Publications

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.

In this chapter, we review the background and history of the discovery of chalcogenide glassy semiconductors. We consider the unique properties of these materials and their practical application. Separately we discuss the electrical properties of chalcogenide glasses and the development of theoretical concepts of the physics of disordered semiconductors. In addition, we review the history of research on switching and memory effects in chalcogenide glassy semiconductors (CGS) and discuss the main approaches to the theoretical description of these phenomena.

Motivated by recent experiments on quantum magnet NiCl2-4SC(NH2)2 (DTN) and its Br-doped counterpart DTNX we propose a theoretical description of optical magnon branch in the antiferromagnet with large single-ion anisotropy in the magnetically ordered phase. In the framework of the 1/S expansion we derive analytical expressions for optical magnon with \bf{k} = 0 energy magnetic field dependence \Delta(h). It is shown that in the linear spin wave approximation \Delta(h) is monotonic without extrema whereas first order in 1/S corrections makes it drastically different function with a minimum near the center of magnetically ordered phase. The latter behaviour was observed in ESR experiments. Moreover, we show that \Delta(h) has nontrivial dependence on the system parameters. It solves the discrepancy between inelastic neutron scattering data where the growth of interaction constants in DTNX with small Br concentration was observed and ESR experiments showing almost unchanged \Delta(h) in comparison with pure DTN.

Online social networks have become an essential communi- cation channel for the broad and rapid sharing of information. Currently, the mechanics of such information-sharing is captured by the notion of cascades, which are tree-like networks comprised of (re)sharing actions. However, it is still unclear what factors drive cascade growth. Moreover, there is a lack of studies outside Western countries and platforms such as Facebook and Twitter. In this work, we aim to investigate what fac- tors contribute to the scope of information cascading and how to predict this variation accurately. We examine six machine learning algorithms for their predictive and interpretative capabilities concerning cascades’ structural metrics (width, mass, and depth). To do so, we use data from a leading Russian-language online social network VKontakte capturing cascades of 4,424 messages posted by 14 news outlets during a year. The results show that the best models in terms of predictive power are Gradient Boosting algorithm for width and depth, and Lasso Regression algorithm for the mass of a cascade, while depth is the least predictable. We find that the most potent factor associated with cascade size is the number of reposts on its origin level. We examine its role along with other factors such as content features and characteristics of sources and their audiences.

We discuss theoretically a frustrated Heisenberg antiferromagnet in magnetic field close to the saturation one. It is demonstrated that a small biaxial anisotropy and/or the magnetic dipolar interaction produce a delicate balance between phases with a commensurate canted, incommensurate helical (conical), and fan spin orderings. As a result, different sequences of phase transitions are realized depending on values of these small anisotropic interactions. We derive analytical expressions for critical fields and ground-state energies of the phases which are in a quantitative agreement with our and previous Monte-Carlo simulations.

The analytical theory of density of states (DOS) in three-dimensional quantum magnets with the bond disorder is proposed based on the self-consistent T-matrix approximation (SCTMA). It successfully describes the DOS both for resonant and non-resonant scattering, whose emergence is governed by the ratio of scattering length and the average distance between impurities, which concentration is denoted as c. Corrections to the quasiparticle band gap in these cases are shown to scale as c^(2/3) and c, respectively. Moreover, the theory yields a semi-circle form of the DOS for the bound states inside the gap, which results in highly nontrivial DOS in the intermediate parameter region between the two limiting cases when the band DOS and the semi-circle overlap. Long-wavelength excitations are discussed. In the resonant regime their damping scales as c^(2/3), which, according to Ioffe-Regel criterion, corresponds to their localization. Applicability of the theory is illustrated by its quantitative agreement with the recent experimental data on spin-dimer system Ba_(3-x)Sr_xCr_2O_8.

