Publications

Photoinduced nonequilibrium processes in nanoscale materials play key roles in photovoltaic and photocatalytic applications. This review summarizes recent theoretical investigations of excited state dynamics in metal halide perovskites (MHPs), carried out using a state-of-the-art methodology combining nonadiabatic molecular dynamics with real-time time-dependent density functional theory. The simulations allow one to study evolution of charge carriers at the ab initio level and in the time-domain, in direct connection with time-resolved spectroscopy experiments. Eliminating the need for the common approximations, such as harmonic phonons, a choice of the reaction coordinate, weak electron–phonon coupling, a particular kinetic mechanism, and perturbative calculation of rate constants, we model full-dimensional quantum dynamics of electrons coupled to semiclassical vibrations. We study realistic aspects of material composition and structure and their influence on various nonequilibrium processes, including nonradiative trapping and relaxation of charge carriers, hot carrier cooling and luminescence, Auger-type charge–charge scattering, multiple excitons generation and recombination, charge and energy transfer between donor and acceptor materials, and charge recombination inside individual materials and across donor/acceptor interfaces. These phenomena are illustrated with representative materials and interfaces. Focus is placed on response to external perturbations, formation of point defects and their passivation, mixed stoichiometries, dopants, grain boundaries, and interfaces of MHPs with charge transport layers, and quantum confinement. In addition to bulk materials, perovskite quantum dots and 2D perovskites with different layer and spacer cation structures, edge passivation, and dielectric screening are discussed. The atomistic insights into excited state dynamics under realistic conditions provide the fundamental understanding needed for design of advanced solar energy and optoelectronic devices.

The structure of experimentally designed solar cells was optimized in terms of the photoactive layer thickness for both organic bulk heterojunction and hybrid perovskite solar cells. The photoactive layer thickness had a totally different behavior on the performance of the organic and hybrid solar cells. Analysis of the optical parameters using transfer matrix modeling within the Maxwell–Garnett effective refractive index model shows that light absorbance and exciton generation rate in the photoactive layer can be used to optimize the thickness range of the photoactive layer. Complete agreement between experimental and simulated data for solar cells with photoactive materials that have very different natures proves the validity of the proposed modeling method. The proposed simple method which is not time-consuming to implement permits to obtain a preliminary assessment of the reasonable range of layer thickness that will be needed for designing experimental samples.

Based on early phenomenological ideas about the operation of superconducting single-photon detectors (SSPD or SNSPD), it was expected that materials with a lower superconducting gap should perform better in the IR range. The plausibility of this concept could be checked using two popular SSPD materials - NbN and WSi films. However, these materials differ strongly in crystallographic structure (polycrystalline B1 versus amorphous), which makes their dependence on disorder different. In our work we present a study of the single-photon response of SSPDs made from two disordered B1 structure superconductors - vanadium nitride and niobium nitride thin films. We compare the intrinsic efficiency of devices made from films with different sheet resistance values. While both materials have a polycrystalline structure and comparable diffusion coefficient values, VN films show metallic behavior over a wide range of sheet resistance, in contrast toNbNfilms with an insulator-like temperature dependence of resistivity, which may be partially due to enhanced Coulomb interaction, leading to different starting points for the normal electron density of states. The results show that even though VN devices are more promising in terms of theoretical predictions, their optimal performance was not reached due to lower values of sheet resistance.

This paper proposes a new algorithm for hiding data in the frequency domain of digital images using the discrete cosine transform. This algorithm uses quantization index modulation (QIM) as the basic embedding operation. Frequency embedding is characterized by the problem of error appearance in a secret message arising directly at the embedding stage. The embedded message is distorted due to the loss of information when restoring integer pixel values from the frequency domain. This problem is significant if the integrity of the transmitted information is critical. For example, an insignificant distortion of an encrypted message leads to the impossibility of decrypting and, consequently, to the loss of all encrypted information. The proposed algorithm solves this problem via an iterative embedding procedure, which corrects the arising errors. Another distinctive feature of this algorithm is the adaptive correction of distortions of the frequency coefficients’ histogram. For this purpose, the embedding procedure adapts to the features of the image and masks the arising distortions. Computing experiments and comparison with the state-of-the-art demonstrate the effectiveness of our algorithm.

The paper presents a new algorithm for embedding information in the frequency domain of the discrete wavelet-transform (DWT) of digital images. A block version of quantization index modulation (QIM) is used as a basic embedding operation. A distinctive feature of the algorithm consists in the adaptive selection of the data block size depending on the local properties of the cover image. It has been shown experimentally that for image areas containing a larger number of edge pixels, it is necessary to select blocks of greater length in the corresponding frequency domains. In addition, the problem of optimization the distortions in the blocks of DWT coefficients is substantiated and solved in order to improve the quality of embedding. A computing model of learning automata is used to solve this problem. The advantage of the obtained algorithm is that the receiver of the stego-image does not need additional information to extract the embedded message. The algorithm is highly efficient in terms of the main criteria of embedding quality and can be used both for embedding digital watermarks and arbitrary messages.

