Diffraction radiation from a charge as radiation from a superluminal source in a vacuum 

An analysis of spectral-angular characteristics of diffraction radiation, both incoherent and coherent, has been performed. It is shown that radiation processes can be interpreted as Cherenkov radiation, which is produced by a region of dynamic polarization moving along a target edge with superluminal velocity vSL. Such radiation is generated if the condition vSL> c is fulfilled, which is the conventional 'threshold' Cherenkov condition. © 2020 Uspekhi Fizicheskikh Nauk and P N Lebedev Physics Institute of the Russian Academy of Sciences.

Prototype of nitro compound vapor and trace detector based on a capacitive MIS sensor 

A prototype of a nitro compound vapor and trace detector, which uses the pyrolysis method and a capacitive gas sensor based on the metal–insulator–semiconductor (MIS) structure type Pd–SiO2 –Si, was developed and manufactured. It was experimentally established that the detection limit of trinitrotoluene trace for the detector prototype is 1 × 10−9 g, which corresponds to concentration from 10−11 g/cm3 to 10−12 g/cm3. The prototype had a response time of no more than 30 s. The possibility of further improving the characteristics of the prototype detector by reducing the overall dimensions and increasing the sensitivity of the MIS sensors is shown. © 2020 by the authors. Licensee MDPI, Basel, Switzerland. 

Virtual tensile test for brittle, plastic, and elastic polymorphs of 4-bromophenyl 4-bromobenzoate 

We report on the development of a novel methodology for computational predictions of the mechanical properties for single crystals. This methodology is based on constrained optimization using dispersion-corrected density functional theory level, and can be dubbed the virtual tensile test. The approach was validated on the example of 4-bromophenyl 4-bromobenzoate, an organic compound known to form three polymorphs with different mechanical characteristics. Each one of these polymorphic crystal structures was stretched stepwise along each crystallographic axis, while the remaining lattice parameters and atomic coordinates were relaxed. The geometrical properties of halogen bonds and the other noncovalent interactions were monitored at each step to understand the nature of mechanical response. The unit cell volumes and lattice energies were plotted as functions of the stretching parameter, and these curves were analyzed in terms of mechanical properties of the brittle, plastic, and elastic polymorphs. Copyright © 2020 American Chemical Society. 

High accuracy machine learning identification of fentanyl-relevant molecular compound classification via constituent functional group analysis 

Fentanyl is an anesthetic with a high bioavailability and is the leading cause of drug overdose death in the U.S. Fentanyl and its derivatives have a low lethal dose and street drugs which contain such compounds may lead to death of the user and simultaneously pose hazards for first responders. Rapid identification methods of both known and emerging opioid fentanyl substances is crucial. In this effort, machine learning (ML) is applied in a systematic manner to identify fentanyl-related functional groups in such compounds based on their observed spectral properties. In our study, accurate infrared (IR) spectra of common organic molecules which contain functional groups that are constituents of fentanyl is determined by investigating the structure–property relationship. The average accuracy rate of correctly identifying the functional groups of interest is 92.5% on our testing data. All the IR spectra of 632 organic molecules are from National Institute of Standards and Technology (NIST) database as the training set and are assessed. Results from this work will provide Artificial Intelligence (AI) based tools and algorithms increased confidence, which serves as a basis to detect fentanyl and its derivatives. © 2020, The Author(s). 

