Publications

Characteristics of a compact III–V optocoupler heterogeneously integrated on a silicon substrate and formed by a 31 μm in diameter microdisk (MD) laser with a closely-spaced 50 μm × 200 μm waveguide photodetector are presented. Both optoelectronic devices were fabricated from the epitaxial heterostroctructures with InGaAs/GaAs quantum well-dot layers. The measured dark current density of the photodetector was as low as 2.1 μA cm−2. The maximum link efficiency determined as the ratio of the photodiode photocurrent increment to the increment of the microlaser bias current was 1%–1.4%. The developed heterogeneous integration of III–V devices to silicon boards by Au-Au thermocompression bonding is useful for avoiding the difficulties associated with III–V epitaxial growth on Si and facilitates integration of several devices with different active layers and waveguides. The application of MD lasers with their lateral light output is promising for simplifying requirements for optical loss at III–V/Si interface.

Big Data refers to the highly growing digital data collections that involve data with different formats, including structured, semi-structured, and unstructured datasets. Analyzing these combinations requires capabilities beyond the traditional database management systems' abilities. Among most sources of big data appears e-markets and social media platforms as significant contributors. This distinction is due to its features that facilitate consumers to express their views or opinions about specific products and services. Customer reviews and ratings become a significant resource for both consumers and firms regarding their plentiful and valuable knowledge. The proposed work introduces a big data framework to analyze such reviews and ratings, starting with data collection from different sources. Followed by integrating the collected data, which comes in different formats, toward the further processing phase. Finally, the analysis and visualization steps to draw the conclusions. Our work was tested on real data collected from active web resources.

 

Hierarchical topic modeling is a potentially powerful instrument for determining topical structures of text collections that additionally allows constructing a hierarchy representing the levels of topic abstractness. However, parameter optimization in hierarchical models, which includes finding an appropriate number of topics at each level of hierarchy, remains a challenging task. In this paper, we propose an approach based on Renyi entropy as a partial solution to the above problem. First, we introduce a Renyi entropy-based metric of quality for hierarchical models. Second, we propose a practical approach to obtaining the “correct” number of topics in hierarchical topic models and show how model hyperparameters should be tuned for that purpose. We test this approach on the datasets with the known number of topics, as determined by the human mark-up, three of these datasets being in the English language and one in Russian. In the numerical experiments, we consider three different hierarchical models: hierarchical latent Dirichlet allocation model (hLDA), hierarchical Pachinko allocation model (hPAM), and hierarchical additive regularization of topic models (hARTM). We demonstrate that the hLDA model possesses a significant level of instability and, moreover, the derived numbers of topics are far from the true numbers for the labeled datasets. For the hPAM model, the Renyi entropy approach allows determining only one level of the data structure. For hARTM model, the proposed approach allows us to estimate the number of topics for two levels of hierarchy.

Signed networks form a particular class of complex networks that has many applications in sociology, recommender and voting systems. The contribution of this paper is twofold. First, we propose an approach aimed at determining the characteristic subgraphs of the network. Second, we apply the developed approach to the analysis of the network describing the Wikipedia adminship elections. It is shown that this network agrees with the status theory if one does not consider strongly tied vertices, i.e., the vertices that are connected in both directions. At the same time, the strongly connected vertices mostly agree with the structural balance theory. This result indicates that there is a substantial difference between single and double connections, the fact that deserves a detailed analysis within a broader context of directed signed networks.

Now a day’s cloud-based instant message services are becoming popular. It can be an efficient way of communication even for events that require a certain level of security like banking, e-commerce applications, e-transactions etc. Cloud can provide a distributed notification of messages along with the queuing them to facilitate the transfer of messages between various applications and connected systems. Cloud message services can be as simple as a pay-as-you-go service, which does not require significant investment and protect its customers, providing a hassle-free and maintenance-free services platform. This research paper proposes a privacy-preserving approach to users utilizing cloud-based services. A cloud-based model is created and simulated. The proposed method is based on a multi-method approach where a virtual system is acting as an agent and is interconnected with other virtual systems. Inside a virtual system, a discrete event model of job processing and privacy preserving is defined as shown in the process flowchart. The proposed model is scalable. To preserve the privacy, Blowfish and Honey Encryption algorithm is used. This research aims to make the current messaging framework more secure, resilient and effective by the application of symmetric-key algorithm amalgamated with Honey key encryption. The performance of the proposed algorithm evaluated and found satisfactory results  

