Spin dynamics and magnetic-field-induced polarization of excitons in ultrathin GaAs/AlAs quantum wells with indirect band gap and type-II band alignment

The exciton spin dynamics are investigated both experimentally and theoretically in two-monolayer-thick GaAs/AlAs quantum wells with an indirect band gap and a type-II band alignment. The magnetic-field-induced circular polarization of photoluminescence, $P_c$, is studied as function of the magnetic field strength and direction as well as sample temperature. The observed nonmonotonic behaviour of these functions is provided by the interplay of bright and dark exciton states contributing to the emission. To interpret the experiment, we have developed a kinetic master equation model which accounts for the dynamics of the spin states in this exciton quartet, radiative and nonradiative recombination processes, and redistribution of excitons between these states as result of spin relaxation. The model offers quantitative agreement with experiment and allows us to evaluate, for the studied structure, the heavy-hole $g$ factor, $g_{hh}=+3.5$, and the spin relaxation times of electron, $\tau_{se} = 33~\mu$s, and hole, $\tau_{sh} = 3~\mu$s, bound in the exciton.Comment: 17 pages, 16 figure

Анализ механизма перехода в предельное состояние жилых домов серии 111 при Спитакском землетрясении 1988 г.

The study aimed at investigating the reaction of aresidential building (Series 111 nine-story block section) to seismic impacts that can lead to destruction of the building. The article presents the results of the retrospective analysis of the seismic resistance of nine-story block sections in the city of Gyumri (Leninakan) under the influence of the 07.12.1988 Spitak earthquake. The seismic resistance of the block section was calculated using the ‘pushover analysis’ method. Direct dynamic calculations were performed using a model of complex elastoplastic deformation of a mechanical system with two degrees of freedom. An external impact was represented by the earthquake records taken at the Ghukasyan station. Macroseismic survey data collected after the 1988 earthquake are presented. The block sections of Series 111 multistory residential buildings were designedbythe Armenian research and design institutions ArmNIISA and Armgosproekt in 1975 for construction in the Armenian Republic on sites with seismicity of 7–8 points. Vibration tests were conducted to test the strength of anine-storey block section in Yerevan in 1976, and it was discovered that the actual seismic resistance of the building did not comply with the current earthquake-resistant construction standards. However, the test results were ignored. On December 7, 1988, the earthquake took place, and the Series 111 buildings collapsed in the cities of Gyumri (Leninakan) and Vanadzor (Kirovokan). In the State Commission Report regarding the Series 111 design, it was concluded that considering the damage due to the violations of the design and construction standards and taking into account the inspection calculations, the accepted design solutions of the residential buildings did not meet the requirements of the regulatory documents for buildings designed to sustain seismicity of 8 points. Such design should not be used for seismic areas and must be critically revised. The retrospective analysis performed by the authors of this article on the basis of experimental data not only confirmed the conclusions of the State Commission, but also made it possible to establish a mechanism for the transition of Series 111 block-sections to the limit state. Unfortunately, the state management of the construction industry failed to use this information and did not take any urgent preventive actions.In fact, the authoritiesof the USSR supported the position of the Gosstroy of Armenia and allowed them to continue the construction of Series 111 residential buildings with minor changes. It was impossible to prevent the Spitak earthquake, but proper preventive actions could have reduced the amount of social and economic damage incurred due to the earthquake. The consequences of obvious design and construction errors discovered 12 years before the Spitak earthquake were ‘written off’and explained by the natural disaster, including underestimated standard seismicity, features of the engineering-geological settings, and the unfavorable spectrum of theactual impact. Thousands of people lost their life, and huge material damage was caused due to the loss of Series 111 residential buildings (300000square meters). Such was the price of professional incompetence and departmental ambitions.Целью настоящей статьи является исследование реакции 9-этажной блок-секции серии 111 на сейсмические воздействия вплоть до разрушения. Изложены результаты ретроспективного анализа сейсмостойкости 9-этажных блок-секций в г. Гюмри (Ленинакане) под воздействием Спитакского землетрясения 07.12.1988 г. Выполнены расчеты сейсмостойкости блок-секции по методу «pushoveranalysis», а также прямой динамический расчет модели сложного упругопластического деформирования механической системы с двумя степенями свободы. В качестве внешнего воздействия использованы записи землетрясения на ст. Гукасян. Приведены данные макросейсмического обследования последствий землетрясения 1988 г. Типовые проекты блок-секций многоэтажных жилых домов серии 111, разработанные в 1975 г. в АрмНИИСА и Армгоспроекте, предназначались для строительства в Армянской ССР на площадках с сейсмичностью 7–8 баллов. Проведенные в 1976 г. в г. Ереване вибрационные испытания 9-этажной блок-секции жилого дома серии 111 показали, что фактическая сейсмостойкость здания не соответствует действующим нормам. Игнорирование результатов вибрационных испытаний привело к тотальному обрушению домов этой серии в городах Гюмри (Ленинакан) и Ванадзор (Кировакан) 7 декабря 1988 г. В Заключении Государственной комиссии в отношении проектов серии 111 отмечено, что характер нарушений норм и данные поверочных расчетов свидетельствуют, что принятые конструктивные решения рассмотренных жилых домов не отвечают требованиям нормативных документов для зданий с расчетной сейсмичностью 8 баллов. Проекты не могут применяться для строительства в сейсмических районах без коренной переработки. Результаты ретроспективного анализа, выполненного авторами статьи на основе экспериментальных данных, не только подтвердили выводы Государственной комиссии, но и позволили установить механизм перехода блок-секции серии 111 в предельное состояние. К сожалению, тогдашнее руководство строительной отраслью страны не сумело использовать эту информацию для принятия срочных превентивных мер. Союзные органы фактически поддержали позицию Госстроя Армении, разрешив продолжить строительство жилых домов серии 111 с незначительными замечаниями. Предотвратить Спитакское землетрясение было невозможно, но социальный и экономический ущерб от него можно было уменьшить за счет превентивных мероприятий. Последствия очевидных проектных и строительных ошибок, вскрытых за 12 лет до Спитакского землетрясения, списали за счет стихии: заниженной нормативной сейсмичности, особенностей инженерно-геологического строения, неблагоприятного спектрального состава воздействия, что действительно имело место. В итоге – тысячи погибших и материальный ущерб от потери жилищного фонда серии 111 в объеме 300 тыс. м2. Такова цена профессиональной некомпетентности и ведомственных амбиций

