3,640 research outputs found
Quantum Breathers in a Nonlinear Lattice
We study nonlinear phonon excitations in a one-dimensional quantum nonlinear
lattice model using numerical exact diagonalization. We find that multi-phonon
bound states exist as eigenstates which are natural counterparts of breather
solutions of classical nonlinear systems. In a translationally invariant
system, these quantum breather states form particle-like bands and are
characterized by a finite correlation length. The dynamic structure factor has
significant intensity for the breather states, with a corresponding quenching
of the neighboring bands of multi-phonon extended states.Comment: 4 pages, RevTex, 4 postscript figures, Physical Relview Letters (in
press
Multiple Histogram Method for Quantum Monte Carlo
An extension to the multiple-histogram method (sometimes referred to as the
Ferrenberg-Swendsen method) for use in quantum Monte Carlo simulations is
presented. This method is shown to work well for the 2D repulsive Hubbard
model, allowing measurements to be taken over a continuous region of
parameters. The method also reduces the error bars over the range of parameter
values due the overlapping of multiple histograms. A continuous sweep of
parameters and reduced error bars allow one to make more difficult
measurements, such as Maxwell constructions used to study phase separation.
Possibilities also exist for this method to be used for other quantum systems.Comment: 4 pages, 5 figures, RevTeX, submitted to Phys. Rev. B Rapid Com
Measurement of a reaction-diffusion crossover in exciton-exciton recombination inside carbon nanotubes using femtosecond optical absorption
Exciton-exciton recombination in isolated semiconducting single-walled carbon
nanotubes was studied using femtosecond transient absorption. Under sufficient
excitation to saturate the optical absorption, we observed an abrupt transition
between reaction- and diffusion- limited kinetics, arising from reactions
between incoherent localized excitons with a finite probability of ~ 0.2 per
encounter. This represents the first experimental observation of a crossover
between classical and critical kinetics in a 1D coalescing random walk, which
is a paradigm for the study of non- equilibrium systems.Comment: Accepted for Physical Review Letters. Title changed, and "exciton
fusion" replaced by "exciton-exciton recombination". Present address of Prof
Tom Brown added. 12 pages plus 3 figure
Two-dimensional Superfluidity and Localization in the Hard-Core Boson Model: a Quantum Monte Carlo Study
Quantum Monte Carlo simulations are used to investigate the two-dimensional
superfluid properties of the hard-core boson model, which show a strong
dependence on particle density and disorder. We obtain further evidence that a
half-filled clean system becomes superfluid via a finite temperature
Kosterlitz-Thouless transition. The relationship between low temperature
superfluid density and particle density is symmetric and appears parabolic
about the half filling point. Disorder appears to break the superfluid phase up
into two distinct localized states, depending on the particle density. We find
that these results strongly correlate with the results of several experiments
on high- superconductors.Comment: 10 pages, 3 figures upon request, RevTeX version 3, (accepted for
Phys. Rev. B
Impact of Smoking Status on Mortality in STEMI Patients Undergoing Mechanical Reperfusion for STEMI: Insights from the ISACS-STEMI COVID-19 Registry
The so-called \"smoking paradox\", conditioning lower mortality in smokers among STEMI patients, has seldom been addressed in the settings of modern primary PCI protocols. The ISACS-STEMI COVID-19 is a large-scale retrospective multicenter registry addressing in-hospital mortality, reperfusion, and 30-day mortality among primary PCI patients in the era of the COVID-19 pandemic. Among the 16,083 STEMI patients, 6819 (42.3%) patients were active smokers, 2099 (13.1%) previous smokers, and 7165 (44.6%) non-smokers. Despite the impaired preprocedural recanalization (p < 0.001), active smokers had a significantly better postprocedural TIMI flow compared with non-smokers (p < 0.001); this was confirmed after adjustment for all baseline and procedural confounders, and the propensity score. Active smokers had a significantly lower in-hospital (p < 0.001) and 30-day (p < 0.001) mortality compared with non-smokers and previous smokers; this was confirmed after adjustment for all baseline and procedural confounders, and the propensity score. In conclusion, in our population, active smoking was significantly associated with improved epicardial recanalization and lower in-hospital and 30-day mortality compared with previous and non-smoking history
