2,127 research outputs found
Fermion-induced quantum criticality in two-dimensional Dirac semimetals: Non-perturbative flow equations, fixed points and critical exponents
We establish a scenario where fluctuations of new degrees of freedom at a
quantum phase transition change the nature of a transition beyond the standard
Landau-Ginzburg paradigm. To this end we study the quantum phase transition of
gapless Dirac fermions coupled to a symmetric order parameter
within a Gross-Neveu-Yukawa model in 2+1 dimensions, appropriate for the
Kekul\'e transition in honeycomb lattice materials. For this model the standard
Landau-Ginzburg approach suggests a first order transition due to the
symmetry-allowed cubic terms in the action. At zero temperature, however,
quantum fluctuations of the massless Dirac fermions have to be included. We
show that they reduce the putative first-order character of the transition and
can even render it continuous, depending on the number of Dirac fermions .
A non-perturbative functional renormalization group approach is employed to
investigate the phase transition for a wide range of fermion numbers. For the
first time we obtain the critical , where the nature of the transition
changes. Furthermore, it is shown that for large the change from the
first to second order of the transition as a function of dimension occurs
exactly in the physical 2+1 dimensions. We compute the critical exponents and
predict sizable corrections to scaling for .Comment: 12+5 pages, 5 figure
Cavitation of Electrons Bubbles in Liquid Helium Below saturation Pressure
We have used a Hartree-type electron-helium potential together with a density
functional description of liquid He and He to study the explosion of
electron bubbles submitted to a negative pressure. The critical pressure at
which bubbles explode has been determined as a function of temperature. It has
been found that this critical pressure is very close to the pressure at which
liquid helium becomes globally unstable in the presence of electrons. It is
shown that at high temperatures the capillary model overestimates the critical
pressures. We have checked that a commonly used and rather simple
electron-helium interaction yields results very similar to those obtained using
the more accurate Hartree-type interaction. We have estimated that the
crossover temperature for thermal to quantum nucleation of electron bubbles is
very low, of the order of 6 mK for He.Comment: 22 pages, 9 figure
Отказоустойчивый вентильный электро-привод для гибридного транспортного средства
Выпускная квалификационная работа содержит 79 страниц, 10 рисунка, 30 таблиц, 24 источников.
Ключевые слова: гибридное транспортное средство, отказоустойчивый вентильный электропривод, математическая модель вентильного двигателя, неполнофазный режим работы, обрыв фазы двигателя, отказ ключа преобразователя частоты.
Был проведён обзор аккумуляторов, обоснован выбор ёмкости аккумуляторной батареи.
Цель работы – разработка отказоустойчивого вентильного электропривода для гибридного транспортного средства, обеспечение алгоритма восстановления работоспособности.
Выпускная квалификационная работа выполнена в текстовом редакторе Microsoft Office Word 2010 с использованием пакетов: Microsoft Visio 2010, Matlab Simulink.Final qualifying work consists of 79 pages, 10 figure, 30 tables, 24 sources.
Key words: hybrid vehicle, failover the valve actuator, the mathematical model of the brushless DC motor, open-phase mode, output phase loss, failure of a key of the frequency Converter.
A review was undertaken of batteries and justifies the choice of battery capacity.
The work purpose – development of fault-tolerant brushless drive for a hybrid vehicle, provision of algorithm restore functionality.
Final qualifying work is executed in a text editor Microsoft Office Word 2010 packages: Microsoft Visio 2010, Matlab Simulink
Cell adhesion and cortex contractility determine cell patterning in the Drosophila retina
Hayashi and Carthew (Nature 431 [2004], 647) have shown that the packing of
cone cells in the Drosophila retina resembles soap bubble packing, and that
changing E- and N-cadherin expression can change this packing, as well as cell
shape.
The analogy with bubbles suggests that cell packing is driven by surface
minimization. We find that this assumption is insufficient to model the
experimentally observed shapes and packing of the cells based on their cadherin
expression. We then consider a model in which adhesion leads to a surface
increase, balanced by cell cortex contraction. Using the experimentally
observed distributions of E- and N-cadherin, we simulate the packing and cell
shapes in the wildtype eye. Furthermore, by changing only the corresponding
parameters, this model can describe the mutants with different numbers of
cells, or changes in cadherin expression.Comment: revised manuscript; 8 pages, 6 figures; supplementary information not
include
The effect of pressure on statics, dynamics and stability of multielectron bubbles
The effect of pressure and negative pressure on the modes of oscillation of a
multi-electron bubble in liquid helium is calculated. Already at low pressures
of the order of 10-100 mbar, these effects are found to significantly modify
the frequencies of oscillation of the bubble. Stabilization of the bubble is
shown to occur in the presence of a small negative pressure, which expands the
bubble radius. Above a threshold negative pressure, the bubble is unstable.Comment: 4 pages, 2 figures, accepted for publication in Physical Review
Letter
The accident insurance as a promising direction for insurance in Russia
Статья посвящена анализу состояния рынка услуг страхования от несчастных случаев. Актуальность исследования определяется тем, что страхование является неотъемлемым атрибутом цивилизованного общества, позволяющим сохранить обеспеченность доходами при неблагоприятных ситуациях. По статистике в России уровень травматизма и непредвиденных обстоятельств весьма велик. Люди традиционно уделяют мало внимания формированию резервов на случай временной или постоянной нетрудоспособности. Цель работы: рассмотреть систему страхования от несчастных случаев. Методы исследования: метод сопоставления, необходимый для выявления главных тенденций на рынке страховых услуг; аналитический метод, позволяющий понять, какие проблемы характерны для этого вида страхования; статистический метод, выявляющий масштабы страхования от несчастных случаев. Результаты: раскрывается сущность и виды страхования от несчастных случаев.The article is devoted to analysis of service market of accident insurance. The relevant of the study is determined by the fact that insurance is an integral attribute of a civilized society that allows you to save security income in adverse situations. According to statistics, in Russia the level of injury and unforeseen circumstances is very great. People traditionally pay little attention to formation of reserves in case of temporary or permanent incapacity for work. The main aim of the study is to consider the system of the accident insurance. Methods: the matching method required to identify the main trends in the insurance market; the analytical technique which allows understanding the typical problems for this type of insurance; the statistical method allows us to identify the extent of accident insurance. Results. The paper reveals the essence and types of accident insurance and considers the model events when the insurance company indemnifies for the damage. The causes of the demand for this type of insurance in Russia are identified. Based on the statistical data the author has determined the proportion of the accident insured people
Substellar companions and the formation of hot subdwarf stars
"Copyright 2011 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics."We give a brief review over the observational evidence for close substellar companions to hot subdwarf stars. The formation of these core helium-burning objects requires huge mass loss of their red giant progenitors. It has been suggested that besides stellar companions substellar objects in close orbits may be able to trigger this mass loss. Such objects can be easily detected around hot subdwarf stars by medium or high resolution spectroscopy with an RV accuracy at the km s(-1)-level. Eclipsing systems of Vir type stick out of transit surveys because of their characteristic light curves. The best evidence that substellar objects in close orbits around sdBs exist and that they are able to trigger the required mass loss is provided by the eclipsing system SDSS J0820+0008, which was found in the course of the MUCHFUSS project. Furthermore, several candidate systems have been discovered.Final Accepted Versio
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