THz parametric gain in semiconductor superlattices in the absence of electric domains

We theoretically show that conditions for THz gain and conditions for formation of destructive electric domains in semiconductor superlattices are fairly different in the case of parametric generation and amplification. Action of an unbiased high-frequency electric field on a superlattice causes a periodic variation of energy and effective mass of miniband electrons. This parametric effect can result in a significant gain at some even harmonic of the pump frequency without formation of electric domains and corruption from pump harmonics.Comment: 4 pages, 3 figures. Accepted to Appl. Phys. Let

ВНУТРЕННЕЕ ЭЛЕКТРИЧЕСКОЕ ПОЛЕ, ЗАХВАТЫВАЮЩИЕ ЭЛЕКТРОНЫ ПУСТОТЫ, МЕРТВЫЕ ЭЛЕКТРОНЫ И УВЕЛИЧЕНИЕ ЭФФЕКТИВНОСТИ ПОЛИМЕРНЫХ СОЛНЕЧНЫХ ЭЛЕМЕНТОВ ПРИ ФТОРИРОВАНИИ

We propose a model that allows an understanding of the nature of electron traps in π-onjugated polymers that are used in photovoltaic devices. It is assumed that the free-cavity voids in the polar π-conjugated polymer exhibit electron-accepting affinity and are filled with electrons, called  erein as ‘dead’ electrons because they are hold by a static random internal electric field and are not driven to the cathode by external built-in voltage. As a result, the dead electrons into electron-trapping voids are unsuitable for use in the external circuit of organic solar cells. As justified here, the exciton dissociation on the surfaces of voids, the capture of drift electrons by voids, the emerging of the dead electrons and their non-geminate recombination are the main obstacles to create highly efficient polymer solar cells. The model of dead electrons allows explaining the increase in the power conversion efficiency of solar cells caused by the polymer fluorination, side-chain polymer manipulation, and dopant-controlled trap-filling. Some characteristics of hybrid P3HT:CdSe solar cells are also analyzed with the help of this model.Предложена модель, позволяющая понять природу электронных ловушек в π-сопряженных полимерах, которые используются в фотоэлектрических устройствах. Предполагается, что пустоты со свободными полостями в полярном π-сопряженном полимере проявляют электроноакцепторные свойства и заполняются электронами, которые названы здесь «мертвыми» электронами, так как они удерживаются статическим случайным внутренним электрическим полем и не направляются к катоду с помощью внешнего напряжения. В результате, захваченные пустотами мертвые электроны непригодны для использования во внешней цепи органических солнечных элементов. Обосновано, что диссоциация экситонов на поверхности пустот, захват электронов пустотами при дрейфе к катоду, появление мертвых электронов и их негеминальная рекомбинация являются главными препятствиями при создании высокоэффективных полимерных солнечных элементов. Модель мертвых электронов позволяет объяснить увеличение эффективности преобразования энергии солнечных элементов, вызванное фторированием полимера, изменением боковых цепей полимера и наполнением ловушек добавками растворителей. Некоторые характеристики гибридных P3HT:CdSe солнечных элементов также анализируются с помощью этой модели.

Theoretical backgrounds of nonlinear THz spectroscopy of semiconductor superlattices

We consider terahertz absorption and gain in a single miniband of semiconductor superlattice subject to a bichromatic electric field in the most general case of commensurate frequencies of the probe and pump fields. Using an exact solution of Boltzmann transport equation, we show that in the small-signal limit the formulas for absorption always contain two distinct terms related to the parametric and incoherent interactions of miniband electrons with the alternating pump field. It provides a theoretical background for a control of THz gain without switching to the negative differential conductivity state. For pedagogical reasons we present derivations of formulas in detail.Comment: 14 page

High-Energy X-ray Radiation Registration Model

It is proved the necessity for carrying out computing experiments according to the mean value and the square of registered X-ray photons absorbed energy of in a scintillation detector. The offered imitation model of the transfer and registration of high-energy X-ray radiation in a sensing volume of CsI and CdWO[4] scintillation detectors is based on a Monte Carlo method. The model considers leakage of secondary photons and electrons. It is offered approaches to justification of adequacy of the developed model of a high-energy X-ray radiation registration