This article analyzes the functional parts and technological software for the developed device for a control of a two-phase synchronous electric motor. An experimental model of a control system for a two-phase synchronous electric motor and software for its operation were designed. The device consists of a source of driving action, a microcontroller that outputs signals to drivers that perform the role of converting the output signals of the microcontroller, filters that smooth out ripples of current flowing through the motor windings, a power source and a two-phase synchronous electric motor. Based on the results obtained, one will plane to create prototypes and conduct their experimental studies in order to further introduce the development into production.

A computer simulation of the depth course of the absorbed dose as a function of the electronic irradiation energy of the acting in the range of 30 – 60 keV was performed, and calculations of the dose accumulation factor under these conditions were performed for polyethylene terephthalate, polymethylmethacrylate, polystyrene and low-density polyethylene, as model polymers of microelectronic device housings. It is shown that the electron energy values corresponding to the maximum dose accumulation factor depend on the polymer density. The conducted studies allow us to determine with great accuracy the conductivity of plastic cases of microelectronic devices under conditions of electronic irradiation, which is of particular interest to exclude the physical possibility of the occurrence of electrostatic discharges that lead to failures of the onboard electronics of spacecraft.

The possibility of implementing a modeling of the flight of a small spacecraft in low earth orbit based on a client-server architecture was researched. Issues such as transmitting and displaying the system state, aggregating states at different points in time, and executing user code were discussed. The conducted studies demonstrate the benefits of using a client-server implementation in solving the problem of simulating the flight of a small spacecraft in near-earth orbit in a world of increasing interest in satellite developments.

Lead-free metal halide perovskites are environmentally friendly and have favorable electro-optical properties; however, their efficiencies are significantly below the theoretical limit. Using ab initio nonadiabatic molecular dynamics, we show that common intrinsic defects accelerate nonradiative charge recombination in CsSnI3 without creating midgap traps. This is in contrast to Pb-based perovskites, in which many defects have little influence on and even prolong carrier lifetimes. Sn-related defects, such as Sn vacancies and replacement of Sn with Cs are most detrimental, since Sn removal breaks the largest number of bonds and strongly perturbs the Sn−I lattice that supports the carriers. The defects increase the nonadiabatic electron-vibrational coupling and interact strongly with free carrier states. Point defects associated with I atoms are less detrimental, and therefore, CsSnI3 synthesis should be performed in Sn rich conditions. The study provides an atomistic rationalization of why lead-free CsSnI3 exhibits lower photovoltaic efficiency compared to some lead-based perovskites.

We present a simple model of the local order in amorphous organic semiconductors, which naturally produces a spatially correlated exponential density of states (DOS). The dominant contribution to the random energy landscape is provided by electrostatic contributions from dipoles or quadrupoles. An assumption of the preferable parallel orientation of neighbor quadrupoles or antiparallel orientation of dipoles directly leads to the formation of the exponential tails of the DOS even for a moderate size of the ordered domains. The insensitivity of the exponential tail formation to the details of the microstructure of the material suggests that this mechanism is rather common in amorphous organic semiconductors.

We design a scheme for detecting a single photon loss from multi-modal quantum signals transmitted via a fiber or in free space. This consists of a special type of unitary coding transformation, the parity controlled-squeezing, applied prior to the transmission on the signal composed by information and ancilla modes. At the receiver, the inverse unitary transformation is applied -decoding, and the ancilla mode is measured via photon detection. The outcome reveals whether a photon loss has occurred. Distortion of the information part of the signal caused by an ancilla photon loss can be corrected via unitary transformation while loss of a photon from the information part of the signal can be detected with the probability exponentially close to unity but cannot be corrected. In contrast to the schemes of decoherence free subspaces and quantum error correction protocols, this method allows one to make use in principle of entire Hilbert space dimensionality. We discuss possible ways of synthesizing the required encoding-decoding transformation.

Automation of control methods of necessary requirements fulfilment or non-fulfilment while ensuring dependability and quality for wireless devices (WD) are necessary at design stages. However, this approach is not used in enterprises. Companies are limited only to expert assessment (non-automated external audit) and assessment of efficiency private criteria in the quality management system (QMS). The existing statistics of failures of radio engineering devices installed on unmanned automatic spacecraft indicates that there are shortcomings of ensuring the dependability and quality of the WD strategy. Therefore, in this paper, the method of dependability assessment of the WD considering the quality management system is proposed. The paper presents a mathematical model of dependability assessment of the WD considering not only the categories of the WD failures but also the private criteria contribution of the QMS functioning efficiency. Automation of the method is achieved by using the developed software.

