Influence of vibrational treatment on thermomechanical response of material under conditions identical to friction stir welding

A molecular dynamics model was constructed to describe material loading on the atomic scale by the mode identical to friction stir welding. It was shown that additional vibration applied to the tool during the loading mode provides specified intensity values and continuous thermomechanical action during welding. An increase in additional vibration intensity causes an increase both in the force acting on the workpiece from the rotating tool and in temperature within the welded area

Features of interface formation in crystallites under mechanically activated diffusion. A molecular dynamics study.

In this paper, we carried out investigation of behavior of the material under loading condition identical those used in FSW using molecular dynamic method. The loading was modelled by a rigid rotating “tool” that movies along boundary between two grains. We considered pairing of two crystallites of copper, crystallites of copper and iron, and two crystallites of aluminum 2024. Analysis of the structure of the sample showed the intermixing and stirring of dissimilar atoms as a result the FSW tool pass at the inter-crystallite boundary. It was shown, that under certain condition of loading when tool passes there a region where atoms can occupying the original position of the crystal lattice. We also show influence of an additional oscillating impact applied to the moving tool on the structure of the resulting weld. The simulation results obtained can be used for understanding the processes realized under mechanically activated diffusion

Infrared photoresistance as a sensitive probe of electronic transport in twisted bilayer graphene

We report on observation of the infrared photoresistance of twisted bilayer graphene (tBLG) under continuous quantum cascade laser illumination at a frequency of 57.1 THz. The photoresistance shows an intricate sign alternating behavior under variations of temperature and back gate voltage, and exhibits giant resonance-like enhancements at certain gate voltages. The structure of the photoresponse correlates with weaker features in the dark dc resistance reflecting the complex band structure of tBLG. It is shown that the observed photoresistance is well captured by a bolometric model describing the electron and hole gas heating, which implies an ultrafast thermalization of the photoexcited electron–hole pairs in the whole range of studied temperatures and back gate voltages. We establish that photoresistance can serve a highly sensitive probe of the temperature variations of electronic transport in tBLG

Effective surface conductivity of optical hyperbolic metasurfaces: from far-field characterization to surface wave analysis

Abstract Metasurfaces offer great potential to control near- and far-fields through engineering optical properties of elementary cells or meta-atoms. Such perspective opens a route to efficient manipulation of the optical signals both at nanoscale and in photonics applications. In this paper we show that a local surface conductivity tensor well describes optical properties of a resonant plasmonic hyperbolic metasurface both in the far-field and in the near-field regimes, where spatial dispersion usually plays a crucial role. We retrieve the effective surface conductivity tensor from the comparative analysis of experimental and numerical reflectance spectra of a metasurface composed of elliptical gold nanoparticles. Afterwards, the restored conductivities are validated by semi-analytic parameters obtained with the nonlocal discrete dipole model with and without interaction contribution between meta-atoms. The effective parameters are further used for the dispersion analysis of surface plasmons localized at the metasurface. The obtained effective conductivity describes correctly the dispersion law of both quasi-TE and quasi-TM plasmons in a wide range of optical frequencies as well as the peculiarities of their propagation regimes, in particular, topological transition from the elliptical to hyperbolic regime with eligible accuracy. The analysis in question offers a simple practical way to describe properties of metasurfaces including ones in the near-field zone with effective conductivity tensor extracting from the convenient far-field characterization

Synthesis of a Filter of Phase-Domanipulated Signals with a Minimum Side-Lobe Level