Interaction of dopants and functional groups adsorbed on the carbon fullerenes: Computational study 

We apply density functional theory to study the effective interaction between dopant atoms (B, N, Si, P) and functional groups (H, F, Cl, OH) on the surface of carbon fullerenes. Both dopant atoms and functional groups strongly interact through the carbon cage even in diametrically opposite positions. Interaction energies distribute in a wide range from 0.1 to 2 eV and non-monotonically depend on fullerene size and distance between dopants or functional groups. Such interaction cannot be described as a simple Coulomb repulsion or sum of dopants binding energies and cage strain energy. We identify some general trends in relative positions of dopants or functional groups in low-energy isomers. Para position of two functional groups is the most feasible for C60 and larger cages. For lower fullerenes, ortho or other spaced positions may be more preferable. The interaction of foreign atoms embedded into the carbon cages is more complicated. The best relative positions intricately depend on the cage size and chemical nature of dopants. As a rule, ortho and para locations are feasible for C60 and larger cages. However, some exceptions are observed. The effect of thermal vibrations on the considered interactions in doped or functionalized fullerenes is negligible in the temperature range from 300 to 1000 K. © 2020 Elsevier B.V. 

Enhancement of spontaneous emission of semiconductor quantum dots inside one-dimensional porous silicon photonic crystals 

Controlling spontaneous emission by modifying the local electromagnetic environment is of great interest for applications in optoelectronics, biosensing and energy harvesting. Although the development of devices based on one-dimensional porous silicon photonic crystals with embedded luminophores is a promising approach for applications, the efficiency of the embedded luminophores remains a key challenge because of the strong quenching of the emission due to the contact of the luminophores with the surface of porous silicon preventing the observation of interesting light-matter coupling effects. Here, we experimentally demonstrate an increase in the quantum dot (QD) spontaneous emission rate inside a porous silicon microcavity and almost an order of magnitude enhancement of QD photoluminescence intensity in the weak light-matter coupling regime. Furthermore, we have demonstrated drastic alteration of the QD spontaneous emission at the edge of the photonic band gap in porous silicon distributed Bragg reflectors and proved its dependence on the change in the density of photonic states. (C) 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement 

Electrical conductivity and magnetoresistance in twisted graphene electrochemically decorated with Co particles 

Application of magnetic metal/graphene hybrid structures in magnetosensorics requires the formation of high-quality low-ohmic (barrier-free) contacts and understanding of mechanisms of electric charge transfer near and through the metal/graphene contact area. In present paper we fabricate samples of twisted graphene electrochemically decorated with Co particles (Co-G/SiO2) which demonstrate perfect ohmic electric contact between Co and graphene sheets. Temperature and magnetic field dependencies of surface resistance for pure twisted graphene (G/SiO2) and Co-G/SiO2 samples are considered within the models of 3D Mott variable range hopping and 2D weak-localization quantum corrections to the Drude conductivity. Phenomenological model is proposed explaining the experimentally observed transition from predominantly negative magnetoresistive effect in weak magnetic fields B (below 1–2 T) to positive magnetoresistance (PMR) at B beyond 5 T assuming the growth of PMR due to the distortion of current-conducting routes under the influence of Lorentz force which originates from the enhancement of large-scale potential relief in Co-G/SiO2 sample. This work considers the new approach to the application of G/SiO2 decoration with Co particles for creation both metallic (distributed, defragmented) shunts and high-quality ohmic electrodes in magnetic sensing. © 2019 Elsevier B.V. 

Ultrasonic-assisted supramolecular solvent liquid-liquid microextraction for determination of manganese and zinc at trace levels in vegetables: Experimental and theoretical studies 

Application of magnetic metal/graphene hybrid structures in magnetosensorics requires the formation of high-quality low-ohmic (barrier-free) contacts and understanding of mechanisms of electric charge transfer near and through the metal/graphene contact area. In present paper we fabricate samples of twisted graphene electrochemically decorated with Co particles (Co-G/SiO2) which demonstrate perfect ohmic electric contact between Co and graphene sheets. Temperature and magnetic field dependencies of surface resistance for pure twisted graphene (G/SiO2) and Co-G/SiO2 samples are considered within the models of 3D Mott variable range hopping and 2D weak-localization quantum corrections to the Drude conductivity. Phenomenological model is proposed explaining the experimentally observed transition from predominantly negative magnetoresistive effect in weak magnetic fields B (below 1–2 T) to positive magnetoresistance (PMR) at B beyond 5 T assuming the growth of PMR due to the distortion of current-conducting routes under the influence of Lorentz force which originates from the enhancement of large-scale potential relief in Co-G/SiO2 sample. This work considers the new approach to the application of G/SiO2 decoration with Co particles for creation both metallic (distributed, defragmented) shunts and high-quality ohmic electrodes in magnetic sensing. © 2019 Elsevier B.V. 

