The decay kinetics of low-temperature exciton photoluminescence in a heterostructure with multiple GaN/AlN monolayer quantum wells, which is prepared by molecular beam epitaxy, is studied. Measured ...radiation decay curves are theoretically simulated within a three-level model. The relaxation of dipole-allowed “bright” excitons spatially confined in GaN monolayers is determined as exciton relaxation with a characteristic time of ~3 ps, which is accompanied by spin flip and by transformation to dipole-forbidden “dark” excitons whose levels lie by ~60 meV below in energy. It has been shown that exciton states at temperatures above 50 K are two-dimensional.
To study the features of the combustion process in a 2CH 10.5/12.0 diesel engine when working on methanol with a portion of diesel fuel in accordance with the purpose, research objectives and ...methodology, tests were conducted at the nominal speed mode at a speed of n = 1800 min−1 and at the maximum torque mode at n = 1400 min−1. Indicator diagrams were taken at the optimal setting of the fuel injection angle, with a constant cyclic supply of diesel fuel and a cyclic supply of methanol, which ensures that the value of the average effective pressure is maintained at the level at which the experimental diesel was shown. Based on the results of processing indicator diagrams, heat release graphs were constructed, the analysis of which is presented in this article.
We demonstrate the results of on THz emission from hybrid THz photoconductive antenna loaded with silver nanoantenna. The results of experimental investigations are in a good agreement with numerical ...simulations presented in our recent work. The conversion efficiency reveals over 5-fold improvement at certain frequencies, if compared with similar photoconductive antenna without silver nanoparticles, while previous results for this type of antenna barely exceeded 2-fold conversion efficiency gain. We propose a cost-effective fabrication procedure to realize such hybrid THz antennas with optimized plasmonic nanostructures via thermal dewetting process, which does not require any post processing and makes the proposed solution very attractive for applications.
Silicon-vacancy (SiV) color centers in diamond offer a promising system for quantum information applications owing to their intense narrowband emission and optically detectable spin states. Here, the ...fluorescent properties of ensembles and single SiV centers in diamonds obtained by high-pressure high-temperature synthesis from adamantane (below referred to as adamantane diamonds) are investigated at liquid-helium temperatures. Ensembles of SiV centers (~10
3
) are studied in large diamond crystals (1–2 µm in size). Despite the large number of excited centers, the fine structure of the zero-phonon line, corresponding to four allowed optical transitions between the ground- and excited-state doublets of the SiV center, can be observed in the fluorescence spectra of these crystals. The width of individual lines is in the range of 60–80 GHz, which is explained by their inhomogeneous broadening. Single SiV centers are studied in diamond crystallites with sizes of about 200 nm. The width of the narrowest transition lines observed in the fluorescence spectra of single SiV centers under resonant excitation is almost three orders of magnitude smaller than linewidths in the spectra of SiV ensembles and is as small as 94 MHz; i.e., these lines are so narrow that their width is determined by the lifetime of the upper state of the transition. Therefore, adamantane nanodiamond features the narrowest emission line width of a single SiV center at cryogenic temperatures among other types of SiV-con-taining nanodiamonds of similar sizes obtained by the high-pressure high-temperature and chemical vapor deposition techniques.
A brief description of the concept of a soft X-ray microscope for the Nanoscope station, which is planned to be installed at the SKIF fourth-generation synchrotron, is given. The microscope is ...designed to study the structure of cells and dynamic processes in them with nanometer spatial resolution. It will use a unique absorption contrast of ~15 between carbon-containing structures and water in the spectral range of the water window, λ = 2.3–4.3 nm, which eliminates the need for contrasting and the use of fluorophores and minimizes the doses of ionizing radiation absorbed in the samples to obtain high-quality 3D images. The scanning and projection schemes of the microscope, their main technical characteristics, including the calculated spectra and parameters of the undulator source are presented, and an estimate of the absorbed doses depending on the resolution is obtained. The main advantage of the proposed concept lies in the use of an objective lens of high-aperture multilayer X-ray mirrors, which makes it possible to clearly visualize the focal section of the sample. Technically simple axial tomography will also be used to reconstruct the three-dimensional structure of frozen or dried samples. In the scanning scheme, due to a low dose of radiation, it will be possible to study living plant cells with a resolution of up to 10 nm, animals with a resolution of up to 80 nm, and cryofixed samples with a resolution of up to 5 nm. In the projection scheme, due to simultaneous observation of the entire focal
XY
section, the time for obtaining three-dimensional images is significantly reduced, but due to a large dose, it will be oriented mainly on the study of fixed samples.
