27 research outputs found
Bosonic Spectral Function and The Electron-Phonon Interaction in HTSC Cuprates
In Part I we discuss accumulating experimental evidence related to the
structure and origin of the bosonic spectral function in high-temperature
superconducting (HTSC) cuprates at and near optimal doping. Some global
properties of the spectral function, such as number and positions of peaks, are
extracted by combining optics, neutron scattering, ARPES and tunnelling
measurements. These methods give convincing evidence for strong electron-phonon
interaction (EPI) with the coupling constant between 1-3 in cuprates near
optimal doping. Here we clarify how these results are in favor of the
Eliashberg-like theory for HTSC cuprates near optimal doping. In Part II we
discuss some theoretical ingredients - such as strong EPI, strong correlations
- which are necessary to explain the experimental results related to the
mechanism of d-wave pairing in optimally doped cuprates. These comprise the
Migdal-Eliashberg theory for EPI in strongly correlated systems which give rise
to the forward scattering peak. The latter is further supported by the weakly
screened Madelung interaction in the ionic-metallic structure of layered
cuprates. In this approach EPI is responsible for the strength of pairing while
the residual Coulomb interaction (by including spin fluctuations) triggers the
d-wave pairing.Comment: 59 pages, 38 figures, review articl
A Review of Computational Stochastic Elastoplasticity
Heterogeneous materials at the micro-structural level are usually subjected to several uncertainties. These materials behave according to an elastoplastic model, but with uncertain parameters. The present review discusses recent developments in numerical approaches to these kinds of uncertainties, which are modelled as random elds like Young's modulus, yield stress etc. To give full description of random phenomena of elastoplastic materials one needs adequate mathematical framework. The probability theory and theory of random elds fully cover that need. Therefore, they are together with the theory of stochastic nite element approach a subject of this review. The whole group of di erent numerical stochastic methods for the elastoplastic problem has roots in the classical theory of these materials. Therefore, we give here the classical formulation of plasticity in very concise form as well as some of often used methods for solving this kind of problems. The main issues of stochastic elastoplasticity as well as stochastic problems in general are stochastic partial di erential equations. In order to solve them we must discretise them. Methods of solving and discretisation are called stochastic methods. These methods like Monte Carlo, Perturbation method, Neumann series method, stochastic Galerkin method as well as some other very known methods are reviewed and discussed here
REPORT ON THE SCIENTIFIC ACTIVITIES OF THE POWER ENGINEERING GROUP AT THE DEPARTMENT OF ELECTROMAGNETIC THEORY OF THE TECHNICAL UNIVERSITY OF BUDAPEST
The paper summarizes the results of the research work of the power engineering group at
the Department of Electromagnetic Theory of the Technical University of Budapest in the fields
of network analysis and electromagnetic field calculations
Recommended from our members
Bosonic spectral function and the electron-phonon interaction in HTSC cuprates
In this paper we discuss experimental evidence related to the structure and origin of the bosonic spectral function α2 F(Ï) in high-temperature superconducting (HTSC) cuprates at and near optimal doping. Global properties of α2 F(Ï), such as number and positions of peaks, are extracted by combining optics, neutron scattering, ARPES and tunnelling measurements. These methods give evidence for strong electron-phonon interaction (EPI) with 1<λep3.5 in cuprates near optimal doping. We clarify how these results are in favor of the modified Migdal-Eliashberg (ME) theory for HTSC cuprates near optimal doping. In Section 2 we discuss theoretical ingredientssuch as strong EPI, strong correlationswhich are necessary to explain the mechanism of d-wave pairing in optimally doped cuprates. These comprise the ME theory for EPI in strongly correlated systems which give rise to the forward scattering peak. The latter is supported by the long-range part of EPI due to the weakly screened Madelung interaction in the ionic-metallic structure of layered HTSC cuprates. In this approach EPI is responsible for the strength of pairing while the residual Coulomb interaction and spin fluctuations trigger the d-wave pairing. Copyright © 2010 E. G. Maksimov et al
Nonlocal Theories in Continuum Mechanics
The purpose of this paper is to explain why the standard continuum theory fails to properly describe certain mechanical phenomena and how the description can be improved by enrichments that incorporate the influence of gradients or weighted spatial averages of strain or of an internal variable. Three typical mechanical problems that require such enrichments are presented: (i) dispersion of short elastic waves in heterogeneous or discrete media, (ii) size effects in microscale elastoplasticity, in particular with the size dependence of the apparent hardening modulus, and (iii) localization of strain and damage in quasibrittle structures and with the resulting transitional size effect. Problems covered in the examples encompass static and dynamic phenomena, linear and nonlinear behavior, and three constitutive frameworks, namely elasticity, plasticity and continuum damage mechanics. This shows that enrichments of the standard continuum theory can be useful in a wide range of mechanical problems.