Molecular Hyperdynamics Coupled with the Nonorthogonal Tight-Binding Approach: Implementation and Validation 

We present the molecular hyperdynamics algorithm and its implementation to the nonorthogonal tight-binding model NTBM and the corresponding software. Due to its multiscale structure, the proposed approach provides the long time scale simulations (more than 1 s), unavailable for conventional molecular dynamics. No preliminary information about the system's potential landscape is needed for the use of this technique. The optimal interatomic potential modification is automatically derived from the previous simulation steps. The average time between adjusted potential energy fluctuations provides an accurate evaluation of physical time during the hyperdynamics simulation. The main application of the presented hyperdynamics method is the study of thermal-induced defects arising in the middle-sized or relatively large atomic systems at low temperatures. To validate the presented method, we apply it to the C60 cage and its derivative C60NH2. Hyperdynamics leads to the same results as a conventional molecular dynamics, but the former possesses much higher performance and accuracy due to the wider temperature region. The coefficient of acceleration achieves 107 and more. Copyright © 2020 American Chemical Society. 

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AA-Stacked Borophene-Graphene Bilayer with Covalent Bonding: Ab Initio Investigation of Structural, Electronic and Elastic properties 

In this Letter, we study the structural, electronic, and elastic properties of single-layer striped borophene stacked on top of single-layer graphene. Through DFT calculation, we show that both the properties of striped borophene and graphene are not fully preserved in the novel heterostructure, which obviously depends on the nature of the chemical bond between the layers. The obtained phonon spectrum confirms the stability of this compound. The divergence of branches in the band structure appears below the Fermi level in the K point in the first Brillouin zone. Moreover, this heterostructure possesses excellent elastic properties and can be considered for use in the field of 2D acousto- and optoelectronics. © 2020 American Chemical Society. 

Spectrophotometric determination of aflatoxin B1 in food sample: Chemometric optimization and theoretical supports for reaction mechanisms and binding regions 

A green, simple, and cheap analytical approach for extraction, preconcentration, and determination of aflatoxin B1 (AFB1) in food samples based on vortex-assisted room temperature ionic liquid-based microextraction (VA-RTIL-ME) was presented. Important parameters including pH, metal amount, ligand amount and vortex time were optimized by using Box–Behnken design. At pH = 5.6, a ternary complex between Zn(II), fluorescein and AFB1 was formed, and extracted into the fine droplets of Tri-n-butyl phosphate (room temperature ionic liquid, extraction solvent) which were dispersed with a vortex (disperser solvent) into the extraction solution. Quantum chemical parameters and tools are widely used to predict the reaction mechanisms of interactions and binding regions of molecules. Via calculated quantum chemical parameters and energy calculations, reactions mechanisms and binding regions for studied molecules were highlighted. Under optimized conditions, linear range, limit detection, enrichment factor were 3–500 ng mL-1, 0.9 ng mL-1 and 140, respectively. Detailed validation studies (accuracy, precision, measurement uncertainty, selectivity, robustness.etc.) were performed under optimum experimental conditions. The good recoveries (93.9–104.3%) and low relative standard deviation (RSD%, 1.5-3.0%) were a good remark of the proposed method. For the reliability of analytical results, the results obtained with the developed method were compared with the standard ELISA test for AFB1. The developed method was successfully applied to the extraction and determination of AFB1 in food samples. © 2020 Elsevier Inc. 