Inelastic interactions of quantum systems with the environment usually wash coherent effects out. In the case of Friedel oscillations, the presence of disorder leads to a fast decay of the oscillation amplitude. Here we show both experimentally and theoretically that in three-dimensional topological insulator Bi2Te3 there is a nesting-induced splitting of coherent scattering vectors which follows a peculiar evolution in energy. The effect becomes experimentally observable when the lifetime of quasiparticles shortens due to disorder. The amplitude of the splitting allows an evaluation of the lifetime of the electrons. A similar phenomenon should be observed in any system with a well-defined scattering vector regardless of its topological properties.

We consider the nanoscale electronic phase separation in a wide class of different materials,  mostly in strongly correlated electron systems. The phase separation turns out to be quite ubiquitous manifesting itself in different situations, where the itineracy of charge carriers competes with their tendency toward localization. The latter is often related to some specific type of magnetic ordering, e.g. antiferromagnetic in manganites and low-spin states in cobaltites. The interplay between the localization induced lowering of potential energy and metallicity (which provides the gain in the kinetic energy) favors an inhomogeneous ground state such as nanoscale ferromagnetic droplets in an antiferromagnetic insulating background. The present review article deals with the advances in the subject of electronic phase separation and formation of different types of nanoscale ferromagnetic (FM) metallic droplets (FM polarons or ferrons) in antiferromagnetically ordered (AFM), charge-ordered (CO), or orbitally-ordered (OO) insulating matrices, as well as the colossal magnetoresistance (CMR) effect and tunneling electron transport in the nonmetallic phase-separated state of complex magnetic oxides. It also touches upon the compounds with spin-state transitions, inhomogeneous phase separated state in strongly correlated multiband systems, and electron polaron effect.  A special, attention is paid to the systems with the imperfect Fermi surface nesting such as chromium alloys, iron-based pnictides, and AA stacked graphene bilayers. 

We consider the nanoscale electronic phase separation in a wide class of different materials, mostly in strongly correlated electron systems. The phase separation turns out to be quite ubiquitous manifesting itself in different situations, where the itineracy of charge carriers competes with their tendency toward localization. The latter is often related to some specific type of magnetic ordering, e.g. antiferromagnetic in manganites and low-spin states in cobaltites. The interplay between the localization-induced lowering of potential energy and metallicity (which provides the gain in the kinetic energy) favors an inhomogeneous ground state such as nanoscale ferromagnetic droplets in an antiferromagnetic insulating background. The present review article deals with the advances in the subject of electronic phase separation and formation of different types of nanoscale ferromagnetic (FM) metallic droplets (FM polarons or ferrons) in antiferromagnetically ordered (AFM), charge-ordered (CO), or orbitally-ordered (OO) insulating matrices, as well as the colossal magnetoresistance (CMR) effect and tunneling electron transport in the nonmetallic phase-separated state of complex magnetic oxides. It also touches upon the compounds with spin-state transitions, inhomogeneous phase-separated state in strongly correlated multiband systems, and electron polaron effect. A special, attention is paid to the systems with the imperfect Fermi surface nesting such as chromium alloys, ironbased pnictides, and AA stacked graphene bilayers. 

The report discusses the application of simulation modeling to dependability prediction of the restored equipment. A formal model is described that allows forming time diagrams of the states of both the components and the equipment as a whole. It is shown that the use of such a model allows us to estimate the availability function and the mean restoration time of the equipment if the law of the distribution of the restoration time of the components differs from the exponential one.

Absolute thickness and free water content gradients in gelatin-based corneal phantoms with physiologically accurate radii of curvature, and aqueous backing were extracted via coherent submillimeter wave reflectometry at 220 330 GHz. Fourier-domain based calibration methods, utilizing temporal and spatial gating, were developed and yielded peak-to-peak amplitude and phase clutter of 10-3 and 0.1, respectively for signal to noise ratios between 40 dB and 50 dB. Twelve phantoms were fabricated. Calibration methods enabled quantification of target sphericity that strongly correlated with optical coherence tomography-based sphericity metrics via image segmentation. Extracted free water volume fraction varied less than 5 % in the 5 phantoms whose fabrication yielded the most spherical geometry. Submillimeter wave-based thickness accuracy was better than 111 m (~/9) with average of 65 m (~/17) and standard deviation of 44 m (~/25) for phantoms with physiologically relevant geometry. Monte Carlo simulations of measurement noise and uncertainty limits agree with experimental data analysis and indicates a lower thickness accuracy limit of 33 m, and water content sensitivities of 0.5 % and 11.8 % for the anterior and posterior segments respectively. Numerical analysis suggests measurement fidelity was SNR limited and identified optical path length ambiguities within the cornea where a continuum of thickness/water gradient pairs produce statistically insignificant differences in complex reflection coefficient for finite SNR. This is the first known submillimeter-wave measurement technique able to extract both the thickness and water content gradients from a soft-tissue phantom, with a water backing, without the need for ancillary measurements.

