-calibration
technique that takes advantage of the simultaneous availability of spatial and
spectral information. In 4 of the 5 objects we find no significant traces of
extended Ly emission beyond the contribution of the QSO nuclei itself,
while in UM 247 there is evidence for a weak and spatially quite compact excess
in the Ly line at several kpc outside the nucleus. For all objects in
our sample we estimated detection limits for extended, smoothly distributed
Ly emission by adding fake nebulosities into the datacubes and trying
to recover them after PSF subtraction. Our observations are consistent with
other studies showing that giant Ly nebulae such as those found
recently around some quasars are very rare. Ly fuzz around typical
radio-quiet QSOs is fainter, less extended and is therefore much harder to
detect. The faintness of these structures is consistent with the idea that
radio-quiet QSOs typically reside in dark matter haloes of modest masses.Comment: 12 Pages, Accepted for publication in A&
Influence of Correlated Hybridization on the Conductance of Molecular Transistors
We study the spin-1/2 single-channel Anderson impurity model with correlated
(occupancy dependent) hybridization for molecular transistors using the
numerical renormalization-group method. Correlated hybridization can induce
nonuniversal deviations in the normalized zero-bias conductance and, for some
parameters, modestly enhance the spin polarization of currents in applied
magnetic field. Correlated hybridization can also explain a gate-voltage
dependence to the Kondo scale similar to what has been observed in recent
experiments.Comment: 4 pages, 5 figure
A Numerical Renormalization Group approach to Green's Functions for Quantum Impurity Models
We present a novel technique for the calculation of dynamical correlation
functions of quantum impurity systems in equilibrium with Wilson's numerical
renormalization group. Our formulation is based on a complete basis set of the
Wilson chain. In contrast to all previous methods, it does not suffer from
overcounting of excitation. By construction, it always fulfills sum rules for
spectral functions. Furthermore, it accurately reproduces local thermodynamic
expectation values, such as occupancy and magnetization, obtained directly from
the numerical renormalization group calculations.Comment: 13 pages, 7 figur
Spinful bosons in an optical lattice
We analyze the behavior of cold spin-1 particles with antiferromagnetic
interactions in a one-dimensional optical lattice using density matrix
renormalization group calculations. Correlation functions and the dimerization
are shown and we also present results for the energy gap between ground state
and the spin excited states. We confirm the anticipated phase diagram, with
Mott-insulating regions of alternating dimerized S=1 chains for odd particle
density versus on-site singlets for even density. We find no evidence for any
additional ordered phases in the physically accessible region, however for
sufficiently large spin interaction, on-site singlet pairs dominate leading,
for odd density, to a breakdown of the Mott insulator or, for even density, a
real-space singlet superfluid.Comment: Minor revisions and clarification
Kondo effect by controlled cleavage of a single molecule contact
Conductance measurements of a molecular wire, contacted between an epitaxial
molecule-metal bond and the tip of a scanning tunneling microscope, are
reported. Controlled retraction of the tip gradually de-hybridizes the molecule
from the metal substrate. This tunes the wire into the Kondo regime in which
the renormalized molecular transport orbital serves as spin impurity at half
filling and the Kondo resonance opens up an additional transport channel.
Numerical renormalization group simulations suggest this type of behavior to be
generic for a common class of metal molecule bonds. The results demonstrate a
new approach to single-molecule experiments with atomic-scale contact control
and prepare the way for the ab initio simulation of many-body transport through
single-molecule junctions.Comment: Main text: 41 pages including references and captions, 9 figures.
Supplementary information: 5 pages including 2 figures New experimental and
theoretical data supporting initial claims are added. The paper has been
reworked from the letter format into a longer versio
The young star cluster system of the Antennae galaxies
āThe original publication is available at www.springerlink.comā. Copyright Springer. DOI: 10.1007/s10509-009-0103-xThe study of young star cluster (YSC) systems, preferentially in starburst and merging galaxies, has seen great interest in the recent past, as it provides important input to models of star formation. However, even some basic properties (such as the luminosity function; LF) of YSC systems are still being debated. Here, we study the photometric properties of the YSC system in the nearest major merger system, the Antennae galaxies. We find evidence for the existence of a statistically significant turnover in the LF.Peer reviewe
Breast Cancer in Young Women - a Clinical Challenge to Be Addressed in a Multidisciplinary Setting
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