1,121 research outputs found
Quantum Corrections to Fidelity Decay in Chaotic Systems
By considering correlations between classical orbits we derive semiclassical
expressions for the decay of the quantum fidelity amplitude for classically
chaotic quantum systems, as well as for its squared modulus, the fidelity or
Loschmidt echo. Our semiclassical results for the fidelity amplitude agree with
random matrix theory (RMT) and supersymmetry predictions in the universal Fermi
golden rule regime. The calculated quantum corrections can be viewed as arising
from a static random perturbation acting on nearly self-retracing interfering
paths, and hence will be suppressed for time-varying perturbations. Moreover,
using trajectory-based methods we show a relation, recently obtained in RMT,
between the fidelity amplitude and the cross-form factor for parametric level
correlations. Beyond RMT, we compute Ehrenfest-time effects on the fidelity
amplitude. Furthermore our semiclassical approach allows for a unified
treatment of the fidelity, both in the Fermi golden rule and Lyapunov regimes,
demonstrating that quantum corrections are suppressed in the latter.Comment: 14 pages, 4 figure
Semiclassical approach to the ac-conductance of chaotic cavities
We address frequency-dependent quantum transport through mesoscopic
conductors in the semiclassical limit. By generalizing the trajectory-based
semiclassical theory of dc quantum transport to the ac case, we derive the
average screened conductance as well as ac weak-localization corrections for
chaotic conductors. Thereby we confirm respective random matrix results and
generalize them by accounting for Ehrenfest time effects. We consider the case
of a cavity connected through many leads to a macroscopic circuit which
contains ac-sources. In addition to the reservoir the cavity itself is
capacitively coupled to a gate. By incorporating tunnel barriers between cavity
and leads we obtain results for arbitrary tunnel rates. Finally, based on our
findings we investigate the effect of dephasing on the charge relaxation
resistance of a mesoscopic capacitor in the linear low-frequency regime
Co dimers on hexagonal carbon rings proposed as subnanometer magnetic storage bits
It is demonstrated by means of density functional and ab-initio quantum
chemical calculations, that transition metal - carbon systems have the
potential to enhance the presently achievable area density of magnetic
recording by three orders of magnitude. As a model system, Co_2-benzene with a
diameter of 0.5 nm is investigated. It shows a magnetic anisotropy in the order
of 0.1 eV per molecule, large enough to store permanently one bit of
information at temperatures considerably larger than 4 K. A similar performance
can be expected, if cobalt dimers are deposited on graphene or on graphite. It
is suggested that the subnanometer bits can be written by simultaneous
application of a moderate magnetic and a strong electric field.Comment: 13 pages, 4 figure
Bench-to-bedside review: β(2)-Agonists and the acute respiratory distress syndrome
The acute respiratory distress syndrome (ARDS) is a devastating constellation of clinical, radiological and pathological signs characterized by failure of gas exchange and refractory hypoxia. Despite nearly 30 years of research, no specific pharmacological therapy has yet proven to be efficacious in manipulating the pathophysiological processes that underlie this condition. Several in vitro and in vivo animal or human studies suggest a potential role for β(2)-agonists in the treatment of ARDS. These agents have been shown to reduce pulmonary neutrophil sequestration and activation, accelerate alveolar fluid clearance, enhance surfactant secretion, and modulate the inflammatory and coagulation cascades. They are also used widely in clinical practice and are well tolerated in critically ill patients. The present review examines the evidence supporting a role for β(2)-agonists as a specific pharmacological intervention in patients with ARDS
National Center for Biomedical Ontology: Advancing biomedicine through structured organization of scientific knowledge
The National Center for Biomedical Ontology is a consortium that comprises leading informaticians, biologists, clinicians, and ontologists, funded by the National Institutes of Health (NIH) Roadmap, to develop innovative technology and methods that allow scientists to record, manage, and disseminate biomedical information and knowledge in machine-processable form. The goals of the Center are (1) to help unify the divergent and isolated efforts in ontology development by promoting high quality open-source, standards-based tools to create, manage, and use ontologies, (2) to create new software tools so that scientists can use ontologies to annotate and analyze biomedical data, (3) to provide a national resource for the ongoing evaluation, integration, and evolution of biomedical ontologies and associated
tools and theories in the context of driving biomedical projects (DBPs), and (4) to disseminate the tools and resources of the Center and to identify, evaluate, and communicate best practices of ontology development to the biomedical community. Through the research activities within the Center, collaborations with the DBPs, and interactions with the biomedical community, our goal is to help scientists to work more effectively in the e-science paradigm, enhancing experiment design, experiment execution, data analysis, information synthesis, hypothesis generation and testing, and understand human disease
The semiclassical origin of curvature effects in universal spectral statistics
We consider the energy averaged two-point correlator of spectral determinants
and calculate contributions beyond the diagonal approximation using
semiclassical methods. Evaluating the contributions originating from
pseudo-orbit correlations in the same way as in [S. Heusler {\textit {et al.}}\
2007 Phys. Rev. Lett. {\textbf{98}}, 044103] we find a discrepancy between the
semiclassical and the random matrix theory result. A complementary analysis
based on a field-theoretical approach shows that the additional terms occurring
in semiclassics are cancelled in field theory by so-called curvature effects.
