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

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

Radiation-Induced "Zero-Resistance State" and the Photon Assisted Transport

We demonstrate that the radiation induced "zero-resistance state" observed in a two-dimensional electron gas is a result of the non-trivial structure of the density of states of the systems and the photon assisted transport. A toy model of a structureless quantum tunneling junction where the system has oscillatory density of states catches most of the important features of the experiments. We present a generalized Kubo-Greenwood conductivity formula for the photon assisted transport in a general system, and show essentially the same nature of the transport anomaly in a uniform system.Comment: 4 pages, 3 figures. Please send comment to [email protected]. This version added a paragraph to discuss the implication of negative conductanc

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.

Efficacy of one-step and multi-step polishing systems in finishing direct composite restoration: a non-randomised controlled experimental trial

Background. The variety of polishing systems and tools available for finishing direct composite restorations may perplex the dentist.Objectives. An effect evaluation in one- and multi-step composite polishing tools using model specimens of GC Gradia Direct and 3M ESPE Filtek Ultimate restoratives.Methods. The prepared specimens of GC Gradia Direct and 3M ESPE Filtek Ultimate restorative composites were exposed in laboratory to the Kenda Maximus, Dentsply PoGo, Kagayaki RoundFlex, Shofu Super-Snap, Kagayaki Ensmart Pin, EVE Composoft polishing systems and Daiyamondo Kagayaki paste. The polishing systems distinguished by brand, technical and performance characters.Surface microgeometry in all 16 samples was estimated in the Laboratory of Optical Metrology, Institute of Design and Technology for Scientific Instrument Engineering, Novosibirsk, using a MNP-1 light interferometric nanoprofile microscope and at the shared core facilities of the Research and Education Centre “Nanomaterial Diagnostics and Properties” of Kuban State University, Krasnodar, using a JEOL JSM-7500F scanning electron microscope.Results. Optical profilometry and scanning electron microscopy were used to estimate surface roughness in 16 specimens exposed to polishing for one minute. The polishing tools Kenda Maximus, Dentsply PoGo, Kagayaki RoundFlex, Shofu Super-Snap, Kagayaki Ensmart Pin, EVE Composoft, as well as Daiyamondo Kagayaki paste used in instrumental polishing, revealed a varied performance. Optical profilometry exhibited the GC Gradia Direct and Filtek Ultimate specimens to possess the lowest average roughness in two cases: 1) after multi-step polishing with Kagayaki Ensmart Pin tools with Daiyamondo Kagayaki paste (average roughness corresponded to Sa — 0.214 pm in GC Gradia Direct and Sa — 0.248 pm — in Filtek Ultimate), 2) in application of the Kenda Maximus monopolishing tool (roughness values of Sa — 0.211 and Sa — 0.242 pm, respectively). Surface roughness after multi-step machining with EVE Composoft silicone polishers was average Sa — 0.579 and Sa — 0.549 pm in both samples and was reported the highest. Scanning electron microscopy confirmed the optical profilometry estimates.Conclusion. The assay showed that the specimen machining with a sole Kenda Maximus diamond abrasive tool and several Kagayaki Ensmart Pin silicone polishing heads followed by a Daiyamondo Kagayaki diamond abrasive paste application produced very similar surface roughness values, which were graded the lowest with GC Gradia Direct and 3M ESPE Filtek Ultimate composite samples using scanning electron microscopy and optical profilometry

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

Plasticity and decomposition of whiskers on electric-induced deformation

The purpose of the work is to study the influence of weak electric fields (1–10 V/cm) on the micro and macroplasticity of whisker crystals of silver azide. The paper considers the processes occurring in the crystals of silver azide on the indentation in noncontact electric field. One of the processes is the indenter-induced movement of unfixed dislocations, which is the evidence of crystals microplasticity, followed by the accumulation of dislocations at the impediments of different nature. Further, this causes the change in linear dimensions of the crystals and the return to original dimensions, which is the evidence of macroplasticity. After deformation, a greater number of dislocations are formed, which stops the formation of the reaction areas where outgassing is observed. A method for controlling the whiskers stability and reactivity using the microindentation in electric field is proposed

Influence irradiation argon ion SnO[2] on optical and electrical characteristics

Tin oxide in the form of films has been deposited by reactive magnetron sputtering on glass substrates a room temperature. Process was carried out in such mode when the deposited films were conductive. The deposited films were irradiated with argon ions. Have been studied happening at that the changes optical and electric properties of films. Have been investigated optical properties of films in the range of 300-1100 nanometers by means of photometry. For research structure of films was used the x-ray diffractometry. Diffractometric researches have shown that the films deposited on a substrate have crystal structure from shares of a quasicrystal phase and after influence of argon ions she completely became quasicrystal. It is established that change transmission of a film correlates with change her electric resistance. Average value transmission in the range of 380-1100 nanometers as well as the electric resistance of a film with growth of irradiation time increases to the values exceeding initial. At the same time at irradiation time ~ 13,2 sec. are observed their slight decrease. To this value of irradiation time there corresponds the minimum value of electric resistance and transmission films. Change of transmission coefficient correlates with change of surface resistance