217,304 research outputs found
Large Deviation Function of the Partially Asymmetric Exclusion Process
The large deviation function obtained recently by Derrida and Lebowitz for
the totally asymmetric exclusion process is generalized to the partially
asymmetric case in the scaling limit. The asymmetry parameter rescales the
scaling variable in a simple way. The finite-size corrections to the universal
scaling function and the universal cumulant ratio are also obtained to the
leading order.Comment: 10 pages, 2 eps figures, minor changes, submitted to PR
Computationally Efficient Nonparametric Importance Sampling
The variance reduction established by importance sampling strongly depends on
the choice of the importance sampling distribution. A good choice is often hard
to achieve especially for high-dimensional integration problems. Nonparametric
estimation of the optimal importance sampling distribution (known as
nonparametric importance sampling) is a reasonable alternative to parametric
approaches.In this article nonparametric variants of both the self-normalized
and the unnormalized importance sampling estimator are proposed and
investigated. A common critique on nonparametric importance sampling is the
increased computational burden compared to parametric methods. We solve this
problem to a large degree by utilizing the linear blend frequency polygon
estimator instead of a kernel estimator. Mean square error convergence
properties are investigated leading to recommendations for the efficient
application of nonparametric importance sampling. Particularly, we show that
nonparametric importance sampling asymptotically attains optimal importance
sampling variance. The efficiency of nonparametric importance sampling
algorithms heavily relies on the computational efficiency of the employed
nonparametric estimator. The linear blend frequency polygon outperforms kernel
estimators in terms of certain criteria such as efficient sampling and
evaluation. Furthermore, it is compatible with the inversion method for sample
generation. This allows to combine our algorithms with other variance reduction
techniques such as stratified sampling. Empirical evidence for the usefulness
of the suggested algorithms is obtained by means of three benchmark integration
problems. As an application we estimate the distribution of the queue length of
a spam filter queueing system based on real data.Comment: 29 pages, 7 figure
Correlation Assisted Phonon Softenings and the Mott-Peierls Transition in VO
To explore the driving mechanisms of the metal-insulator transition (MIT) and
the structural transition in VO2, we have investigated phonon dispersions of
rutile VO2 (R-VO2) in the DFT and the DFT+U (U : Coulomb correlation) band
calculations. We have found that the phonon softening instabilities occur in
both cases, but the softened phonon mode only in the DFT+U describes properly
both the MIT and the structural transition from R-VO2 to monoclinic VO2
(M1-VO2). This feature demonstrates that the Coulomb correlation effect plays
an essential role of assisting the Peierls transition in R-VO2. We have also
found from the phonon dispersion of M1-VO2 that M1 structure becomes unstable
under high pressure. We have predicted a new phase of VO2 at high pressure that
has a monoclinic CaCl2-type structure with metallic nature
Compositional changes on GaN surfaces under low-energy ion bombardment studied by synchrotron-based spectroscopies
We have investigated compositional changes on GaNsurfaces under Ar-ion bombardment using synchrotron-based high-resolution x-rayphotoemission (PES) and near-edge x-ray absorption fine structure(NEXAFS)spectroscopy. The low-energy ion bombardment of GaN produces a Ga-rich surface layer which transforms into a metallic Ga layer at higher bombarding energies. At the same time, the photoemissionspectra around N 1s core levels reveal the presence of both uncoordinated nitrogen and nitrogen interstitials, which we have analyzed in more details by x-rayabsorption measurements at N K edge. We have demonstrated that PES and NEXAFS provide a powerful combination for studying the compositional changes on GaNsurfaces. A mechanism for the relocation and loss of nitrogen during ion bombardment in agreement with some recent experimental and theoretical studies of defect formation in GaN has been proposed.P.N.K.D. is grateful for the financial
support of the Australian Research Council
The Preference for Round Number Prices
This study determines if a preference for round prices exists in the wheat market and how wheat sales react to price movements around whole dollar amounts. The results show round prices are slightly more prevalent than non-round prices and that transactions increase when price moves above a whole dollar amount.Demand and Price Analysis,
Determining Returns to Storage: USDA Data versus Micro Level Data
USDA data are commonly used to determine producers' returns to storage. Aggregating data may result in a loss of information, leading to underestimated returns. This study compares USDA and elevator data from Oklahoma to determine how much USDA data underestimates returns. Results indicate USDA data only slightly underestimate returns to storage.Research Methods/ Statistical Methods,
20 K superconductivity in heavily electron doped surface layer of FeSe bulk crystal
A superconducting transition temperature Tc as high as 100 K was recently
discovered in 1 monolayer (1ML) FeSe grown on SrTiO3 (STO). The discovery
immediately ignited efforts to identify the mechanism for the dramatically
enhanced Tc from its bulk value of 7 K. Currently, there are two main views on
the origin of the enhanced Tc; in the first view, the enhancement comes from an
interfacial effect while in the other it is from excess electrons with strong
correlation strength. The issue is controversial and there are evidences that
support each view. Finding the origin of the Tc enhancement could be the key to
achieving even higher Tc and to identifying the microscopic mechanism for the
superconductivity in iron-based materials. Here, we report the observation of
20 K superconductivity in the electron doped surface layer of FeSe. The
electronic state of the surface layer possesses all the key spectroscopic
aspects of the 1ML FeSe on STO. Without any interface effect, the surface layer
state is found to have a moderate Tc of 20 K with a smaller gap opening of 4
meV. Our results clearly show that excess electrons with strong correlation
strength alone cannot induce the maximum Tc, which in turn strongly suggests
need for an interfacial effect to reach the enhanced Tc found in 1ML FeSe/STO.Comment: 5 pages, 4 figure
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