Distinction between the Poole-Frenkel and tunneling models of electric field-stimulated carrier emission from deep levels in semiconductors

The enhancement of the emission rate of charge carriers from deep-level defects in electric field is routinely used to determine the charge state of the defects. However, only a limited number of defects can be satisfactorily described by the Poole-Frenkel theory. An electric field dependence different from that expected from the Poole-Frenkel theory has been repeatedly reported in the literature, and no unambiguous identification of the charge state of the defect could be made. In this article, the electric field dependencies of emission of carriers from DX centers in AlxGa1-xAs:Te, Cu pairs in silicon, and Ge:Hg have been studied applying static and terahertz electric fields, and analyzed by using the models of Poole-Frenkel and phonon assisted tunneling. It is shown that phonon assisted tunneling and Poole-Frenkel emission are two competitive mechanisms of enhancement of emission of carriers, and their relative contribution is determined by the charge state of the defect and by the electric-field strength. At high-electric field strengths carrier emission is dominated by tunneling independently of the charge state of the impurity. For neutral impurities, where Poole-Frenkel lowering of the emission barrier does not occur, the phonon assisted tunneling model describes well the experimental data also in the low-field region. For charged impurities the transition from phonon assisted tunneling at high fields to Poole-Frenkel effect at low fields can be traced back. It is suggested that the Poole-Frenkel and tunneling models can be distinguished by plotting logarithm of the emission rate against the square root or against the square of the electric field, respectively. This analysis enables one to unambiguously determine the charge state of a deep-level defect

Muon Spin Relaxation Study of (La, Ca)MnO3

We report predominantly zero field muon spin relaxation measurements in a series of Ca-doped LaMnO_3 compounds which includes the colossal magnetoresistive manganites. Our principal result is a systematic study of the spin-lattice relaxation rates 1/T_1 and magnetic order parameters in the series La_{1-x}Ca_xMnO_3, x = 0.0, 0.06, 0.18, 0.33, 0.67 and 1.0. In LaMnO_3 and CaMnO_3 we find very narrow critical regions near the Neel temperatures T_N and temperature independent 1/T_1 values above T_N. From the 1/T_1 in LaMnO_3 we derive an exchange integral J = 0.83 meV which is consistent with the mean field expression for T_N. All of the doped manganites except CaMnO_3 display anomalously slow, spatially inhomogeneous spin-lattice relaxation below their ordering temperatures. In the ferromagnetic (FM) insulating La_{0.82}Ca_{0.18}MnO_3 and ferromagnetic conducting La_{0.67}Ca_{0.33}MnO_3 systems we show that there exists a bi-modal distribution of \muSR rates \lambda_f and \lambda_s associated with relatively 'fast' and 'slow' Mn fluctuation rates, respectively. A physical picture is hypothesized for these FM phases in which the fast Mn rates are due to overdamped spin waves characteristic of a disordered FM, and the slower Mn relaxation rates derive from distinct, relatively insulating regions in the sample. Finally, likely muon sites are identified, and evidence for muon diffusion in these materials is discussed.Comment: 21 pages, 17 figure

Использование принципа клиентоориентированности при формировании стратегии развития предприятий

Материалы межвуз. науч. конф. студентов, магистрантов и аспирантов, Гомель, 15 мая 2008 г