В статье показана возможность обнаружения воздушных объектов с помощью макета кластера радиолокационной станции вертикального зондирования (РЛС ВЗ) с проведением натурных испытаний и оценкой достижимых характеристик обнаружения. В макете в качестве зондирующего сигнала применялся 13-элементный код Баркера. Принятый сигнал сжимается в согласованном фильтре приемного устройства, в результате чего вокруг основного отклика формируются боковые лепестки. Количество боковых лепестков N – 1, где N – число дискрет ФКМ‑сигнала. Боковые лепестки имеют одинаковую амплитуду и треугольную форму, а их уровень в N раз меньше уровня основного отклика [1, 2]. Наличие боковых лепестков нежелательно по причине возможного маскирования сигнала, отраженного от цели с малой эффективной площадью рассеяния (ЭПР), боковыми лепестками сигналов, отраженных от цели с большой ЭПР, что зачастую наблюдается в зоне отражений от местных предметов. Таким образом, существует риск, что цель с малой ЭПР не будет обнаружена приемником. Кроме того, боковые лепестки вносят вклад в увеличение уровня шумов при обнаружении цели в области отражений от местных предметов, гидрометеообразований и т.п. Учитывая это, в макете для значительного уменьшения уровня боковых лепестков было разработано и применено устройство подавления боковых лепестков (УПБЛ) с использованием инверсных фильтров [3]. В предлагаемом УПБЛ используется обратная фильтрация с помощью цифровых фильтров, благодаря чему боковые лепестки сжатого в приемнике радиолокационного импульса теоретически могут быть полностью устранены, а в практическом случае происходит значительное подавление боковых лепестковThe article shows the possibility of detecting airborne objects using a model of a vertical sounding radar cluster (VZ radar) with field tests and an assessment of the achievable detection characteristics. In the layout, a 13-element Barker code was used as a probing signal. The received signal is compressed in the receiver’s matched filter, resulting in sidelobes around the main response. The number of side lobes is N – 1, where N is the number of discretes of the PCM signal. The side lobes have the same amplitude and triangular shape, and their level is N times less than the level of the main response [1, 2]. The presence of side lobes is undesirable due to the possible masking of a signal reflected from a target with a small effective scattering area (ESR) by side lobes of signals reflected from a target with a large ESR, which is often observed in the area of reflections from local objects. Thus, there is a risk that a target with a low RCS will not be detected by the receiver. In addition, side lobes contribute to an increase in the noise level when a target is detected in the area of reflections from local objects, hydrometeorological formations, etc. With this in mind, in the layout, to significantly reduce the level of side lobes, a side-lobe suppression device (SBSL) was developed and used using inverse filters [3]. The proposed UPBL uses inverse filtering with digital filters, due to which the side lobes of the radar pulse compressed in the receiver can theoretically be completely eliminated, and in the practical case there is a significant suppression of the side lobe

Spatial Expansion of Islamic Extremism in the Lake Chad Basin: Current Situation and Prospective Directions

The article discusses the expansion of the Islamic extremist groups (especially Boko Haram) in the Lake Chad basin countries. The geopolitical zones and states of Nigeria, regions of Niger and Cameroon, macro-regions of Chad were selected as the territorial range. The religious affiliation data has been compiled from the DHS-database. Income levels and literacy rates were evaluated indirectly using body mass index and the degree of age-heaping (modified Whipple's index), respectively. A hierarchical cluster analysis, has allowed us to categorize the territorial-administrative units into four groups by the probability of new Islamic extremist groups appearing there. The article clearly shows that Boko Haram may expand in the Western and North-Western directions. Meanwhile, the new cells are more likely to form inside Nigeria than outside it. Thus, in the near future, theexpansion of Islamic extremist organizations in the Lake Chad basin countries will occur at the local level. Keywords: Lake Chad basin countries, Islamic Extremism, Boko Haram, expansion

Terahertz Magnetospectroscopy of Cyclotron Resonances from Topological Surface States in Thick Films of CdxHg1 – xTe

Herein, studies of the cyclotron resonance (CR) in thick Cd????Hg1−????Te films with different cadmium contents corresponding to materials with and without band inversion, as well as critical content corresponding to an almost linear energy dispersion are presented. The results demonstrate that the formation of 2D topological surface states requires sharp interfaces between layers with and without band inversion, in which case the corresponding CR is clearly observed for the out-of-plane orientation of magnetic field but does not show up for an in-plane orientation. In contrast, all samples having more conventional technological design with smooth interfaces (i.e., containing regions of Cd????Hg1−????Te with gradually changing Cd content x) show equally pronounced CR in both in-plane and out-of-plane magnetic field revealing that CR is excited in effectively 3D states. Modeling of the surface states for different film designs supports main observations. In all samples, additional broad helicity-independent resonances are observed, which are attributed to photoionization and magnetic freeze-out of impurity states