14,451 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
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
Mechanical Design of the MID Split-and-Delay Line at the European XFEL
A new split-and-delay line (SDL) is under development for the Materials Imaging and Dynamics (MID) end station at the European XFEL.* The device utilises Bragg reflection to provide pairs of X-ray pulses with an energy of (5 - 10) keV and a continuously tunable time delay of (-10 - 800) ps - thus allowing zero-crossing of the time delay. The mechanical concept features separate positioning stages for each optical element. Those are based on a serial combination of coarse motion axes and a fine alignment 6 DoF Cartesian parallel kinematics**. That allows to meet the contradictory demands of a fast long-range travel of up to 1000 mm and in the same time a precise alignment with a resolution in the nanometer range. Multiple laser interferometers monitor the position of the optical elements and allow an active control of their alignment. All optical elements and mechanics will be installed inside an UHV chamber, including the interferometer and about 100 stepper motors. With this paper we present the mechanical design for the SDL. It will additionally show the design of a prototype of a positioning stage which allows extensive testing of the implemented concepts and techniques
Neutrino fluence after r-process freeze-out and abundances of Te isotopes in presolar diamonds
Using the data of Richter et al. (1998) on Te isotopes in diamond grains from
a meteorite, we derive bounds on the neutrino fluence and the decay timescale
of the neutrino flux relevant for the supernova r-process. Our new bound on the
neutrino fluence F after freeze-out of the r-process peak at mass number A =
130 is more stringent than the previous bound F < 0.045 (in units of 10**37
erg/cm**2) of Qian et al. (1997) and Haxton et al. (1997) if the neutrino flux
decays on a timescale tau > 0.65 s. In particular, it requires that a fluence
of F = 0.031 be provided by a neutrino flux with tau < 0.84 s. Such a fluence
may be responsible for the production of the solar r-process abundances at A =
124-126 (Qian et al. 1997; Haxton et al. 1997). Our results are based on the
assumption that only the stable nuclei implanted into the diamonds are retained
while the radioactive ones are lost from the diamonds upon decay after
implantation (Ott 1996). We consider that the nanodiamonds are condensed in an
environment with C/O > 1 in the expanding supernova debris or from the exterior
H envelope. The implantation of nuclei would have occurred 10**4-10**6 s after
r-process freeze-out. This time interval may be marginally sufficient to permit
adequate cooling upon expansion for the formation of diamond grains. The
mechanisms of preferential retention/loss of the implanted nuclei are not well
understood.Comment: AASTeX, 11 pages, 3 Postscript figure
Galactic Archaeology with CoRoT and APOGEE: Creating mock observations from a chemodynamical model
In a companion paper, we have presented the combined
asteroseismic-spectroscopic dataset obtained from CoRoT lightcurves and APOGEE
infra-red spectra for 678 solar-like oscillating red giants in two fields of
the Galactic disc (CoRoGEE). We have measured chemical abundance patterns,
distances, and ages of these field stars which are spread over a large radial
range of the Milky Way's disc. Here we show how to simulate this dataset using
a chemodynamical Galaxy model. We also demonstrate how the observation
procedure influences the accuracy of our estimated ages.Comment: 5 pages, 6 figures. To appear in Astronomische Nachrichten, special
issue "Reconstruction the Milky Way's History: Spectroscopic surveys,
Asteroseismology and Chemo-dynamical models", Guest Editors C. Chiappini, J.
