Negative magneto-resistance of electron gas in a quantum well with parabolic potential

We have studied the electrical conductivity of the electron gas in parallel electric and magnetic fields directed along the plane of a parabolic quantum well (across the profile of the potential). We found a general expression for the electrical conductivity applicable for any magnitudes of the magnetic field and the degree of degeneration of the electron gas. A new mechanism of generation of the negative magnetoresistance has been revealed. It has been shown that in a parabolic quantum well with a non-degenerated electron gas the negative magnetoresistance results from spin splitting of the levels of the size quantization.Comment: 15 pages, 3 figure

Positive Magneto-Resistance in Quasi-1D Conductors

We present here a simple qualitative model that interpolates between the high and low temperature properties of quasi-1D conductors. At high temperatures we argue that transport is governed by inelastic scattering whereas at low temperatures the conductance decays exponentially with the electron dephasing length. The crossover between these regimes occurs at the temperature at which the elastic and inelastic scattering times become equal. This model is shown to be in quantitative agreement with the organic conductor $TTT_2I_{3-\delta}$. Within this model, we also show that on the insulating side, the positive magnetoresistance of the form $(H/T)^2$ observed in $TTT_2I_{3-\delta}$ and other quasi-1D conductors can be explained by the role spin-flip scattering plays in the electron dephasing rate.Comment: 4 pages, Latex, no figure

Stimulated Raman Scattering in Melilite‐Type Crystals Ca2MgSi2O7 and Ca2Ga2SiO7

χ(3)‐nonlinear optical interactions in two melilite‐type stimulated Raman scattering (SRS)‐active non‐centrosymmetric crystals, Ca2MgSi2O7 and Ca2Ga2SiO7, formerly known as Nd3+‐laser media, are presented. Under picosecond pumping at 1.064 and 0.532 µm cascaded and cross‐cascaded effects occur in these tetragonal silicates. Besides the SRS‐promoting phonon modes with energy of ωSRS1 ≈ 908 cm−1 and ωSRS2 ≈ 668 cm−1 for Ca2MgSi2O7, and ωSRS1 ≈ 720 cm−1 and ωSRS2 ≈ 550 cm−1 for Ca2Ga2SiO7, respectively, combined phonon modes are observed. For Ca2MgSi2O7 new data in a broad wavelength range of refractive indices and their dispersion are given as well. The observed χ(3)‐nonlinear properties expand the functionality of the studied silicates and foreshadow their use in self‐frequency Raman laser converters (self‐SRS lasers)

On the propagation of Voigt waves in energetically active materials

If a dissipative anisotropic dielectric material, characterized by the permittivity matrix $\underline{\underline{\epsilon}}$, supports Voigt-wave propagation, then so too does the analogous active material characterized by the permittivity matrix $\underline{\underline{{\tilde{\epsilon}}}}$, where $\underline{\underline{{\tilde{\epsilon}}}}$ is the hermitian conjugate of $\underline{\underline{\epsilon}}$. Consequently, a dissipative material that supports Voigt-wave propagation can give rise to a material that supports the propagation of Voigt waves with attendant linear gain in amplitude with propagation distance, by infiltration with an active dye

Comparative characterization of different kinds of chromatographic quantification using the double standard addition method

Различные варианты обработки результатов количественного газохроматографического анализа способом двойной стандартной добавки сопоставлены по точности. Три основных из них: I – простое сравнение данных, получаемых с использованием однократной и двойной добавок, II – аппроксимация зависимости m(S) в координатах «площадь пика определяемого компонента» (S) – «масса добавки» (mдоб) методом наименьших квадратов по уравнению линейной регрессии и III–вычисление количеств определяемых компонентов (mx) по каждой из стандартных добавок с последующей линейной экстраполяцией их значений на «нулевую» стандартную добавку, mx(mдоб® 0). Показано, что результаты определений в различных вариантах стандартных добавок сопоставимы по точности, но несколько занижены относительно заданных количеств аналитов. Главной причиной таких систематических погрешностей является испарение растворителя при последовательном дозировании проб одних и тех же образцов в хроматограф. В результате площади пиков, определяемые после ввода стандартных добавок в образцы, оказываются несколько завышенными, что и приводит к занижению результатов. Второй (менее значимый) фактор – незначительное увеличение объема образцов за счет добавок определяемых компонентов. Отмечено, что погрешности определений различными вариантами способа стандартной добавки не превышают случайных составляющих погрешностей. Лучшие результаты (с учетом знаков отклонений) обеспечивает вычисление содержания определяемого аналита методом двойной стандартной добавки с экстраполяцией результатов на «нулевую» величину добавки. Для исключения влияния «человеческого фактора» (увеличение точности результатов в ходе анализа серий однотипных образцов за счет опыта аналитиков) все параллельные определения были проведены студентами бакалавриата Института химии Санкт-Петербургского государственного университета в ходе выполнения ими лабораторных работ. Такая орга­низация экспериментов повышает их достоверность, поскольку исключает зависимость результатов от различий в квалификации аналитиков.Different algorithms for processing the quantitative gas chromatographic analysis data using the double standard addition method are compared for their accuracy. Three principal approaches are possible for such processing: I – simple comparison of values determined by single and double standard additions, II – approximation of «peak area of analyte» (S) – «mass of standard addition» (madd) dependence by the least squares method [linear regression, m(S)], and III – independent quantification of analyte with both standard additions followed by the linear extrapolation of two subresults on the socalled «zero standard addition», mx(madd ® 0). It is concluded that the quantitation results obtained using the various modes of the method are comparable in accuracy, but somewhat underestimated relative to the specified amounts of analytes. The principal reason of such systematic errors is the evaporation of the solvent during the successive injecting of the same samples into the gas chromatograph. Due to this reason the peak areas, measured after the standard addition, appear to be slightly increased and this leads to the systematic underestimation of the results. The second (less important) factor is the small increase of the sample volumes due to the addition of the components to be determined. It is confirmed that the systematic errors of different modes of standard addition are not exceeding the values of their random uncertainties. The optimal results (considering their signs of deviations) are provided using the double standard addition method with extrapolation of subresults on «zero standard addition». In order to exclude the possible influence of «human factor» (increasing the re­sults precision during the series of analyses of similar samples due to the rising experience of analytical chemists) all parallel measurements have been per­for­med by bachelor students of the Chemistry Institute of the St. Petersburg State University in the course of their laboratory practical works in chromatography. Such organization of experiments increases their credibility as it excluded the dependence of the results on the qualification of chemists.Студенческая лабораторная работа, результаты которой составили предмет настоящего сообщения, выполнена с использованием оборудования Ресурсного Центра «Методы анализа состава вещества» Санкт-Петербургского государственного университета. Авторы благодарят сотрудников Центра за содействие.The students’ work, results of which are discussed in the current paper, was carried out using the equipment of the “Methods of analysis of substance’s composition” Resource Centre at the St. Petersburg State University. The authors are grateful to the staff of this Center for assistance