Relationships between the seasonal dynamics of soil fungi biomass and environmental factors in predominating forest types in the Bryansk woodlands (European Russia)

Being the crucial part of the forest soil's microbial pool, soil fungi in general and mycorrhizal fungi in particular are an important study object when it comes to forest ecosystems sustainability and preservation. Thus, the study of ectomycorrhizal fungi has been carried out in the Bryanskiy Les State Nature Biosphere Reserve, located in the south-eastern part of the Bryansk woodlands (European Russia). Forest types featured in the study are the local predominating types, namely green-moss-fructiculose pine forests and polydominant deciduous broadleaved nemoral-herbaceous forests with spruce. This study was aimed to assess seasonal dynamics of soil fungi' biomass overall and ectomycorrhizal fungi in particular over the course of the 2017 vegetation period (May – November) and its dependence on biotic and abiotic environmental factors, such as soil water content, temperature and vegetation. The vegetation period was divided into three periods of observation, namely an early (May – July), middle (July – September) and late (September – November) one. The method used to assess the fungal biomass was direct microscopic observation using the fluorescein diacetate staining. In order to estimate the ectomycorrhizal fungi biomass separately, trenching and in-growth mesh bags were employed. The obtained results suggest that the soil fungi biomass steadily increases over the vegetation period in both studied forest types. This is mostly affected by the forest type, available water amount and seasonal changes, while the temperature's impact is less pronounced. On average, the soil fungi biomass was higher in broadleaved forests than in pine forests (2.288 mg C × g-1 soil vs. 1.672 mg C × g-1 soil, respectively), with non-ectomycorrhizal component having comparable biomass. The dynamics of biomass differed in the two forest types. However, noticeable differences (p < 0.1) between the two forest types have only been recorded during the July – September period. The biomass of ectomycorrhizal fungi is smaller than the biomass of non-mycorrhizal fungi, but at the same time it is less affected by changes in moisture. Besides that, the study has shown that the forest litter characteristics can greatly affect the dynamics of the fungal biomass

Analysis of the mechanism of transition to the limit state of Series 111 residential buildings during the 1988 Spitak earthquake

The study aimed at investigating the reaction of aresidential building (Series 111 nine-story block section) to seismic impacts that can lead to destruction of the building. The article presents the results of the retrospective analysis of the seismic resistance of nine-story block sections in the city of Gyumri (Leninakan) under the influence of the 07.12.1988 Spitak earthquake. The seismic resistance of the block section was calculated using the ‘pushover analysis’ method. Direct dynamic calculations were performed using a model of complex elastoplastic deformation of a mechanical system with two degrees of freedom. An external impact was represented by the earthquake records taken at the Ghukasyan station. Macroseismic survey data collected after the 1988 earthquake are presented. The block sections of Series 111 multistory residential buildings were designedbythe Armenian research and design institutions ArmNIISA and Armgosproekt in 1975 for construction in the Armenian Republic on sites with seismicity of 7–8 points. Vibration tests were conducted to test the strength of anine-storey block section in Yerevan in 1976, and it was discovered that the actual seismic resistance of the building did not comply with the current earthquake-resistant construction standards. However, the test results were ignored. On December 7, 1988, the earthquake took place, and the Series 111 buildings collapsed in the cities of Gyumri (Leninakan) and Vanadzor (Kirovokan). In the State Commission Report regarding the Series 111 design, it was concluded that considering the damage due to the violations of the design and construction standards and taking into account the inspection calculations, the accepted design solutions of the residential buildings did not meet the requirements of the regulatory documents for buildings designed to sustain seismicity of 8 points. Such design should not be used for seismic areas and must be critically revised. The retrospective analysis performed by the authors of this article on the basis of experimental data not only confirmed the conclusions of the State Commission, but also made it possible to establish a mechanism for the transition of Series 111 block-sections to the limit state. Unfortunately, the state management of the construction industry failed to use this information and did not take any urgent preventive actions.In fact, the authoritiesof the USSR supported the position of the Gosstroy of Armenia and allowed them to continue the construction of Series 111 residential buildings with minor changes. It was impossible to prevent the Spitak earthquake, but proper preventive actions could have reduced the amount of social and economic damage incurred due to the earthquake. The consequences of obvious design and construction errors discovered 12 years before the Spitak earthquake were ‘written off’and explained by the natural disaster, including underestimated standard seismicity, features of the engineering-geological settings, and the unfavorable spectrum of theactual impact. Thousands of people lost their life, and huge material damage was caused due to the loss of Series 111 residential buildings (300000square meters). Such was the price of professional incompetence and departmental ambitions