Interaction of dopants and functional groups adsorbed on the carbon fullerenes: Computational study 

We apply density functional theory to study the effective interaction between dopant atoms (B, N, Si, P) and functional groups (H, F, Cl, OH) on the surface of carbon fullerenes. Both dopant atoms and functional groups strongly interact through the carbon cage even in diametrically opposite positions. Interaction energies distribute in a wide range from 0.1 to 2 eV and non-monotonically depend on fullerene size and distance between dopants or functional groups. Such interaction cannot be described as a simple Coulomb repulsion or sum of dopants binding energies and cage strain energy. We identify some general trends in relative positions of dopants or functional groups in low-energy isomers. Para position of two functional groups is the most feasible for C60 and larger cages. For lower fullerenes, ortho or other spaced positions may be more preferable. The interaction of foreign atoms embedded into the carbon cages is more complicated. The best relative positions intricately depend on the cage size and chemical nature of dopants. As a rule, ortho and para locations are feasible for C60 and larger cages. However, some exceptions are observed. The effect of thermal vibrations on the considered interactions in doped or functionalized fullerenes is negligible in the temperature range from 300 to 1000 K. © 2020 Elsevier B.V. 

Probing dense QCD matter in the laboratory - The CBM experiment at FAIR 

The 'Facility for Antiproton and Ion Research' (FAIR) in Darmstadt will provide unique research opportunities for the investigation of fundamental open questions related to nuclear physics and astrophysics, including the exploration of QCD matter under extreme conditions, which governs the structure and dynamics of cosmic objects and phenomena like neutron stars, supernova explosions, and neutron star mergers. The physics program of the Compressed Baryonic Matter (CBM) experiment is devoted to the production and investigation of dense nuclear matter, with a focus on the high-density equation-of-state (EOS), and signatures for new phases of dense QCD matter. According to the present schedule, the CBM experiment will receive the first beams from the FAIR accelerators in 2025. This article reviews promising observables, outlines the CBM detector system, and presents results of physics performance studies. © 2020 IOP Publishing Ltd. 

Method of formation of super-smooth optical surfaces using GCIB and ANAB processing 

Optical glass–ceramic is a promising material for the elements of precision optical systems operating in a wide temperature range. However, conventional methods such as the chemical–mechanical or ion polishing do not allow to form a substrate surface with less than 0.2 nm root mean square roughness. The method of gas cluster ion beam (GCIB) and accelerated neutral atom beam (ANAB) processing of glass–ceramic substrates is proposed to obtain super-smooth optical surfaces. The experimental samples were characterized by atomic force microscopy and X-ray diffractometry. It is shown that the surface is modified with the disappearance of linear defects (scratches) and the roughness parameters are improved. Only residual chaotic relief with the average roughness value (Ra) of 0.2 – 0.3 nm is observed after GCIB treatment. Subsequent ANAB processing allows to decrease the Ra parameter down to 0.15 nm. The improvement of surface characteristics is confirmed by the analysis of PSD-function in the investigated range of spatial frequencies. As a result of this work the method for super-smooth optical substrates fabrication was developed and the characteristics of obtained surfaces were investigated. © 2020 Elsevier B.V.

Silicon rebirth: Ab initio prediction of metallic sp3-hybridized silicon allotropes 

We report the prediction of metallic quasione-dimensional sp3-hybridized silicon allotropes in the form of prismanes. Silicon prismanes or polysilaprismanes are the silicon nanotubes of a special type constructed from the dehydrogenated molecules of cyclosilanes (silicon rings). By means of density functional theory, the electronic, geometry, energy, and some mechanical properties of these tubes are investigated. Our results show that silicon polyprismanes are thermodynamically stable compounds, and the character of the energy spectrum, as well as the behavior of transmission function near the Fermi level, illustrate that they exhibit non-typical for the silicon systems metallic nature. Moreover, the metallic state of polysilaprismanes is resistant to the mechanical stresses applied along their main axis. Unusual properties predicted in the presented study discover new prospects of application of silicon nanostructures as the basic elements of future micro- and nanoelectronics, as well as in energy, metrology, medical, and information technologies. © 2019 Elsevier B.V. 

