Parallelization of the Wolff Single-Cluster Algorithm

A parallel [open multiprocessing (OpenMP)] implementation of the Wolff single-cluster algorithm has been developed and tested for the three-dimensional (3D) Ising model. The developed procedure is generalizable to other lattice spin models and its effectiveness depends on the specific application at hand. The applicability of the developed methodology is discussed in the context of the applications, where a sophisticated shuffling scheme is used to generate pseudorandom numbers of high quality, and an iterative method is applied to find the critical temperature of the 3D Ising model with a great accuracy. For the lattice with linear size L=1024, we have reached the speedup about 1.79 times on two processors and about 2.67 times on four processors, as compared to the serial code. According to our estimation, the speedup about three times on four processors is reachable for the O(n) models with n ≥ 2. Furthermore, the application of the developed OpenMP code allows us to simulate larger lattices due to greater operative (shared) memory available

Lorentzian geometry and variability reduction in airplane boarding : slow passengers first outperforms random boarding

Airlines use different boarding policies to organize the queue of passengers waiting to enter the airplane. We analyze three policies in the many-passenger limit by a geometric representation of the queue position and row designation of each passenger and apply a Lorentzian metric to calculate the total boarding time. The boarding time is governed by the time each passenger needs to clear the aisle, and the added time is determined by the aisle-clearing time distribution through an effective aisle-clearing time parameter. The nonorganized queues under the common random boarding policy are characterized by large effective aisle-clearing time. We show that, subject to a mathematical assumption which we have verified by extensive numerical computations in all realistic cases, the average total boarding time is always reduced when slow passengers are separated from faster passengers and the slow group is allowed to enter the airplane first. This is a universal result that holds for any combination of the three main governing parameters: the ratio between effective aisle-clearing times of the fast and the slow groups, the fraction of slow passengers, and the congestion of passengers in the aisle. Separation into groups based on aisle-clearing time allows for more synchronized seating, but the result is nontrivial, as the similar fast-first policy—where the two groups enter the airplane in reverse order—is inferior to random boarding for a range of parameter settings. The asymptotic results conform well with discrete-event simulations with realistic numbers of passengers. Parameters based on empirical data, with hand luggage as criteria for separating passengers into the slow and fast groups, give an 8% reduction in total boarding time for slow first compared to random boarding

Probabilistic Description of Traffic Breakdowns

We analyze the characteristic features of traffic breakdown. To describe this phenomenon we apply to the probabilistic model regarding the jam emergence as the formation of a large car cluster on highway. In these terms the breakdown occurs through the formation of a certain critical nucleus in the metastable vehicle flow, which enables us to confine ourselves to one cluster model. We assume that, first, the growth of the car cluster is governed by attachment of cars to the cluster whose rate is mainly determined by the mean headway distance between the car in the vehicle flow and, may be, also by the headway distance in the cluster. Second, the cluster dissolution is determined by the car escape from the cluster whose rate depends on the cluster size directly. The latter is justified using the available experimental data for the correlation properties of the synchronized mode. We write the appropriate master equation converted then into the Fokker-Plank equation for the cluster distribution function and analyze the formation of the critical car cluster due to the climb over a certain potential barrier. The further cluster growth irreversibly gives rise to the jam formation. Numerical estimates of the obtained characteristics and the experimental data of the traffic breakdown are compared. In particular, we draw a conclusion that the characteristic intrinsic time scale of the breakdown phenomenon should be about one minute and explain the case why the traffic volume interval inside which traffic breakdown is observed is sufficiently wide.Comment: RevTeX 4, 14 pages, 10 figure

Non-perturbative approaches in nanoscience and corrections to finite-size scaling

Please see http://www.birs.ca/workshops/2016/16w5069/Programme16w5069.pdfNon UBCUnreviewedAuthor affiliation: IMSIT at Liepaja UniversityFacult

Radiative Phonon-Assisted and Auger Recombination in Si Nanocrystals

Recent analysis of the literature shows that the photoluminescence (PL) of Si nanocrystals and porous silicon is caused by phonon-assisted exciton radiative recombination, as well as by direct radiative electron transfer from the second to the first conduction sub-band, which is related to the Auger recombination. The PL decay curve for porous silicon after excitation with ultraviolet laser pulse has been established experimentally. We have constructed continuity equations for the first and the second conduction sub-bands, including radiative phonon-assisted exciton recombination, Auger recombination and direct radiative transition from the second to the first conduction sub-band. The solution of these equations yields the theoretical PL decay curve. These equations contain adjustable parameters. We have estimated their values by using the least-squares fit of the theoretical curve to the experimental data. We have shown that in our case both radiative components are approximately equal at the beginning, but the relative part of the phonon-assisted recombination increases with time and finally tends to 100%. Using the obtained values of the fit parameters, we have constructed the decay curves at different excitation intensities. We conclude that, at a small excitation power, the decay curve is exponential, the phonon-assisted radiative exciton recombination is dominating, and quantum efficiency is close to one. At a high excitation power, the decay curve is approximately stretched-exponential, the direct radiative transitions from the second to the first conduction sub-band are dominating, and quantum efficiency is about 1/3 for porous silicon sample

Increase of Solar Cell Efficiency in Graded Band Gap Structure

The photo current-voltage characteristic of a solar cell with graded band gap is calculated numerically based on the drift-diffusion equation and Poisson equation. The calculated efficiency of the CdTe solar cell with p-n junction located in 1μm depth increases remarkably when the band gap of the front n-type layer is graded. The effect is strong for high surface recombination velocity S and is remarkable even at S=0 : the calculated efficiency increases from 19.6% to 24.3%

Formation of Buried Layers by Laser Radiation

New conception and both experimental anid theoretical results regarding buried layers formation and control of their depth anid thickness are presented. Effect of the CO2laser radiation on distribution and state of oxygen or nitrogen atoms, implanted in silicon wafer, is investigated experimentally. A model is proposed and equations are obtained descrbing the process of buried layer's foruation from impurities, introduced inside the crystal, in presence of the temperature gradient. An interaction between impurities is taken into account

Comment on "How skew distributions emerge in evolving systems" by Choi M. Y. et al.

Power-law distributions and other skew distributions, observed in various models and real systems, are considered. As an example, critical exponents determined from highly accurate experimental data very close to the λ-transition point in liquid helium are discussed in some detail. A model, describing evolving systems with increasing number of elements, is considered to study the distribution over element sizes. Stationary power-law distributions are found. Certain non-stationary skew distributions are obtained and analyzed, based on exact solutions.Validerad; 2010; 20100908 (weber