6,561 research outputs found
Real-time dynamics in Quantum Impurity Systems: A Time-dependent Numerical Renormalization Group Approach
We develop a general approach to the nonequilibrium dynamics of quantum
impurity systems for arbitrary coupling strength. The numerical renormalization
group is used to generate a complete basis set necessary for the correct
description of the time evolution. We benchmark our method with the exact
analytical solution for the resonant-level model. As a first application, we
investigate the equilibration of a quantum dot subject to a sudden change of
the gate voltage and external magnetic field. Two distinct relaxation times are
identified for the spin and charge dynamics.Comment: 5 pages, 5 figure
The temperature dependent bandstructure of a ferromagnetic semiconductor film
The electronic quasiparticle spectrum of a ferromagnetic film is investigated
within the framework of the s-f model. Starting from the exact solvable case of
a single electron in an otherwise empty conduction band being exchange coupled
to a ferromagnetically saturated localized spin system we extend the theory to
finite temperatures. Our approach is a moment-conserving decoupling procedure
for suitable defined Green functions. The theory for finite temperatures
evolves continuously from the exact limiting case. The restriction to zero
conduction band occupation may be regarded as a proper model description for
ferromagnetic semiconductors like EuO and EuS. Evaluating the theory for a
simple cubic film cut parallel to the (100) crystal plane, we find some marked
correlation effects which depend on the spin of the test electron, on the
exchange coupling, and on the temperature of the local-moment system.Comment: 11 pages, 9 figure
Kondo-lattice model: Application to the temperature-dependent electronic structure of EuO(100) films
We present calculations for the temperature-dependent electronic structure
and magnetic properties of thin ferromagnetic EuO films. The treatment is based
on a combination of a multiband-Kondo lattice model with first-principles
TB-LMTO band structure calculations. The method avoids the problem of
double-counting of relevant interactions and takes into account the correct
symmetry of the atomic orbitals. We discuss the temperature-dependent
electronic structures of EuO(100) films in terms of quasiparticle densities of
states and quasiparticle band structures. The Curie temperature T_C of the EuO
films turns out to be strongly thickness-dependent, starting from a very low
value = 15K for the monolayer and reaching the bulk value at about 25 layers
Studies on the Helminth Fauna of Alaska. XXVII. The Occurrence of Larvae of \u3ci\u3eTrichinella spiralis\u3c/i\u3e in Alaskan Mammals
In 1948 much interest in trichinosis in arctic regions was aroused, particularly by the findings of Thorborg et al. (1948), who investigated serious outbreaks occurring among the Eskimo of West Greenland during 1947. Consequently, with the founding of the Arctic Health Research Center in the autumn of 1948, a study of trichinosis in Alaska was the first project to be initiated by the Zoonotic Disease Section (formerly Animal-borne Disease Section) of this Center.
Field work was begun in January, 1949, and a preliminary note on trichinosis in Alaskan mammals was published by Brandly and Rausch (1950). The subject of trichinosis in arctic regions was reviewed by Connell (1949). The survey to determine the prevalence of T. spiralis in mammals in Alaska was terminated in the spring of 1953; this paper reports the results of this work
Functional consequences of sphingomyelinase-induced changes in erythrocyte membrane structure.