Electron Phonon Interaction and Strong Correlations in High-Temperature Superconductors: One can not avoid unavoidable
The important role of the electron-phonon interaction (EPI) in explaining the
properties of the normal state and pairing mechanism in high-T
superconductors (HTSC) is discussed. A number of experimental results are
analyzed such as: dynamical conductivity, Raman scattering, neutron scattering,
ARPES, tunnelling measurements, isotope effect and etc. They give convincing
evidence that the EPI is strong and dominantly contributes to pairing in HTSC
oxides. It is argued that strong electronic correlations in conjunction with
the pronounced (in relatively weakly screened materials) EPI are unavoidable
ingredients for the microscopic theory of pairing in HTSC oxides. I present the
well defined and controllable theory of strong correlations and the EPI. It is
shown that strong correlations give rise to the pronounced \textit{forward
scattering peak} in the EPI - the FSP theory. The FSP theory explains in a
consistent way several (crucial) puzzles such as much smaller transport
coupling constant than the pairing one (), which are
present if one interprets the results in HTSC oxides by the old
Migdal-Eliashberg theory for the EPI. The ARPES shift puzzle where the nodal
kink at 70 meV is unshifted in the superconducting state, while the anti-nodal
one at 40 meV is shifted can be explained at present only by the FSP theory. A
number of other interesting predictions of the FSP theory are also discussed.Comment: 84 pages, 27 figures, APS Proceeding
Detecção e agrupamento de contornos
A detecção de contornos a partir de imagens digitais Ă© um procedimento do qual resulta informação essencial para muitos algoritmos de visĂŁo por computador. A natureza das imagens digitais bidimensionais: a sua relativamente baixa resolução; a amostragem espacial e em amplitude; a presença de ruĂdo; a falta de informação em profundidade; as oclusĂ”es, etc., e a importĂąncia dos contornos como informação bĂĄsica para muitos outros algoritmos a montante, fazem com que a detecção de contornos seja um problema apenas parcialmente resolvido, com mĂșltiplas abordagens e dando origem desde hĂĄ algumas dĂ©cadas a larga quantidade de publicaçÔes. Continua a ser um tema actual de investigação como se comprova pela quantidade e qualidade das publicaçÔes cientĂficas mais actuais nesta ĂĄrea.
A tese discute a detecção de contornos nas suas fases clĂĄssicas: a estimação da amplitude do sinal que aponta a presença de um ponto de contorno; a prĂ©-classiïŹcação dos pontos da imagem com base nos sinais estimados e o posterior agrupamento dos pontos de contorno individuais em segmentos de curvas de contorno.
PropĂ”e-se, nesta tese: um mĂ©todo de projecto de estimadores de presença de pontos de contorno baseado na utilização de equaçÔes integrais de Fredholm; um classiïŹcador nĂŁo-linear que utiliza informação de pontos vizinhos para a tomada de decisĂŁo, e uma metodologia de agrupamento de pontos de contorno com crescimento iterativo com uma função de custo com suporte local. A metodologia de extracção das propriedades baseada na equação integral de Fredholm de primeira ordem permite uma anĂĄlise uniïŹcadora de vĂĄrios mĂ©todos previamente propostos na literatura sobre o assunto.
O procedimento de classiïŹcação dos pontos de contorno baseia-se na anĂĄlise das sequĂȘncias ordenadas das amplitudes do gradiente na vizinhança do ponto de contorno. O procedimento Ă© estudado com base nas funçÔes densidade de distribuição das estatĂsticas ordenadas dos pontos de contorno vizinhos e na assunção de que os pontos de um mesmo contorno possuem distribuiçÔes ordenadas similares. A fase ïŹnal da detecção de contornos Ă© realizada com um procedimento de agrupamento de contornos em que se constrĂłi uma hipĂłtese de vizinhança para eventual crescimento do contorno e em que se estima o melhor ponto para agregação ao contorno.
Os resultados experimentais para os métodos propostos são apresentados e analisados com imagens reais e sintéticas