Recent reinforcement learning studies extensively explore the interplay between cooperative and competitive behaviour in mixed environments. Unlike cooperative environments where agents strive towards a common goal, mixed environments are notorious for the conflicts of selfish and social interests. As a consequence, purely rational agents often struggle to maintain cooperation. A prevalent approach to induce cooperative behaviour is to assign additional rewards based on other agents' well-being. However, this approach suffers from the issue of multi-agent credit assignment, which can hinder performance. This issue is efficiently alleviated in cooperative setting with such state-of-the-art algorithms as QMIX and COMA. Still, when applied to mixed environments, these algorithms may result in unfair allocation of rewards. We propose BAROCCO, an extension of these algorithms capable to balance individual and social incentives. The mechanism behind BAROCCO is to train two distinct but interwoven components that jointly affect agents' decisions. We experimentally confirm the advantages of BAROCCO.

Recent Raman data on nanocrystallite arrays are revised within the microscopic theory for Raman peaks positions and broadening (linewidth). The theory combines the elasticity theory-like approach for optical phonons used in order to evaluate the Raman peaks structure and the Green's function method applied for the phonon lines broadening. These theories are supported by the atomistic calculations within the dynamical matrix method for optical phonons and by the bond polarization model used to calculate the Raman intensities. The experimental data on four various nanopowders are analyzed with the use of this theory. The large width of the Raman peak in nanoparticles as compared with the corresponding peak in bulk materials and the width inverse dependence on the particle size previously observed by other researchers are explained within the framework of the theory. It is shown that the theory is capable to extract confidently from the Raman data four important microscopic characteristics of the nanopowder including the mean particle size, the variance of the particle size distribution function, the strength of intrinsic disorder in the particle, and the effective faceting number that parameterizes the particle shape.

We report on single photon emitters for the green-yellow spectral range, which comprise a CdSe/ZnSe quantum dot placed inside a semiconductor tapered nanocolumn acting as a multimode nanoantenna. Despite the presence of many optical modes inside, such a nanoantenna is able to collect the quantum dot radiation and ensure its effective output. We demonstrate periodic arrays of such emitters, which are fabricated by focused ion beam etching from a II-VI/III-V heterostructure grown using molecular beam epitaxy. With non-resonant optical pumping, the average count rate of emitted single photons exceeds 5 MHz with the second-order correlation function g(2) (0) = 0.25 at 220 K. Such single photon emitters are promising for secure free space optical communication lines. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.

Chekhov's gun is a dramatic principle stating that every element in a story must be necessary, and irrelevant elements should be removed. This paper presents a new natural language processing task — Chekhov's gun recognition or (CGR) — recognition of entities that are pivotal for the development of the plot. Though similar to classical Named Entity Recognition (NER) it has profound differences and is crucial for the tasks of narrative processing, since Chekhov's guns have a profound impact on the causal relationship in a story. The paper presents a new benchmark dataset for the CGR task that includes 5550 descriptions with one or more Chekhov's Gun in each and validates the task on two more datasets available in the natural language processing (NLP) literature.

The topological Hall effect is used extensively to study chiral spin textures in various materials. However, the factors controlling its magnitude in technologically-relevant thin films remain uncertain. Using variable-temperature magnetotransport and real-space magnetic imaging in a series of Ir/Fe/Co/Pt heterostructures, here we report that the chiral spin fluctuations at the phase boundary between isolated skyrmions and a disordered skyrmion lattice result in a power-law enhancement of the topological Hall resistivity by up to three orders of magnitude. Our work reveals the dominant role of skyrmion stability and configuration in determining the magnitude of the topological Hall effect.

This work develops a new method, based on the use of Gustafson’s integrals and on the evaluation of singular integrals, allowing one to establish the unitarity of the separation of variables transform for infinite-dimensional representations of rank one quantum integrable models. We examine in detail the case of the SL(2, R) spin chains.