microRNAs regulate cell-to-cell variability of endogenous target gene expression in developing mouse thymocytes
The development and homeostasis of multicellular organisms relies on gene regulation within individual constituent cells. Gene regulatory circuits that increase the robustness of gene expression frequently incorporate microRNAs as post-transcriptional regulators. Computational approaches, synthetic gene circuits and observations in model organisms predict that the co-regulation of microRNAs and their target mRNAs can reduce cell-to-cell variability in the expression of target genes. However, whether microRNAs directly regulate variability of endogenous gene expression remains to be tested in mammalian cells. Here we use quantitative flow cytometry to show that microRNAs impact on cell-to-cell variability of protein expression in developing mouse thymocytes. We find two distinct mechanisms that control variation in the activation-induced expression of the microRNA target CD69. First, the expression of miR-17 and miR-20a, two members of the miR-17-92 cluster, is coregulated with the target mRNA Cd69 to form an activation-induced incoherent feed-forward loop. Another microRNA, miR-181a, acts at least in part upstream of the target mRNA Cd69 to modulate cellular responses to activation. The ability of microRNAs to render gene expression more uniform across mammalian cell populations may be important for normal development and for disease
Carbon nanotube thin film strain sensors: comparison between experimental tests and numerical simulations
Carbon nanotubes can be randomly deposited in polymer thin film matrices to form nanocomposite strain sensors. However, a computational framework that enables the direct design of these nanocomposite thin films is still lacking. The objective of this study is to derive an experimentally validated and two-dimensional numerical model of carbon nanotube-based thin film strain sensors. This study consisted of two parts. First, multi-walled carbon nanotube (MWCNT)-Pluronic strain sensors were fabricated using vacuum filtration, and their physical, electrical, and electromechanical properties were evaluated. Second, scanning electron microscope images of the films were used for identifying topological features of the percolated MWCNT network, where the information obtained was then utilized for developing the numerical model. Validation of the numerical model was achieved by ensuring that the area ratios (of MWCNTs relative to the polymer matrix) were equivalent for both the experimental and modeled cases. Strain sensing behavior of the percolation-based model was simulated and then compared to experimental test results
Hall effect of charge carriers in a correlated system
The dynamical Hall response in a correlated electronic system is analysed
within the linear response theory for tight binding models. At the d.c.
Hall constant for a single quasiparticle is expressed explicitly via the charge
stiffness, and a semiclassical result is recovered. As expected a hole-like
response is found for the mobile hole introduced into a quantum
antiferromagnet, as represented by the model.Comment: 4 pages, RevTeX, no figure
New measurement of via neutron capture on hydrogen at Daya Bay
This article reports an improved independent measurement of neutrino mixing
angle at the Daya Bay Reactor Neutrino Experiment. Electron
antineutrinos were identified by inverse -decays with the emitted
neutron captured by hydrogen, yielding a data-set with principally distinct
uncertainties from that with neutrons captured by gadolinium. With the final
two of eight antineutrino detectors installed, this study used 621 days of data
including the previously reported 217-day data set with six detectors. The
dominant statistical uncertainty was reduced by 49%. Intensive studies of the
cosmogenic muon-induced Li and fast neutron backgrounds and the
neutron-capture energy selection efficiency, resulted in a reduction of the
systematic uncertainty by 26%. The deficit in the detected number of
antineutrinos at the far detectors relative to the expected number based on the
near detectors yielded in the
three-neutrino-oscillation framework. The combination of this result with the
gadolinium-capture result is also reported.Comment: 26 pages, 23 figure
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