38,278 research outputs found
Two-Photon Beatings Using Biphotons Generated from a Two-Level System
We propose a two-photon beating experiment based upon biphotons generated
from a resonant pumping two-level system operating in a backward geometry. On
the one hand, the linear optical-response leads biphotons produced from two
sidebands in the Mollow triplet to propagate with tunable refractive indices,
while the central-component propagates with unity refractive index. The
relative phase difference due to different refractive indices is analogous to
the pathway-length difference between long-long and short-short in the original
Franson interferometer. By subtracting the linear Rayleigh scattering of the
pump, the visibility in the center part of the two-photon beating interference
can be ideally manipulated among [0, 100%] by varying the pump power, the
material length, and the atomic density, which indicates a Bell-type inequality
violation. On the other hand, the proposed experiment may be an interesting way
of probing the quantum nature of the detection process. The interference will
disappear when the separation of the Mollow peaks approaches the fundamental
timescales for photon absorption in the detector.Comment: to appear in Phys. Rev. A (2008
Rotating system for four-dimensional transverse rms-emittance measurements
Knowledge of the transverse four-dimensional beam rms-parameters is essential
for applications that involve lattice elements that couple the two transverse
degrees of freedom (planes). Of special interest is the removal of inter-plane
correlations to reduce the projected emittances. A dedicated ROtating System
for Emittance measurements (ROSE) has been proposed, developed, and
successfully commissioned to fully determine the four-dimensional beam matrix.
This device has been used at the High Charge injector (HLI) at GSI using a beam
line which is composed of a skew quadrupole triplet, a normal quadrupole
doublet, and ROSE. Mathematical algorithms, measurements, and results for ion
beams of 83Kr13+ at 1.4 MeV/u are reported in this paper.Comment: 11 pages, 10 figure
Electrically induced tunable cohesion in granular systems
Experimental observations of confined granular materials in the presence of
an electric field that induces cohesive forces are reported. The angle of
repose is found to increase with the cohesive force. A theoretical model for
the stability of a granular heap, including both the effect of the sidewalls
and cohesion is proposed. A good agreement between this model and the
experimental results is found. The steady-state flow angle is practically
unaffected by the electric field except for high field strengths and low flow
rates.Comment: accepted for publication in "Journal of Statistical Mechanics: Theory
and Experiment
Control of porous structure in flexible silicone aerogels produced from methyltrimethoxysilane (MTMS): the effect of precursor concentration in sol–gel solutions
Controllable nanoporous structure in MTMS-based silicone aerogels is required to improve their thermal conductivity. Silicone aerogels were formed in a two-step acid–base catalysed sol–gel process combined with supercritical drying. The influence of MTMS concentration, specifically the molar ratio of methanol:MTMS and water:MTMS in the sol–gel process was studied in relation to the porous structure of resultant silicone aerogels. Samples were characterised to determine the dimensions of micro, meso and macro-pore structure by means of both nitrogen gas adsorption–desorption for detection of pores 300 nm. Porosity, pore volume distribution and Brunau er–Emmitt–Teller surface area in the silicone aerogels were all found to be influenced by adjustment of the molar ratio of methanol:MTMS and the molar ratio of water:MTMS during sol–gel processing
Incorporation of Density Matrix Wavefunctions in Monte Carlo Simulations: Application to the Frustrated Heisenberg Model
We combine the Density Matrix Technique (DMRG) with Green Function Monte
Carlo (GFMC) simulations. The DMRG is most successful in 1-dimensional systems
and can only be extended to 2-dimensional systems for strips of limited width.