Weighted constellations give graphical representations of weighted branched coverings of the Riemann sphere. They were introduced to provide a combinatorial inter-pretation of the 2D Toda τ-functions of hypergeometric type serving as generating functions for weighted Hurwitz numbers in the case of polynomial weight generating functions. The product over all vertex and edge weights of a given weighted constellation, summed over all configurations, reproduces the τ-function. In the present work, this is generalized to constellations in which the weighting parameters are determined by a rational weight generating function. The associated τ-function may be expressed as a sum over the weights of doubly labelled weighted constellations, with two types of weighting parameters associated to each equivalence class of branched coverings. The double labelling of branch points, re-ferred to as “colour” and “flavour” indices, is required by the fact that, in the Taylor expansion of the weight generating function, a particular colour from amongst the denominator parameters may appear multiply, and the flavour labels indicate this multiplicity. © 2021 European Mathematical Society.

The exciton valley dynamics in van der Waals heterostructures with transition metal dichalcogenide monolayers is driven by the long-range exchange interaction between the electron and the hole in the exciton. It couples the states active in the opposite circular polarizations resulting in the longitudinal-transverse splitting of excitons propagating in the monolayer plane. Here we study theoretically the effect of the dielectric environment on the long-range exchange interaction and demonstrate how the encapsulation in hexagonal boron nitride modifies the exciton longitudinal-transverse splitting. We calculate the exciton spin-valley polarization relaxation due to the long-range exchange interaction and demonstrate that the variation of the monolayer environment results in significant, up to fivefold, enhancement of the exciton valley polarization lifetime.

In the paper, the differential games on networks with partner sets are considered. The payoffs of a given player depend on his actions and the actions of the players from his partner set. The cooperative version of the game is proposed, and a special type of characteristic function is introduced. It is proved the constructed cooperative game is convex. Using the properties of the payoff functions and the constructed characteristic function, the Shapley Value and $\tau$-value are computed. It is also proved that in this special class of differential games the Shapley value is time-consistent.

Polarized small-angle neutron scattering was used for studying of critical fluctuations in the Dzyaloshinskii–Moriya helimagnets Mn_(1 – x)Fe_xSi with x = 0.10, 0.15, 0.20. The Mn_(1 – x)Fe_xSi compounds are helically ordered below Tc and show a helical fluctuation regime above Tc in a wide range up to TDM. The critical temperatures Tc and TDM decrease with x and tend to 0 at x = 0.11 and 0.17, respectively. It was shown experimentally that three samples reveal properties of fluctuations in different regimes. The sample with x = 0.10 provides sharp narrow peak in polarized SANS maps for temperatures near Tc and in the ordered phase, whereas in the one with x = 0.15 critical fluctuations are suppressed by the disorder which destroys long-range magnetic order, corresponding scattering peaks being substantially wider than for x = 0.10. For the sample with x = 0.20 even at lowest temperatures only traces of the half-moon scattering patterns are visible. The degree of the scattering polarization is close to 1 for all three samples meaning that the corresponding helical fluctuations are chiral. The Mn_(1 – x)Fe_xSi compounds represent an example of the system where ferromagnetic exchange approaches zero but Dzyaloshinskii–Moriya interaction is finite and provides chiral rotation of spins in magnetic fluctuations. We argue at the qualitative level that observed peculiarities can be attributed to defect antiferromagnetic bonds which are added to the system by Fe ions and lead to finite correlation length of the spiral at small temperatures for x > x_c.

The equilibrium distributions of the misfit dislocation density rho(z) and elastic stresses epsilon(z) are calculated along the direction of the epitaxial growth of the metamorphic InAlAs/GaAs(001) layer with higher In content (to 87 mol %) and various profiles of varying the composition: step, linear, and root. The calculations are performed using the method based on iteration searching for the minimum total energy of the system. It is shown that the largest differences between various constructions of the buffer layer are observed in the character of distributions rho(z), rather than epsilon(z). Unlike the traditional constructions with a step and linear gradients of the composition, which are characterized by a quite homogeneous distribution of misfit dislocations, in a buffer layer with a root composition gradient, the main part of such dislocations is concentrated in the lower part of the layer near the heteroboundary with a GaAs substrate, and their density sharply decreases by more than one order of value along the layer thickness, achieving the value minimum for all abovementioned constructions. In spite of the fact that the important effect of interacting the dislocations to each other is not taken into account in this work, the calculations enable us to establish the main peculiarities of the distributions rho(z) and epsilon(z) in various metamorphic buffer InAlAs layers, which were observed experimentally before. Thus, this approach can be effectively used when designing optimal constructions of the device metamorphic heterostructures.