Новые полимерные ПАВ на основе разветвленных силатрансодержащих полиэфиров

Objectives. Biologically active polymeric surfactants are a new promising class of macromolecules that can find application in medicine, cosmetology, and agriculture. In this study, a number of new biologically active amphiphilic polymers based on branched silatrane-containing polyesters and polyethers were obtained, and their surface-active properties were investigated.Methods. The branched polymers were represented by polyethers and polyesters, obtained respectively via the anionic polymerization of 1,2-epoxypropanol or a combination of equilibrium polycondensation and ring opening polymerization. The polymers were modified with 3-isocyanopropylsilatrane and trimethylethoxysilane to obtain the amphiphilic compounds containing silatrane groups bonded to the polymer backbone by the urethane bond. The structure of the synthesized polymer silatranes was confirmed via nuclear magnetic resonance spectroscopy and gel permeation chromatography. The surface active properties of all the copolymers obtained were investigated in connection with their obvious amphiphilicity. In particular, the formation of micelles in aqueous solutions is such a property. The critical micelle concentrations were determined by a method of quenching the fluorescence of the polymers.Results. It was shown that the values of the critical micelle concentrations and the hydrophilic-lipophilic balance values of polymers determined by the Griffin equation correlate well with each other. A linear relationship between the hydrophilic-lipophilic balance and the critical micelle concentrations was established. At the same time, polyether-based polymers generally showed higher critical micelle concentrations than polyester-based polymers, although the hydrophilic-lipophilic balance values for polymers of different series, but with close degrees of substitution, were close. It was found that the use of all synthesized polymers as stabilizers of direct and reverse emulsions leads to an increase in the aggregative stability of both types of emulsions. The stability of emulsions depended both on the degree of substitution of peripheral hydroxyl groups of polymers by silatranes and on the molecular weight and structure of the branched block of polymers. The stability of direct emulsions increased for all polymers, while that of inverse emulsions decreased with an increasing degree of substitution of hydroxyl groups by silatranes. The increase of the branched block molecular weight led to an increase of droplet sizes for both direct and inverse emulsions. The smallest droplet size for direct and inverse emulsions was obtained using polymers with low molecular weight branched polyester blocks as surfactants.Conclusions. The results obtained prove the possibility of creating polymer surfactants containing silatrane groups. By varying the structure of the polymer, its molecular weight and the degree of substitution of peripheral functional groups, it is possible to obtain surfactants with desired surface properties.Цели. Биологически активные полимерные ПАВ являются новым многообещающим классом макромолекул, которые могут найти применение в медицине, косметологии, сельском хозяйстве. В данном исследовании был получен ряд новых амфифильных полимеров на основе разветвленных силатран-содержащих полиэфиров и исследованы их поверхностно-активные свойства.Методы. Разветвленные полимеры были представлены простыми и сложными полиэфирами, которые получали соответственно способом анионной полимеризации 1,2-эпоксипропанола либо комбинацией равновесной поликонденсации и полимеризации с раскрытием цикла. Для получения амфифильных соединений, содержащих силатрановые группы, связанные с полимерным каркасом уретановой связью, полимеры были модифицированы 3-изоцианопропилсилатраном и триметилэтоксисиланом. Структура синтезированных полимерных силатранов была подтверждена методами ЯМР-спектроскопии и гель-проникающей хроматографии. Поверхностно-активные свойства всех полученных сополимеров были исследованы в связи с их очевидной амфифильностью, в частности, таким свойством является образование мицелл в водных растворах. Методом гашения флуоресценции полимеров были определены величины критических концентраций мицеллообразования (ККМ).Результаты. Показано, что величины ККМ и определенные в соответствие с уравнением Гриффина величины гидрофильно-липофильного баланса (ГЛБ) для полимеров коррелируют, при этом была установлена линейная зависимость между указанными величинами. Полимеры на основе простых полиэфиров в целом показывали более высокие значения ККМ, чем полимеры на основе сложных полиэфиров, хотя величины ГЛБ для полимеров разных серий, но с близкими степенями замещения были близки. Было обнаружено, что использование всех синтезированных полимеров в качестве стабилизаторов прямых и обратных эмульсий приводит к увеличению агрегативной устойчивости эмульсий обоих типов. Устойчивость эмульсий зависела как от степени замещения периферийных гидроксильных групп полимеров силатранами, так и от молекулярной массы и строения разветвленного блока полимеров. Для всех полимеров устойчивость прямых эмульсий возрастала, а обратных эмульсий – снижалась с увеличением степени замещения гидроксильных групп силатранами. С увеличением молекулярной массы разветвленного блока размеры капель как прямых, так и обратных эмульсий увеличивались. Наименьший размер капель прямой и обратной эмульсии был получен при использовании в качестве ПАВ полимеров с низкомолекулярными разветвленными блоками на основе сложных эфиров.Заключение. Полученные результаты показывают возможность создания полимерных ПАВ, содержащих силатрановые группы. Варьируя строение полимера, его молекулярную массу и степень замещения периферийных функциональных групп, возможно получение ПАВ с заданными поверхностными свойствами

Identification of IKr Kinetics and Drug Binding in Native Myocytes

Determining the effect of a compound on IKr is a standard screen for drug safety. Often the effect is described using a single IC50 value, which is unable to capture complex effects of a drug. Using verapamil as an example, we present a method for using recordings from native myocytes at several drug doses along with qualitative features of IKr from published studies of HERG current to estimate parameters in a mathematical model of the drug effect on IKr. IKr was recorded from canine left ventricular myocytes using ruptured patch techniques. A voltage command protocol was used to record tail currents at voltages from −70 to −20 mV, following activating pulses over a wide range of voltages and pulse durations. Model equations were taken from a published IKr Markov model and the drug was modeled as binding to the open state. Parameters were estimated using a combined global and local optimization algorithm based on collected data with two additional constraints on IKrI–V relation and IKr inactivation. The method produced models that quantitatively reproduce both the control IKr kinetics and dose dependent changes in the current. In addition, the model exhibited use and rate dependence. The results suggest that: (1) the technique proposed here has the practical potential to develop data-driven models that quantitatively reproduce channel behavior in native myocytes; (2) the method can capture important drug effects that cannot be reproduced by the IC50 method. Although the method was developed for IKr, the same strategy can be applied to other ion channels, once appropriate channel-specific voltage protocols and qualitative features are identified