Molecular Hyperdynamics Coupled with the Nonorthogonal Tight-Binding Approach: Implementation and Validation 

We present the molecular hyperdynamics algorithm and its implementation to the nonorthogonal tight-binding model NTBM and the corresponding software. Due to its multiscale structure, the proposed approach provides the long time scale simulations (more than 1 s), unavailable for conventional molecular dynamics. No preliminary information about the system's potential landscape is needed for the use of this technique. The optimal interatomic potential modification is automatically derived from the previous simulation steps. The average time between adjusted potential energy fluctuations provides an accurate evaluation of physical time during the hyperdynamics simulation. The main application of the presented hyperdynamics method is the study of thermal-induced defects arising in the middle-sized or relatively large atomic systems at low temperatures. To validate the presented method, we apply it to the C60 cage and its derivative C60NH2. Hyperdynamics leads to the same results as a conventional molecular dynamics, but the former possesses much higher performance and accuracy due to the wider temperature region. The coefficient of acceleration achieves 107 and more. Copyright © 2020 American Chemical Society.

Effect of polaron formation on electronic, charge and magnetic properties of Nb12O29 

We present the ab initio study of different phases of Nb12O29, which has been done in the framework of density functional theory using the onsite Hubbard-U correction applied to the Nb-d states. We vary the U parameter in between 0 and 7 eV and suggest U = 3.7–5.0 eV to be most appropriate for the description of Nb12O29. We show that one cannot get any adequate description of this oxide using the crystallographic unit cell but at least 3 times larger supercell is needed increased along the shortest lattice parameter b to accommodate lattice distortions associated with polaron formation. Our results obtained for the enlarged cells show the possibility of simultaneous co-existence of localized states (polaron formation) and delocalized states providing metal-like properties of Nb12O29, in qualitative agreement with available experimental data. © 2019 Elsevier B.V. 

Energy band gap tuning in Te-doped WS2/WSe2 heterostructures 

Understanding the possibility of band-gap engineering in multilayers composed of two-dimensional materials is extremely important for modeling and creation of novel electronic and photonic devices. Stacking of WS2 and WSe2 monolayers looks particularly attractive for applications due to direct gap of resulting heterostructure, especially taking into account the indirect-gap nature of their bulk-state counterparts. We performed a theoretical investigation of chalcogen atoms replacement in WS2/WSe2 heterostructure by isovalent Te atoms in order to reveal its effects on the band gap, electronic structure and density of states. The doped heterostructures were found to preserve semiconductor properties, whereas the gap changed its nature from direct to indirect in dependence on the position and the distance between substituting Te atoms. Te atoms in the S atom positions led preferably to an indirect gap and increased its value as compared to the pristine material; upon substitution of Se atoms, the direct gap of the heterostructure is preserved but with a small reduction, whereas the substitution of both S and Se atoms changed the gap in a different way depending on Te position. This information makes possible the creation of multilayered structures with tunable gap important for a novel generation of electronic and photonic devices. © 2020, Springer Science+Business Media, LLC, part of Springer Nature. 

Mechanically driven circulation in a rotating cylinder/disk device 

The circulation flow of a viscous incompressible fluid in the cylindrical containers bounded by the ends with the rotating and fixed disks are explored. The main goal of the research is aimed to study influence of rotation of casing and end disk on a circulation flow in a cylinder/disk device. This problem relates to inducing a circulation flow in a mechanical centrifuge for mixture separation. To solve this complex problem, an approximate calculation approach is proposed. It is based both on the integral relationships reducing to the balance of the azimuthal moment friction forces acting on the rotating volume and continuity of a circulation flow. To evaluate the friction forces at the ends of a cylinder, the case of infinitely extended disks is considered. For this case, the approximate analytical solutions of the hydrodynamic problem for the turbulent boundary layers on the rotating and fixed extended disks are obtained. In contrast to the previously published works, the inertial convective term associated with the angular momentum transfer from the external flow to the boundary layer in the equations of motion and the symmetrical radial velocity profile are taken into account. © 2020 Elsevier Masson SAS

