161 research outputs found
Polaron features for long-range electron-phonon interaction
The polaron features for long-range electron-phonon interaction are
investigated by extending a variational approach previously proposed for the
study of systems with local coupling. The ground-state spectral weight, the
average kinetic energy, the mean number of phonons, and the electron-lattice
correlation function are discussed for a wide range of model parameters
focusing on the adiabatic regime and comparing the results with the short-range
case (Holstein model). A strong mixing of electronic and phononic degrees of
freedom for small values of the electron-phonon coupling constant is found in
the adiabatic case due to the long-range interaction. Finally a polaron "phase
diagram" is proposed.Comment: 4 figs., to appear in J. Phys.:Condens. Matte
Instabilities for a relativistic electron beam interacting with a laser irradiated plasma
The effects of a radiation field (RF) on the unstable modes developed in
relativistic electron beam--plasma interaction are investigated assuming that
, where is the frequency of the RF and
is the plasma frequency. These unstable modes are parametrically
coupled to each other due to the RF and are a mix between two--stream and
parametric instabilities. The dispersion equations are derived by the
linearization of the kinetic equations for a beam--plasma system as well as the
Maxwell equations. In order to highlight the effect of the radiation field we
present a comparison of our analytical and numerical results obtained for
nonzero RF with those for vanishing RF. Assuming that the drift velocity
of the beam is parallel to the wave vector of the
excitations two particular transversal and parallel configurations of the
polarization vector of the RF with respect to are
considered in detail. It is shown that in both geometries resonant and
nonresonant couplings between different modes are possible. The largest growth
rates are expected at the transversal configuration when is
perpendicular to . In this case it is demonstrated that in general
the spectrum of the unstable modes in -- plane is split into two
distinct domains with long and short wavelengths, where the unstable modes are
mainly sensitive to the beam or the RF parameters, respectively. In parallel
configuration, , and at short wavelengths
the growth rates of the unstable modes are sensitive to both beam and RF
parameters remaining insensitive to the RF at long wavelengths.Comment: 23 pages, 5 figure
One- and two-photon resonant spectroscopy of hydrogen and anti-hydrogen atoms in external electric fields
The resonant spectra of hydrogen and anti-hydrogen atoms in the presence of
an external electric field are compared theoretically. It is shown that
nonresonant corrections to the transition frequency contain terms linear in the
electric field. The existence of these terms does not violate space and time
parity and leads to a difference in the resonant spectroscopic measurements for
hydrogen and anti-hydrogen atoms in an external electric field. The one-photon
1s-2p and the two-photon 1s-2s resonances are investigated
Концептуальные основы технологии песчаного дисперсно-армированного асфальтобетона
A problem of efficient resource usage in road branch continues to be one of the most complicated issues and requires an intensification in investigation process pertaining to possibilities for production of road construction materials of low resource intensity with high physical and mechanical properties. Technogenic wastes of the Belarusian enterprises are rather various and they need a detailed investigation. Application of such methods as IR spectrometry, probe microscopy, study of of geometric characteristics of particles and fibers make it possible to determine more active centres and reveal micro-defects that influence on strength of adhesion bond at the boundary of “fiber – binder” and physical and mechanical properties of ready-mixed asphalt concrete. Nature of basalt fiber presupposes mainly physical character of adhesion interaction at the boundary of phase separation. An increase of technogenic waste activity to enhance adhesion contacts up to chemisorption level is possible only due to preliminary fiber processing which includes cleaning, removal of foreign inclusions, etching, drying, probable sorting-out and fluffing. Industrial approbation of such technological process is not possible without development of a corresponding module or a plant. Disperse reinforment causes changes in composition and technology of sand asphalt concrete. An increase in specific surface of an aggregate, necessity of uniform distribution of fiber in terms of volume determine the required need in a binder, procedure and regimes for component mixing. Grain composition of the aggregate can be represented by crush screening and natural sand of mixture of these materials. Requirements to properties of sand disperse-reinforced asphalt concrete are formed on the basis of operational conditions and layer arrangement of the material in the design of a surface dressing. The disperse-reinforced sand asphalt concrete can perform functions of a superfine protective layer, a levelling layer or a crack stopping layer which is resistant to fatigue crack formation.