Nanosized Borides and Carbides for Electroplating. Metal-Matrix Coatings: Specifications, Performance Evaluation

This paper summarizes experience of application of nano-sized carbides and borides of titanium and chromium, silicon carbide as components of electro-depositable coating compositions based on nickel, zinc, and chromium. Basic physical and mechanical properties of the coatings are determined. Technological and economic evaluation is completed; practicability of high-cost nano-diamonds substitution for nano-sized borides and carbides is justified

Use of Technogenic Silica Fume and Brown Coal Semi-Coke in the Technology of Silicon Carbide

The paper describes thermodynamic experiments to determine the optimal temperature and time modes for the carbide production process from the briquette charge comprising silica fume and brown coal semi-coke, conditions for chemical enriching of silicon carbide, its phase, chemical and granulometric compositions and particle morphology

ЗАТУХАНИЕ S1-СОСТОЯНИЯ ПЕРИДИНИНА В СРЕДАХ РАЗЛИЧНОЙ ПОЛЯРНОСТИ И КОНФОРМЕРЫ

Theory of nonradiative transitions based on the model of the orientation broadening of electronic levels has been used to interpret the known data for the medium polarity effect on the lifetime of the S1(Ag−) state of peridinin. The findings allow assuming that the molecules of pigment heavily interact with the internal electric field produced by environment. In consequence of this, peridinin at the excited S1(Ag−) state exists in the form of different conformers I and II with permanent dipole moments of 40.7 and 42.8 D, respectively. Their structure is distinguished by turning the epoxycyclohexane ring relative to the polyene chain through about 180°. Conformer I is dominant in nonpolar and low polar media and is at the intramolecular charge transfer state S1(Ag−/ICT), − whereas conformer II exists in polar media and, as suggested, belongs to the charge separated state S1(Ag−/ICT).Теория безызлучательных переходов, основанная на модели ориентационного уширения электронных уровней, использована для интерпретации литературных данных по влиянию полярности среды на длительность S1(Ag−)-состояния каротиноида перидинина. Сделан вывод, что молекулы перидинина сильно взаимодействуют с внутренним электрическим полем, создаваемым окружением. Вследствие этого в возбужденном S1(Ag−)-состоянии перидинин существует в виде двух конформеров I и II. Конформер I доминирует в нейтральных и слабо полярных средах и имеет постоянный дипольный момент (ПДМ) 40,7 Д. Его состояние классифицировано как состояние с внутримолекулярным переносом заряда S1(Ag−/ ICT). Конформер II существует в сильно полярных средах и имеет ПДМ 42,8 Д. Состояние конформера II рассматривается как состояние с разделенным зарядом S1(Ag−/ ICT)

The Role of Outer Membrane Vesicles of Agents of Particularly Dangerous Infections in the Pathogenesis and Immunogenesis of Infectious Process

The literature review is devoted to the modern concepts of the vesiculation phenomenon and its biological role in pathogenic bacteria – causative agents of particularly dangerous human infections. Data on the production, structure, composition, and functions of the outer membrane vesicles (OMV) of bacteria have been summarized. In recent years, the interest of researchers in the formation of spherical structures (so called bubbles or vesicles) from outer membrane of gram-negative bacteria has significantly increased. Such structures are surrounded by the double layer of a phospholipid membrane, the outer layer of which is enriched with lipopolysaccharide molecules. The inner space of vesicles could include various antigens, receptors, adhesins, toxins, enzymes, porins, etc. The formation of vesicles by the outer membranes of bacteria is recognized as a normal physiological manifestation of bacterial activity aimed at adaptation to environmental conditions. The investigation of the biological role of OMV showed their connection with the pathogenesis and immunogenesis of bacterial diseases. The review provides information on the peculiarity of induction, OMV composition and their participation in the processes of patho- and immunogenesis of severe infections caused by groups I–II PBA – the gram-negative causative agents of plague, tularemia, brucellosis, glanders, melioidosis, cholera, and formation of extracellular vesicles in a gram-positive anthrax pathogen. The particular attention is paid to the issue of developing safe and effective next-generation vaccine preparations based on bacterial vesicles