Nonstable Latchups in CMOS ICs under Pulsed Laser Irradiation 

This article concerns experimental and simulation results on nonstable latchups (SLs) in CMOS integrated circuits (ICs) under pulsed laser irradiation. Different transient responses in elements of the p-n-p-n structure and irregular ionization distribution on the IC surface are the main reasons for non-SLs. Radiation experimental test results are presented as well as a discussion of non-SL mechanisms. © 1963-2012 IEEE. 

New high-efficiency resonant O-type devices as the promising sources of microwave power 

New O-type high-power vacuum resonant microwave devices are considered in this study: COM klystrons, CSM klystrons and resotrodes. All these devices can output a large amount of power (up to units of MW and higher) with an efficiency of up to 90%. Such devices are promising microwave sources for industrial microwave technologies as well as for microwave energy. The principle of GSP-equivalence for klystrons is described herein, allowing a complete physical analog of this device with other parameters to be created. The existing mathematical and computer models of klystrons are analyzed. The processes of stage-by-stage optimization and the embedding procedure, which leads to COM and to CSM klystrons, are considered. Resotrodes, IOT-type devices with energy regeneration in the input circuit, are also considered. It is shown that these devices can combine high power with an efficiency of up to 90% and a gain of more than 30 dB. Resotrodes with 0-regeneration can be effective sources of radio frequency (RF) power in the range of 20 to 200 MHz. Resotrodes with 2π-regeneration are an effective source of RF/microwave energy in the range of 200 MHz to 1000 MHz. © 2020 by the authors. Licensee MDPI, Basel, Switzerland. 

Hole hopping in dimers of: N, N ′ di(1-naphthyl)- N, N ′-diphenyl-4,4′-diamine (α-NPD): A theoretical study 

Hole-hopping parameters for Marcus-like charge transport, Marcus hole hopping rates, and hole mobilities are calculated for a series of model dimers of a typical hole-transporting material α-NPD using multireference quantum chemistry. The parameters are extracted from the two-state energy profiles built for charge hopping between two states with a hole localized on each of the monomers. The dependence of the hopping integral on the intermolecular arrangement in the dimer is studied. It is shown that at short intermolecular distances strong orbital interactions between molecules cause a drastic increase in the hopping integral and, therefore, in the hopping rate. This journal is © the Owner Societies.

Detection of ultra-high-energy gamma rays from the Crab Nebula: physical implications 

The Crab Nebula is an extreme particle accelerator that boosts the energy of electrons up to a few PeV (10(15) eV), close to the maximum energy allowed theoretically. The physical conditions in the acceleration site and the nature of the acceleration process itself remain highly uncertain. The key information about the highest-energy accelerated particles is contained in the synchrotron and inverse Compton (IC) channels of radiation at energies above 1 MeV and 100 TeV, respectively. A recent report of the detection of an ultra-high-energy gamma-ray signal from the Crab Nebula up to 300 TeV allows us to determine the energy distribution of the highest-energy electrons and to derive the magnetic field strength in the acceleration region, B <= 120 mu G, in a parameter-freeway. This estimate brings new constraints on the properties of non-thermal particle distributions and places important constraints on the magnetohydrodynamic models for the Crab Nebula, in particular on the feasible magnetization and anisotropy of the pulsar wind. The calculations of synchrotron and IC emission show that future observations with instruments that allow detection of the Crab Nebula above 300 TeV and above 1 MeV will clarify the conditions that allow acceleration of electrons beyond PeV energies in the Crab Nebula. In particular, we will be able to verify the hypothetical multicomponent composition of the electron energy distribution, and we will determine the magnetic field strength in the regions responsible for the acceleration of PeV electrons.