GFMC is not restricted to low dimensions but is limited by the efficiency of
the sampling. This limitation is crucial when the system exhibits a so-called
sign problem, which on the other hand is not a particular obstacle for the
DMRG. We show how to combine the virtues of both methods by using a DMRG
wavefunction as guiding wave function for the GFMC. This requires a special
representation of the DMRG wavefunction to make the simulations possible within
reasonable computational time. As a test case we apply the method to the
2-dimensional frustrated Heisenberg antiferromagnet. By supplementing the
branching in GFMC with Stochastic Reconfiguration (SR) we get a stable
simulation with a small variance also in the region where the fluctuations due
to minus sign problem are maximal. The sensitivity of the results to the choice
of the guiding wavefunction is extensively investigated. We analyse the model
as a function of the ratio of the next-nearest to nearest neighbor coupling
strength. We observe in the frustrated regime a pattern of the spin
correlations which is in-between dimerlike and plaquette type ordering, states
that have recently been suggested. It is a state with strong dimerization in
one direction and weaker dimerization in the perpendicular direction.Comment: slightly revised version with added reference
Interaction of Individual Skyrmions in Nanostructured Cubic Chiral Magnet
We report the direct evidence of field-dependent character of the interaction
between individual magnetic skyrmions as well as between skyrmions and edges in
B20-type FeGe nanostripes observed by means of high resolution Lorentz
transmission electron microscopy. It is shown that above certain critical
values of external magnetic field the character of such long-range skyrmion
interactions change from attraction to repulsion. Experimentally measured
equilibrium inter-skyrmion and skrymion-edge distances as function of applied
magnetic field shows quantitative agreement with the results of micromagnetic
simulations. Important role of demagnetizing fields and internal symmetry of
three-dimensional magnetic skyrmions are discussed in details.Comment: accepted in PR
SDSS J075101.42+291419.1: A Super-Eddington Accreting Quasar with Extreme X-ray Variability
We report the discovery of extreme X-ray variability in a type 1 quasar: SDSS
J. It has a black hole mass of
measured from reverberation mapping (RM), and the black hole is accreting with
a super-Eddington accretion rate. Its XMM-Newton observation in 2015 May
reveals a flux drop by a factor of with respect to the Swift
observation in 2013 May when it showed a typical level of X-ray emission
relative to its UV/optical emission. The lack of correlated UV variability
results in a steep X-ray-to-optical power-law slope () of
-1.97 in the low X-ray flux state, corresponding to an X-ray weakness factor of
36.2 at rest-frame 2 keV relative to its UV/optical luminosity. The mild
UV/optical continuum and emission-line variability also suggest that the
accretion rate did not change significantly. A single power-law model modified
by Galactic absorption describes well the keV spectra of the X-ray
observations in general. The spectral fitting reveals steep spectral shapes
with . We search for active galactic nuclei (AGNs) with such
extreme X-ray variability in the literature and find that most of them are
narrow-line Seyfert 1 galaxies and quasars with high accretion rates. The
fraction of extremely X-ray variable objects among super-Eddington accreting
AGNs is estimated to be . We discuss two possible scenarios,
disk reflection and partial covering absorption, to explain the extreme X-ray
variability of SDSS J. We propose a possible origin for the
partial covering absorber, which is the thick inner accretion disk and its
associated outflow in AGNs with high accretion rates.Comment: 15 pages, 9 figures, accepted for publication in Ap
Case-control study of stroke and the quality of hypertension control in north west England
Objective: To examine the risk of stroke in relation to quality of hypertension control in routine general practice across an entire health district.
Design: Population based matched case-control study.
Setting: East Lancashire Health District with a participating population of 388,821 aged < or = 80.
Subjects: Cases were patients under 80 with their first stroke identified from a population based stroke register between 1 July 1994 and 30 June 1995. For each case two controls matched with the case for age and sex were selected from the same practice register. Hypertension was defined as systolic blood pressure > or = 160 mm Hg or diastolic blood pressure > or = 95 mm Hg, or both, on at least two occasions within any three month period or any history of treatment with antihypertensive drugs.
Main outcome measures: Prevalence of hypertension and quality of control of hypertension assessed by using the mean blood pressure recorded before stroke) and odds ratios of stroke (derived from conditional logistic regression).
Results: Records of 267 cases and 534 controls were examined; 61% and 42% of these subjects respectively were hypertensive. Compared with non-hypertensive subjects hypertensive patients receiving treatment whose average pre-event systolic blood pressure was controlled to or = 160 mm Hg) or untreated had progressively raised odds ratios of 1.6, 2.2, 3.2, and 3.5 respectively. Results for diastolic pressure were similar; both were independent of initial pressures before treatment. Around 21% of strokes were thus attributable to inadequate control with treatment, or 46 first events yearly per 100,000 population aged 40-79.
Conclusions: Risk of stroke was clearly related to quality of control of blood pressure with treatment. In routine practice consistent control of blood pressure to below 150/90 mm Hg seems to be required for optimal stroke prevention
Numerical simulation study of the dynamical behavior of the Niedermayer algorithm
We calculate the dynamic critical exponent for the Niedermayer algorithm
applied to the two-dimensional Ising and XY models, for various values of the
free parameter . For we regain the Metropolis algorithm and for
we regain the Wolff algorithm. For , we show that the mean
size of the clusters of (possibly) turned spins initially grows with the linear
size of the lattice, , but eventually saturates at a given lattice size
, which depends on . For , the Niedermayer
algorithm is equivalent to the Metropolis one, i.e, they have the same dynamic
exponent. For , the autocorrelation time is always greater than for
(Wolff) and, more important, it also grows faster than a power of .
Therefore, we show that the best choice of cluster algorithm is the Wolff one,
when compared to the Nierdermayer generalization. We also obtain the dynamic
behavior of the Wolff algorithm: although not conclusive, we propose a scaling
law for the dependence of the autocorrelation time on .Comment: Accepted for publication in Journal of Statistical Mechanics: Theory
and Experimen
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