Montalb\'an, and M. Steffe
On X-ray-singularities in the f-electron spectral function of the Falicov-Kimball model
The f-electron spectral function of the Falicov-Kimball model is calculated
within the dynamical mean-field theory using the numerical renormalization
group method as the impurity solver. Both the Bethe lattice and the hypercubic
lattice are considered at half filling. For small U we obtain a single-peaked
f-electron spectral function, which --for zero temperature-- exhibits an
algebraic (X-ray) singularity () for . The
characteristic exponent depends on the Coulomb (Hubbard) correlation
U. This X-ray singularity cannot be observed when using alternative
(Keldysh-based) many-body approaches. With increasing U, decreases and
vanishes for sufficiently large U when the f-electron spectral function
develops a gap and a two-peak structure (metal-insulator transition).Comment: 8 pages, 8 figures, revte
Stellar Populations and Star Cluster Formation in Interacting Galaxies with the Advanced Camera for Surveys
Pixel-by-pixel colour-magnitude and colour-colour diagrams - based on a
subset of the Hubble Space Telescope Advanced Camera for Surveys Early Release
Observations - provide a powerful technique to explore and deduce the star and
star cluster formation histories of the Mice and the Tadpole interacting
galaxies. In each interacting system we find some 40 bright young star clusters
(20 <= F606W (mag) <= 25, with a characteristic mass of ~3 x 10^6 Msun), which
are spatially coincident with blue regions of active star formation in their
tidal tails and spiral arms. We estimate that the main events triggering the
formation of these clusters occurred ~(1.5-2.0) x 10^8 yr ago. We show that
star cluster formation is a major mode of star formation in galaxy
interactions, with >= 35% of the active star formation in encounters occurring
in star clusters. This is the first time that young star clusters have been
detected along the tidal tails in interacting galaxies. The tidal tail of the
Tadpole system is dominated by blue star forming regions, which occupy some 60%
of the total area covered by the tail and contribute ~70% of the total flux in
the F475W filter (decreasing to ~40% in F814W). The remaining pixels in the
tail have colours consistent with those of the main disk. The tidally triggered
burst of star formation in the Mice is of similar strength in both interacting
galaxies, but it has affected only relatively small, spatially coherent areas.Comment: 23 pages in preprint form, 6 (encapsulated) postscript figures;
accepted for publication in New Astronomy; ALL figures (even the grey-scale
ones) need to be printed on a colour printer style files included; for
full-resolution paper, see http://www.ast.cam.ac.uk/STELLARPOPS/ACSpaper
Isotope shifts of the (3s3p)P - (3s4s)S Mg I transitions
We report measurements of the isotope shifts of the (3s3p)P -
(3s4s)S Mg I transitions for the stable isotopes Mg (I=0),
Mg (I=5/2) and Mg (I=0). Furthermore the Mg S
hyperfine coefficient A(S) = (-321.6 1.5) MHz is extracted and
found to be in excellent agreement with state-of-the-art theoretical
predictions giving A(S) = -325 MHz and B(S)
MHz. Compared to previous measurements, the data presented in this work is
improved up to a factor of ten.Comment: 4 pages, 4 figures submitted to PR
Recommended from our members
Tau and atrophy: domain-specific relationships with cognition.
BackgroundLate-onset Alzheimer's disease (AD) is characterized by primary memory impairment, which then progresses towards severe deficits across cognitive domains. Here, we report how performance in cognitive domains relates to patterns of tau deposition and cortical thickness.MethodsWe analyzed data from 131 amyloid-β positive participants (55 cognitively normal, 46 mild cognitive impairment, 30 AD) of the Alzheimer's Disease Neuroimaging Initiative who underwent magnetic resonance imaging (MRI), flortaucipir (FTP) positron emission tomography, and neuropsychological testing. Surface-based vertex-wise and region-of-interest analyses were conducted between FTP and cognitive test scores, and between cortical thickness and cognitive test scores.ResultsFTP and thickness were differentially related to cognitive performance in several domains. FTP-cognition associations were more widespread than thickness-cognition associations. Further, FTP-cognition patterns reflected cortical systems that underlie different aspects of cognition.ConclusionsOur findings indicate that AD-related decline in domain-specific cognitive performance reflects underlying progression of tau and atrophy into associated brain circuits. They also suggest that tau-PET may have better sensitivity to this decline than MRI-derived measures of cortical thickness
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