Inflammation enhances the secretion of sphingomyelinases (SMases). SMases catalyze the hydrolysis of sphingomyelin into phosphocholine and ceramide. In erythrocytes, ceramide formation leads to exposure of the removal signal phosphatidylserine (PS), creating a potential link between SMase activity and anemia of inflammation. Therefore, we studied the effects of SMase on various pathophysiologically relevant parameters of erythrocyte homeostasis. Time-lapse confocal microscopy revealed a SMase-induced transition from the discoid to a spherical shape, followed by PS exposure, and finally loss of cytoplasmic content. Also, SMase treatment resulted in ceramide-associated alterations in membrane-cytoskeleton interactions and membrane organization, including microdomain formation. Furthermore, we observed increases in membrane fragility, vesiculation and invagination, and large protein clusters. These changes were associated with enhanced erythrocyte retention in a spleen-mimicking model. Erythrocyte storage under blood bank conditions and during physiological aging increased the sensitivity to SMase. A low SMase activity already induced morphological and structural changes, demonstrating the potential of SMase to disturb erythrocyte homeostasis. Our analyses provide a comprehensive picture in which ceramide-induced changes in membrane microdomain organization disrupt the membrane-cytoskeleton interaction and membrane integrity, leading to vesiculation, reduced deformability, and finally loss of erythrocyte content. Understanding these processes is highly relevant for understanding anemia during chronic inflammation, especially in critically ill patients receiving blood transfusions
Improvement for Quenched Wilson Fermions
We briefly describe some of our recent results for the mass spectrum and
matrix elements using improved fermions for quenched QCD. Where possible
a comparison is made between improved and Wilson fermions.Comment: 6 pages, Latex, 11 figures, epsf.sty and buckow1.sty needed
(buckow1.sty included). Talk presented at the 31st Ahrenshoop Symposium on
the Theory of Elementary Particles, September 1997, Buckow, German
Numerical Renormalization Group for Impurity Quantum Phase Transitions: Structure of Critical Fixed Points
The numerical renormalization group method is used to investigate zero
temperature phase transitions in quantum impurity systems, in particular in the
particle-hole symmetric soft-gap Anderson model. The model displays two stable
phases whose fixed points can be built up of non-interacting single-particle
states. In contrast, the quantum phase transitions turn out to be described by
interacting fixed points, and their excitations cannot be described in terms of
free particles. We show that the structure of the many-body spectrum of these
critical fixed points can be understood using renormalized perturbation theory
close to certain values of the bath exponents which play the role of critical
dimensions. Contact is made with perturbative renormalization group
calculations for the soft-gap Anderson and Kondo models. A complete description
of the quantum critical many-particle spectra is achieved using suitable
marginal operators; technically this can be understood as epsilon-expansion for
full many-body spectra.Comment: 14 pages, 12 figure
Nucleon structure in terms of OPE with non-perturbative Wilson coefficients
Lattice calculations could boost our understanding of Deep Inelastic
Scattering by evaluating moments of the Nucleon Structure Functions. To this
end we study the product of electromagnetic currents between quark states. The
Operator Product Expansion (OPE) decomposes it into matrix elements of local
operators (depending on the quark momenta) and Wilson coefficients (as
functions of the larger photon momenta). For consistency with the matrix
elements, we evaluate a set of Wilson coefficients non-perturbatively, based on
propagators for numerous momentum sources, on a 24^3 x 48 lattice. The use of
overlap quarks suppresses unwanted operator mixing and lattice artifacts.
Results for the leading Wilson coefficients are extracted by means of Singular
Value Decomposition.Comment: 7 pages, 3 figures, contribution to the XXVI International Symposium
on Lattice Field Theory, July 14-19 Williamsburg, Virginia, US
Quantum limit in resonant vacuum tunneling transducers
We propose an electromechanical transducer based on a resonant-tunneling
configuration that, with respect to the standard tunneling transducers, allows
larger tunneling currents while using the same bias voltage. The increased
current leads to an increase of the shot noise and an increase of the momentum
noise which determine the quantum limit in the system under monitoring.
Experiments with micromachined masses at 4.2 K could show dominance of the
momentum noise over the Brownian noise, allowing observation of the
quantum-mechanical noise at the mesoscopic scale
Observation of twin beam correlations and quadrature entanglement by frequency doubling in a two-port resonator
We demonstrate production of quantum correlated and entangled beams by second
harmonic generation in a nonlinear resonator with two output ports. The output
beams at wavelength 428.5 nm exhibit 0.9 dB of nonclassical intensity
correlations and 0.3 dB of entanglement.Comment: 5 pages, 7 figure
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