The proportion of people living with HIV (PLWH) in need of antiretroviral therapy (ART) is growing rapidly in Russia. Successful treatment outcomes reduces disease progression and contributes to HIV ...epidemic control. We conducted a pilot study following 100 PLWH newly found eligible for ART in St. Petersburg, Russia. We determined the proportion of PLWH who initiated ART, remained in treatment, and achieved an undetectable VL during 6-month follow up. Semi-structured interviews were conducted prior the initiation of ART and progress along the cascade was assessed through medical chart review. Individual characteristics associated with successful ART outcomes were assessed as part of efforts to generate hypotheses. Almost all (96%) participants initiated ART, full retention was demonstrated by 80%, among whom 71% achieved undetectable VL. Optimal retention was associated with older age and higher education (p < 0.05). There was no significant difference in ART outcomes between those who used illicit drugs and those had not. Interventions to improve treatment effectiveness should emphasize that initiation, optimal retention and achieving an undetectable VL are independent of drug abuse status. However, our pilot study highlights the need for the further research in the examining links between individual and structural factors and ART effectiveness.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, OILJ, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK, VSZLJ
We present theoretical optimization of the design of a quantum well (QW) heterostructure based on AlGaN alloys, aimed at achievement of the maximum possible internal quantum efficiency of emission in ...the mid-ultraviolet spectral range below 300 nm at room temperature. A sample with optimized parameters was fabricated by plasma-assisted molecular beam epitaxy using the submonolayer digital alloying technique for QW formation. High-angle annular dark-field scanning transmission electron microscopy confirmed strong compositional disordering of the thus-fabricated QW, which presumably facilitates lateral localization of charge carriers in the QW plane. Stress evolution in the heterostructure was monitored in real time during growth using a multibeam optical stress sensor intended for measurements of substrate curvature. Time-resolved photoluminescence spectroscopy confirmed that radiative recombination in the fabricated sample dominated in the whole temperature range up to 300 K. This leads to record weak temperature-induced quenching of the QW emission intensity, which at 300 K does not exceed 20% of the low-temperature value.
A real opportunity for applying traditional optical methods to soft X-ray and extreme UV (ultraviolet) radiation bands has appeared thanks to recent successes in the area of multilayer-mirror ...deposition and procedures for fabricating supersmooth and highly precise substrates of mirrors. The implementation of this opportunity opens up fundamentally new prospectss in the nanodiagnostics of substances, micro- and nanoelectronics, microbiology, solar astronomy and other applications. The main directions in multilayer X-ray optics developed at the Institute for the Physics of Microstructures, Russian Academy of Sciences, are presented and the aspects of the use thereof in science and technology are considered. The main problems arising during the fabrication of multilayer interference structures for the soft X-ray and extreme UV bands are discussed. The main results obtained recently in the scope of each direction of investigation are presented. Plans for the future development of these directions are discussed.
Sedimentary waves are represented by a series of wavelike accumulative features with various degrees of morphological expression. These morphological forms are superimposed on the underlying surface ...with a clear stratigraphic unconformity. The hydrophysical processes are the main relief-forming factors of such features. During expeditions 38 (2018) and 41 (2019) of the R/V
Akademik Nikolay Strakhov
, the seafloor was mapped between the Kanin Peninsula and inlet of Cheshskaya Bay and new data were acquired on the morphology sedimentary waves. New data obtained in summer (July–August) 2019 show rapid changes in the natural hydrodynamic environment influencing the alternation of the sedimentary waves. The occurrence of hazardous natural processes, geological and geomorphological risks, and the level of threats were identified, especially with respect to the prolonged navigation period along the Northern Sea Route.
The study is devoted to the development of a microscope operating at a wavelength of 3.37 nm in the water window. A Schwarzschild lens with a multilayer coating is used as an image-forming objective ...lens. The microscope has a theoretical resolution of up to 13 nm and a field of view of 7.3–97.5 µm depending on the magnification. A low depth of focus (±37.5 nm) allows one to implement the
z-
tomography of samples. A schematic diagram of the microscope, the result of aspherizing the mirror collector, and the results of measuring the roughness of the mirror substrates are given. A three-point method for mounting the mirrors and also a magnetic damper operating on Foucault currents are described. The exposure time is estimated.
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