Проблема эффективного использования ресурсов в дорожной отрасли остается одной из самых сложных, поэтому требуется интенсификация процесса исследования возможностей производства дорожно-строительных материалов пониженной ресурсоемкости с повышенными физико-механическими свойствами. Техногенные отходы предприятий Беларуси достаточно разнообразны, и необходимо их изучение. Использование методов ИК-спектрометрии, зондовой микроскопии, изучение геометрических характеристик частиц и волокон позволяют определить наиболее активные центры и выявить микродефекты, влияющие на прочность адгезионной связи на границе «волокно – вяжущее» и физико-механические свойства готового асфальтобетона. Природа базальтового волокна предполагает в основном физический характер адгезионного взаимодействия на границе раздела фаз. Повышение активности техногенного отхода с целью усиления адгезионных контактов вплоть до хемосорбционного уровня возможно только при предварительной обработке волокна, которая будет включать очистку, удаление инородных включений, травление волокна, сушку, возможную сортировку и вспушивание. Промышленная апробация такого технологического процесса невозможна без разработки соответствующего модуля или установки. Дисперсное армирование вызывает изменения в составах и технологии песчаного асфальтобетона. Увеличение удельной поверхности заполнителя, необходимость равномерного распределения волокна по объему определяют потребность в вяжущем, порядок и режимы перемешивания компонентов. Зерновой состав заполнителя может быть представлен как отсевом дробления, так и природным песком или смесью этих материалов. Требования к свойствам песчаного дисперсно-армированного асфальтобетона формируются в зависимости от условий эксплуатации и расположения слоя этого материала в конструкции дорожной одежды. Дисперсно-армированный песчаный асфальтобетон может выполнять функции сверхтонкого защитного слоя, выравнивающего слоя или трещинопрерывающей прослойки, слоя, устойчивого к усталостному трещинообразованию
Catalog of dessins d'enfants with \le 4 edges
In this work all the dessins d'enfant with no more than 4 edges are listed
and their Belyi pairs are computed. In order to enumerate all dessins the
technique of matrix model computations was used. The total number of dessins is
134; among them 77 are spherical, 53 of genus 1 and 4 of genus 2. The orders of
automorphism groups of all the dessins are also found.
Dessins are listed by the number of edges. Dessins with the same number of
edges are ordered lexicographically by their lists of 0-valencies. The
corresponding matrix model for any list of 0-valencies is given and computed.
Complex matrix models for dessins with 1 -- 3 edges are used. For the dessins
with 4 edges we use Hermitian matrix model, correlators for which are computed
in [1].Comment: 64 pages, 134 figure
Adaptation of the Landau-Migdal Quasiparticle Pattern to Strongly Correlated Fermi Systems
A quasiparticle pattern advanced in Landau's first article on Fermi liquid
theory is adapted to elucidate the properties of a class of strongly correlated
Fermi systems characterized by a Lifshitz phase diagram featuring a quantum
critical point (QCP) where the density of states diverges. The necessary
condition for stability of the Landau Fermi Liquid state is shown to break down
in such systems, triggering a cascade of topological phase transitions that
lead, without symmetry violation, to states with multi-connected Fermi
surfaces. The end point of this evolution is found to be an exceptional state
whose spectrum of single-particle excitations exhibits a completely flat
portion at zero temperature. Analysis of the evolution of the temperature
dependence of the single-particle spectrum yields results that provide a
natural explanation of classical behavior of this class of Fermi systems in the
QCP region.Comment: 26 pages, 14 figures. Dedicated to 100th anniversary of A.B.Migdal
birthda
Vibration induced memory effects and switching in ac-driven molecular nanojunctions
We investigate bistability and memory effects in a molecular junction weakly
coupled to metallic leads with the latter being subject to an adiabatic
periodic change of the bias voltage. The system is described by a simple
Anderson-Holstein model and its dynamics is calculated via a master equation
approach. The controlled electrical switching between the many-body states of
the system is achieved due to polaron shift and Franck-Condon blockade in the
presence of strong electron-vibron interaction. Particular emphasis is given to
the role played by the excited vibronic states in the bistability and
hysteretic switching dynamics as a function of the voltage sweeping rates. In
general, both the occupation probabilities of the vibronic states and the
associated vibron energy show hysteretic behaviour for driving frequencies in a
range set by the minimum and maximum lifetimes of the system. The consequences
on the transport properties for various driving frequencies and in the limit of
DC-bias are also investigated.Comment: 15 pages, 20 figures, published versio
Stripes, Vibrations and Superconductivity
We propose a model of a spatially modulated collective charge state of
superconducting cuprates. The regions of higher carrier density (stripes) are
described in terms of Luttinger liquids and the regions of lower density as a
two-dimensional interacting bosonic gas of d_{x^2-y^2} hole pairs. The
interactions among the elementary excitations are repulsive and the transition
to the superconducting state is driven by decay processes. Vibrations of the
CCS and the lattice, although not participating directly in the binding
mechanism, are fundamental for superconductivity. The superfluid density and
the lattice have a strong tendency to modulation implying a still unobserved
dimerized stripe phase in cuprates. The phase diagram of the model has a
crossover from 1D to 2D behavior and a pseudogap region where the amplitude of
the order parameters are finite but phase coherence is not established. We
discuss the nature of the spin fluctuations and the unusual isotope effect
within the model.Comment: 51 pages, 20 figures. Post-March Meeting version: New references are
added, some of the typos are corrected, and a few new discussions are
include
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