We give the semiclassical interpretation of the curvature effects in terms of
contributions from multiple transversals of periodic orbits around shorter
periodic orbits and discuss the consistency of our results with previous
approaches
Exposure to an enriched CO 2 atmosphere alters carbon assimilation and allocation in a pine forest ecosystem
We linked a leaf-level CO 2 assimilation model with a model that accounts for light attenuation in the canopy and measurements of sap-flux-based canopy conductance into a new canopy conductance-constrained carbon assimilation (4C-A) model. We estimated canopy CO 2 uptake ( A nC ) at the Duke Forest free-air CO 2 enrichment (FACE) study. Rates of A nC estimated from the 4C-A model agreed well with leaf gas exchange measurements ( A net ) in both CO 2 treatments. Under ambient conditions, monthly sums of net CO 2 uptake by the canopy ( A nC ) were 13% higher than estimates based on eddy-covariance and chamber measurements. Annual estimates of A nC were only 3% higher than carbon (C) accumulations and losses estimated from ground-based measurements for the entire stand. The C budget for the Pinus taeda component was well constrained (within 1% of ground-based measurements). Although the closure of the C budget for the broadleaf species was poorer (within 20%), these species are a minor component of the forest. Under elevated CO 2 , the C used annually for growth, turnover, and respiration balanced only 80% of the A nC . Of the extra 700 g C m −2  a −1 (1999 and 2000 average), 86% is attributable to surface soil CO 2 efflux. This suggests that the production and turnover of fine roots was underestimated or that mycorrhizae and rhizodeposition became an increasingly important component of the C balance. Under elevated CO 2 , net ecosystem production increased by 272 g C m −2  a −1 : 44% greater than under ambient CO 2 . The majority (87%) of this C was sequestered in a moderately long-term C pool in wood, with the remainder in the forest floor–soil subsystem.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/73982/1/j.1365-2486.2003.00662.x.pd
TIGRFAMs and Genome Properties: tools for the assignment of molecular function and biological process in prokaryotic genomes
TIGRFAMs is a collection of protein family definitions built to aid in high-throughput annotation of specific protein functions. Each family is based on a hidden Markov model (HMM), where both cutoff scores and membership in the seed alignment are chosen so that the HMMs can classify numerous proteins according to their specific molecular functions. Most TIGRFAMs models describe ‘equivalog’ families, where both orthology and lateral gene transfer may be part of the evolutionary history, but where a single molecular function has been conserved. The Genome Properties system contains a queriable set of metabolic reconstructions, genome metrics and extractions of information from the scientific literature. Its genome-by-genome assertions of whether or not specific structures, pathways or systems are present provide high-level conceptual descriptions of genomic content. These assertions enable comparative genomics, provide a meaningful biological context to aid in manual annotation, support assignments of Gene Ontology (GO) biological process terms and help validate HMM-based predictions of protein function. The Genome Properties system is particularly useful as a generator of phylogenetic profiles, through which new protein family functions may be discovered. The TIGRFAMs and Genome Properties systems can be accessed at and
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