A method for the fabrication of high-Q crystalline whispering gallery mode optical microresonators using diamond turning and further asymptotic abrasive polishing is developed and described. The proposed method allows the fabrication of microresonators with a predefined geometry and a Q-factor higher than 107. A step-by-step fabrication procedure is presented, important parameters ensuring the optimal quality of the surface of the fabricated microresonators are determined, the inspection procedure for the principal parameters is described, and a review of the fabrication of microresonators from different materials is presented.

Magnetic topological semimetals (MTSs) are quantum materials highly desirable for spintronics. We report the synthesis, the crystal structure, the chemical bonding analysis, the magneto(transport) properties, and the bulk and surface electronic structures of GdBiTe. It is a high-Z isostructural analogue of the archetypical nodal-line TS ZrSiS and a recently discovered MTS LnSbTe (Ln = Ce, Gd). GdBiTe crystallizes in the nonsymmorphic space group P4/nmm (No. 129) with a = 4.3706(2) Å and c = 9.2475(7) Å. Chemical bonding analysis describes it as a layered structure of alternating weakly bonded double-stacked covalent [GdTe] layers and planar square [Bi] nets. GdBiTe exhibits an antiferromagnetic transition at TN = 15 K, and an additional transition, possibly a spin reorientation into a canted antiferromagnetic state, occurs below ca. 5 K. The electrical resistivity is compatible with a semimetallic behavior above TN. The Hall coefficient is negative, reflecting an electron-like character of the transport in a semimetal. The magnetoresistance presents a negative contribution at temperatures lower than ca. 30 K, consistent with the freezing of spin fluctuations due to the applied field. First-principles calculations identify a collinear antiferromagnetic ground state with the Gd(III) magnetic moments coupled ferromagnetically in-plane (easy axis along [100]) and antiferromagnetically along the c axis. This spin alignment differs from the reported LnSbTe and enables new scenarios of symmetry breaking due to magnetic order and spin−orbit coupling in a symmetryprotected topological semimetal. GdBiTe hosts exotic topological features resulting from an interplay of lattice symmetry, magnetism, and topology. Its collinear antiferromagnetic phase exhibits fingerprints of the nodal lines in the bulk electronic spectrum confined to kz = π/c planes and a surface state with a symmetry-protected crossing at the (010) face, whereas the paramagnetic phase is a weak topological insulator and a higher-order topological insulator with topologically protected surface states at the (100), (010), and (001) planes. GdBiTe is also better suited for topological transport properties than LnSbTe thanks to a gapped trivial electron pocket at the Γ point. The isostructural LaBiTe was synthesized as a single-phase powder (sp. gr. P4/nmm; a = 4.48819(9) Å, and c = 9.5501(3) Å). Its bulk and surface electronic spectra are similar to the nonmagnetic case of GdBiTe.

In the paper we demonstrate that the thermal doping of SiO2 film by phosphorus, causing formation of thin film of phospho-silicate glass on its surface, allows to rise charge stability of gate dielectric of MIS structure. We have ascertained that a presence of the film of phospho-silicate glass has given a possibility to significantly lower local injection currents flowing within defects because of electron capturing by traps located in the film of phospho-silicate glass what results in the rising of energy barrier. As a result, amount of the structures that comes to a state of breakdown at low values of charge injected into the dielectric under high fields noticeably reduces. We show that heating processes of injected electrons lowers in the films of phospho-silicate glass and this results in increasing of charge stability of the gate dielectric under high-field injection.

We report our study of detection efficiency (DE) saturation in wavelength range 400–1550 nm for the NbN superconducting microstrip single-photon detectors (SMSPDs) featuring the strip width up to 3 µm. We observe an expected decrease of the DE saturation plateau with the increase of photon wavelength and decrease of film sheet resistance. At 1.7 K temperature DE saturation can be clearly observed at 1550 nm wavelength in strip with the width up to 2 µm when sheet resistance of the film is above 630 Ω sq−1. In such strips the length of the saturation plateau almost does not depend on the strip width. We used these films to make meander-shaped detectors with the light sensitive area from 20 × 20 µm2 to a circle 50 µm in diameter. In the latter case, the detector with the strip width of 0.49 µm demonstrates saturation of DE up to 1064 nm wavelength. Although DE at 1310 and 1550 nm is not saturated, it is as high as 60%. The response time is limited by the kinetic inductance and equals to 20 ns (by 1/e decay), timing jitter is 44 ps. When coupled to multi-mode fibre large-area meanders demonstrate significantly higher dark count rate which we attribute to thermal background photons, thus advanced filtering technique would be required for practical applications.