2,596 research outputs found
Dynamics of a large-spin-boson system in the strong coupling regime
We investigate collective effects of an ensemble of biased two level systems
interacting with a bosonic bath in the strong coupling regime. The two level
systems are described by a large pseudo-spin J. An equation for the expectation
value M(t) of the z-component of the pseudo spin is derived and solved
numerically for an ohmic bath at T=0. In case of a large cut-off frequency of
the spectral function, a Markov approximation is justified and an analytical
solution is presented. We find that M(t) relaxes towards a highly correlated
state with maximum value for large times. However, this relaxation is
extremely slow for most parameter values so as if the system was "frozen in" by
interaction with the bosonic bath.Comment: 4 pages, 2 figures, to be published in proceedings of MB1
The development of precipitation-hardened chromium-base alloys Final report
Precipitation with refractory metal carbides for creep resistant chromium-base alloy
Nonequilibrium Quantum Phase Transitions in the Dicke Model
We establish a set of nonequilibrium quantum phase transitions in the Dicke
model by considering a monochromatic nonadiabatic modulation of the atom-field
coupling. For weak driving the system exhibits a set of sidebands which allow
the circumvention of the no-go theorem which otherwise forbids the occurence of
superradiant phase transitions. At strong driving we show that the system
exhibits a rich multistable structure and exhibits both first- and second-order
nonequilibrium quantum phase transitions.Comment: 4 pages, 3 Figures, and supplementary material. This new version
contains corrected typos, new references and new versions of the figures.
Published by Physical Review Letter
Atrial high-rate episodes: prevalence, stroke risk, implications for management, and clinical gaps in evidence
Self-terminating atrial arrhythmias are commonly detected on continuous rhythm monitoring, e.g. by pacemakers or defibrillators. It is unclear whether the presence of these arrhythmias has therapeutic consequences. We sought to summarize evidence on the prevalence of atrial high-rate episodes (AHREs) and their impact on risk of stroke. We performed a comprehensive, tabulated review of published literature on the prevalence of AHRE. In patients with AHRE, but without atrial fibrillation (AF), we reviewed the stroke risk and the potential risk/benefit of oral anticoagulation. Atrial high-rate episodes are found in 10-30% of AF-free patients. Presence of AHRE slightly increases stroke risk (0.8% to 1%/year) compared with patients without AHRE. Atrial high-rate episode of longer duration (e.g. those >24 h) could be associated with a higher stroke risk. Oral anticoagulation has the potential to reduce stroke risk in patients with AHRE but is associated with a rate of major bleeding of 2%/year. Oral anticoagulation is not effective in patients with heart failure or survivors of a stroke without AF. It remains unclear whether anticoagulation is effective and safe in patients with AHRE. Atrial high-rate episodes are common and confer a slight increase in stroke risk. There is true equipoise on the best way to reduce stroke risk in patients with AHRE. Two ongoing trials (NOAH-AFNET 6 and ARTESiA) will provide much-needed information on the effectiveness and safety of oral anticoagulation using non-vitamin K antagonist oral anticoagulants in patients with AHRE.info:eu-repo/semantics/publishedVersio
Universal Conductance and Conductivity at Critical Points in Integer Quantum Hall Systems
The sample averaged longitudinal two-terminal conductance and the respective
Kubo-conductivity are calculated at quantum critical points in the integer
quantum Hall regime. In the limit of large system size, both transport
quantities are found to be the same within numerical uncertainty in the lowest
Landau band, and , respectively. In
the 2nd lowest Landau band, a critical conductance is
obtained which indeed supports the notion of universality. However, these
numbers are significantly at variance with the hitherto commonly believed value
. We argue that this difference is due to the multifractal structure
of critical wavefunctions, a property that should generically show up in the
conductance at quantum critical points.Comment: 4 pages, 3 figure
Modeling Disordered Quantum Systems with Dynamical Networks
It is the purpose of the present article to show that so-called network
models, originally designed to describe static properties of disordered
electronic systems, can be easily generalized to quantum-{\em dynamical}
models, which then allow for an investigation of dynamical and spectral
aspects. This concept is exemplified by the Chalker-Coddington model for the
Quantum Hall effect and a three-dimensional generalization of it. We simulate
phase coherent diffusion of wave packets and consider spatial and spectral
correlations of network eigenstates as well as the distribution of
(quasi-)energy levels. Apart from that it is demonstrated how network models
can be used to determine two-point conductances. Our numerical calculations for
the three-dimensional model at the Metal-Insulator transition point delivers
among others an anomalous diffusion exponent of .
The methods presented here in detail have been used partially in earlier work.Comment: 16 pages, Rev-TeX. to appear in Int. J. Mod. Phys.
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