70,794 research outputs found
On Stochastic Error and Computational Efficiency of the Markov Chain Monte Carlo Method
In Markov Chain Monte Carlo (MCMC) simulations, the thermal equilibria
quantities are estimated by ensemble average over a sample set containing a
large number of correlated samples. These samples are selected in accordance
with the probability distribution function, known from the partition function
of equilibrium state. As the stochastic error of the simulation results is
significant, it is desirable to understand the variance of the estimation by
ensemble average, which depends on the sample size (i.e., the total number of
samples in the set) and the sampling interval (i.e., cycle number between two
consecutive samples). Although large sample sizes reduce the variance, they
increase the computational cost of the simulation. For a given CPU time, the
sample size can be reduced greatly by increasing the sampling interval, while
having the corresponding increase in variance be negligible if the original
sampling interval is very small. In this work, we report a few general rules
that relate the variance with the sample size and the sampling interval. These
results are observed and confirmed numerically. These variance rules are
derived for the MCMC method but are also valid for the correlated samples
obtained using other Monte Carlo methods. The main contribution of this work
includes the theoretical proof of these numerical observations and the set of
assumptions that lead to them
Photoinjector-generation of a flat electron beam with transverse emittance ratio of 100
The generation of a flat electron beam directly from a photoinjector is an
attractive alternative to the electron damping ring as envisioned for linear
colliders. It also has potential applications to light sources such as the
generation of ultra-short x-ray pulses or Smith-Purcell free electron lasers.
In this Letter, we report on the experimental generation of a flat-beam with a
measured transverse emittance ratio of for a bunch charge of
nC; the smaller measured normalized root-mean-square emittance is
m and is limited by the resolution of our experimental setup.
The experimental data, obtained at the Fermilab/NICADD Photoinjector
Laboratory, are compared with numerical simulations and the expected scaling
laws.Comment: 5 pages, 3 figure
Nearly Antiferromagnetic Fermi Liquids: A Progress Report
I describe recent theoretical and experimental progress in understanding the
physical properties of the two dimensional nearly antiferromagnetic Fermi
liquids (NAFL's) found in the normal state of the cuprate superconductors. In
such NAFL's, the magnetic interaction between planar quasiparticles is strong
and peaked at or near the commensurate wave vector, . For
the optimally doped and underdoped systems, the resulting strong
antiferromagnetic correlations produce three distinct magnetic phases in the
normal state: mean field above , pseudoscaling between and
, and pseudogap below . I present arguments which suggest that the
physical origin of the pseudogap found in the quasiparticle spectrum below
is the formation of a precursor to a spin-density-wave-state, describe
the calculations based on this scenario of the dynamical spin susceptibility,
Fermi surface evolution, transport, and Hall effect, and summarize the
experimental evidence in its support.Comment: LATEX + PS figures. To appear in the proceedings of the
Euroconference on "Correlations in Unconventional Quantum Liquids," Evora,
Portugal, October 199
Non-specific cellular uptake of surface-functionalized quantum dots
We report a systematic empirical study of nanoparticle internalization into
cells via non-specific pathways. The nanoparticles were comprised of commercial
quantum dots (QDs) that were highly visible under a fluorescence confocal
microscope. Surface-modified QDs with basic biologically-significant moieties,
e.g. carboxyl, amino, streptavidin were used, in combination with the surface
derivatization with polyethylene glycol (PEG) in a range of immortalized cell
lines. Internalization rates were derived from image analysis and a detailed
discussion about the effect of nanoparticle size, charge and surface groups is
presented. We find that PEG-derivatization dramatically suppresses the
non-specific uptake while PEG-free carboxyl and amine functional groups promote
QD internalization. These uptake variations displayed a remarkable consistency
across different cell types. The reported results are important for experiments
concerned with cellular uptake of surface-functionalized nanomaterials, both
when non-specific internalization is undesirable and also when it is intended
for material to be internalized as efficiently as possible.
Published article at: http://iopscience.iop.org/0957-4484/21/28/285105/Comment: 14 pages 7 figure
Molecular production at a wide Feshbach resonance in Fermi-gas of cooled atoms
The problem of molecular production from degenerate gas of fermions at a wide
Feshbach resonance, in a single-mode approximation, is reduced to the linear
Landau-Zener problem for operators. The strong interaction leads to significant
renormalization of the gap between adiabatic levels. In contrast to static
problem the close vicinity of exact resonance does not play substantial role.
Two main physical results of our theory is the high sensitivity of molecular
production to the initial value of magnetic field and generation of a large BCS
condensate distributed over a broad range of momenta in inverse process of the
molecule dissociation.Comment: 4 pages, no figure
Decoherence-Based Quantum Zeno Effect in a Cavity-QED System
We present a decoherence-based interpretation for the quantum Zeno effect
(QZE) where measurements are dynamically treated as dispersive couplings of the
measured system to the apparatus, rather than the von Neumann's projections. It
is found that the explicit dependence of the survival probability on the
decoherence time quantitatively distinguishes this dynamic QZE from the usual
one based on projection measurements. By revisiting the cavity-QED experiment
of the QZE [J. Bernu, et al., Phys. Rev. Lett, 101, 180402 (2008)], we suggest
an alternative scheme to verify our theoretical consideration that frequent
measurements slow down the increase of photon number inside a microcavity due
to the nondemolition couplings with the atoms in large detuning.Comment: 4 pages, 3 figure
Recommended from our members
Studies on Temperature and Strain Sensitivities of a Few-mode Critical Wavelength Fiber Optic Sensor
This paper studied the relationship between the temperature/strain wavelength sensitivity of a fiber optic in-line Mach-Zehnder Interferometer (MZI) sensor and the wavelength separation of the measured wavelength to the critical wavelength (CWL) in a CWL-existed interference spectrum formed by interference between LP01 and LP02 modes. The in-line MZI fiber optic sensor has been constructed by splicing a section of specially designed few-mode fiber (FMF), which support LP01 and LP02 modes propagating in the fiber, between two pieces of single mode fiber. The propagation constant difference, Δβ, between the LP01 and LP02 modes, changes non-monotonously with wavelength and reaches a maximum at the CWL. As a result, in sensor operation, peaks on the different sides of the CWL then shift in opposite directions, and the associated temperature/strain sensitivities increase significantly when the measured wavelength points become close to the CWL, from both sides of the CWL. A theoretical analysis carried out has predicted that with this specified FMF sensor approach, the temperature/strain wavelength sensitivities are governed by the wavelength difference between the measured wavelength and the CWL. This conclusion was seen to agree well with the experimental results obtained. Combining the wavelength shifts of the peaks and the CWL in the transmission spectrum of the SFS structure, this study has shown that this approach forms the basis of effective designs of high sensitivity sensors for multi-parameter detection and offering a large measurement range to satisfy the requirements needed for better industrial measurements
- …