428 research outputs found
Existence of the critical endpoint in the vector meson extended linear sigma model
The chiral phase transition of the strongly interacting matter is
investigated at nonzero temperature and baryon chemical potential mu_B within
an extended (2+1) flavor Polyakov constituent quark-meson model which
incorporates the effect of the vector and axial vector mesons. The effect of
the fermionic vacuum and thermal fluctuations computed from the grand potential
of the model is taken into account in the curvature masses of the scalar and
pseudoscalar mesons. The parameters of the model are determined by comparing
masses and tree-level decay widths with experimental values in a
chi^2-minimization procedure which selects between various possible assignments
of scalar nonet states to physical particles. We examine the restoration of the
chiral symmetry by monitoring the temperature evolution of condensates and the
chiral partners' masses and of the mixing angles for the pseudoscalar eta-eta'
and the corresponding scalar complex. We calculate the pressure and various
thermodynamical observables derived from it and compare them to the continuum
extrapolated lattice results of the Wuppertal-Budapest collaboration. We study
the T-mu_B phase diagram of the model and find that a critical end point exists
for parameters of the model, which give acceptable values of chi^2.Comment: 21 pages, 8 color eps figures, published versio
Design and Optimization of Electromagnetic Band Gap Structures
Dizertační práce pojednává o návrhu a optimalizaci periodických struktur s elektromagnetickým zádržným pásmem (EBG – electromagnetic band gap) pro potlačení povrchových vln šířících se na elektricky tlustých dielektrických substrátech. Nepředvídatelné chování elektromagnetických vlastností těchto struktur v závislosti na parametrech elementární buňky činí jejích syntézi značně komplikovanou. Bez patřičného postupu je návrh EBG struktur metodou pokusu a omylu. V první části práce jsou shrnuty základní poznatky o šíření elektromagnetických vln v tzv. metamateriálech. Následně je diskutován správný způsob výpočtu disperzního diagramu ve vybraných komerčních programech. Jádrem dizertace je automatizovaný návrh a optimalizace EBG struktur využitím různých globálních optimalizačních algoritmů. Praktický význam vypracované metodiky je předveden na návrhových příkladech periodických struktur s redukovanými rozměry, dvoupásmovými EBG vlastnostmi, simultánním EBG a AMC (artificial magnetic conductor – umělý magnetický vodič) chováním a tzv. superstrátu. Poslední kapitola je věnována experimentálnímu ověření počítačových modelů.The thesis deals with the design and optimization of periodic structures for surface waves suppression on electrically dense dielectric substrates. The design of such structures is rather complicated due to the large factor of uncertainty how the electromagnetic band gap (EBG) properties change depending on the unit cell geometry. Without a proper approach, the design of EBGs is based on trial-and-error. In this thesis, the basic theory of electromagnetic wave propagation in metamaterials is presented first. Second, the correct dispersion diagram computation in the selected full-wave software tools is discussed. The main attention is turned then to the automated design and optimization of EBG structures using different global evolutionary algorithms. The practical exploitation of the developed technique is demonstrated on design examples of reduced-size and dual-band EBGs, periodic structures with simultaneous electromagnetic band gap and artificial magnetic conductor (AMC) properties and periodic structures acting as superstrates. The last chapter of the thesis is devoted to the experimental verification of computer models.
Reflected Spectrally Negative Stable Processes and their Governing Equations
This paper explicitly computes the transition densities of a spectrally
negative stable process with index greater than one, reflected at its infimum.
First we derive the forward equation using the theory of sun-dual semigroups.
The resulting forward equation is a boundary value problem on the positive
half-line that involves a negative Riemann-Liouville fractional derivative in
space, and a fractional reflecting boundary condition at the origin. Then we
apply numerical methods to explicitly compute the transition density of this
space-inhomogeneous Markov process, for any starting point, to any desired
degree of accuracy. Finally, we discuss an application to fractional Cauchy
problems, which involve a positive Caputo fractional derivative in time
The Effect of Microstructure on the Local Wear Behavior of Heat Treated Structural Steel
It is known that the friction and wear properties of metals and alloys show a strong correlation with their chemical composition, hardness and microstructure.The aim of this work was to analyse and evaluate the correlations between the microstructure and the wear properties of low alloyed, heat treated structural steel during dry friction. Three kinds of hypoeutectoid structural steel with different microstructure were studied. For experimental purposes, a new type of micro-scale, ball-cratering tribometer and a proper wear-kinetic model based on an ordinary differential equation have been constructed. It was verified that if the process parameters (load, angular speed) are constant, the solution of the wear-kinetic differential equation could be expressed in a simple, closed form. Additionally, it was shown that (i) the heat treated steels have the highest wear resistance if the microstructure consists of only one hard martensitic phase, (ii) in case of microstructures consisting of two different phases (fer
rite-pearlite, bainite, and spheroidite) the wear resistance decreases in the following order: bainite, ferrite-pearlite, and spheroidite
Seeded intervals and noise level estimation in change point detection: A discussion of Fryzlewicz (2020)
In this discussion, we compare the choice of seeded intervals and that of
random intervals for change point segmentation from practical, statistical and
computational perspectives. Furthermore, we investigate a novel estimator of
the noise level, which improves many existing model selection procedures
(including the steepest drop to low levels), particularly for challenging
frequent change point scenarios with low signal-to-noise ratios.Comment: To appear in the Journal of the Korean Statistical Societ
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