We investigated the low-temperature helical magnetic structures in a thin plate of chiral magnet FeGe, paying special attention to the variation of the magnetic modulation period with the angle of an applied magnetic field. A unique vector-field setup allowed us to continuously rotate the helical texture in the sample plane while measuring high-resolution resonant small-angle x-ray scattering patterns. The experiments and supporting micromagnetic simulations show that the direction of helical propagation can be robustly controlled by directional magnetic field training. The observed anisotropy of the helical modulation period is explained naturally by inclusion of higher-order anisotropic exchange in the Bak-Jensen model. The observation and estimation of this interaction provides a pathway to refine theoretical models of cubic chiral magnets and design spintronics devices based on the propagation switching of helical textures.

Electronic states in a novel type of quantum-size heterostructures referred to as InGaAs quantum welldots (QWDs) were experimentally studied using absorption in stripe waveguides of different lengths based on a single, double, five, and ten QWD layers. The value of the modal absorption was measured to be 70 cm−1 and 90 cm−1 for ground-state transition and high-energy one, respectively. The structure of electronic states in the QWDs is also analyzed by polarization-resolved waveguide absorption and the dependence of a polarization degree on the chip length is discussed. TM polarization of the heavyhole- based optical transition photoresponse observed in the long waveguides is attributed to the light depolarization due to the scattering on the QWD heterointerfaces.

Motivated by recent experimental results for GdRu2Si2 [Khanh et al., Nat. Nanotechnol. 15, 444 (2020)], in which a nanometric square skyrmion lattice was observed, we propose a simple analytical mean-field description of the high-temperature part of the phase diagram of centrosymmetric tetragonal frustrated antiferromagnets with dipolar interaction in the external magnetic field. Dipolar forces provide momentum-dependent biaxial anisotropy in reciprocal space. It is shown that in a tetragonal lattice, in the large part of the Brillouin zone, for mutually perpendicular modulation vectors in the ab plane this anisotropy has mutually perpendicular easy axes and collinear middle axes, which leads to double-Q modulated spin structure stabilization. In the large part of its stability region, the latter turns out to be a square skyrmion lattice with a topological charge of ±1 per magnetic unit cell, which is determined by the frustrated exchange coupling and thus nanometer sized. Easy and middle axes can be swapped in the presence of additional single-ion easy-axis anisotropy. This results in the different phase diagram. It is argued that the latter case is relevant to GdRu2Si2.

Two laser heterostructures with active region based on seven InGaAs quantum wells and on InGaAs/InGaAlAs superlattice were grown on InP substrates by molecular beam epitaxy. Both active regions were designed for vertical-cavity surface-emitting lasers of 1535- 1565 nm spectral range and had total thickness about 80-90 nm. Characteristics of edgeemitting laser diodes fabricated from grown laser heterostructures were studied and compared.

7th International School and Conference "Saint-Petersburg OPEN 2020" on Optoelectronics, Photonics, Engineering and Nanostructures was held on April 27 - 30, 2020. The Organizer of the conference is the Alferov Federal State Budgetary Institution of Higher Education and Science Saint Petersburg National Research Academic University of the Russian Academy of Sciences. Initially, the School and Conference was supposed to be held in full-time format at the Alferov Academic University (Saint-Petersburg, Russia), as it happened in the past. However, due to the restrictions imposed by the city authorities on holding mass events due to the threat of the spread of the COVID-19 infection, the conference committees decided to move the conference to the online format. The conference consisted of poster reports presented by the participants and online oral presentations by invited speakers. Posters and video reports of the participants were posted on the conference website. Invited speakers made their presentations online. During their speeches, participants could discuss and ask questions in the chat. The School and Conference included a series of invited talks given by leading professors with the aim to introduce young scientists with actual problems and major advances in physics and technology.

A new method for deriving estimates of the rate of convergence of optimal methods for solving problems of smooth (strongly) convex stochastic optimization is described. The method is based on the results of stochastic optimization derived from results on the convergence of optimal methods under the conditions of inexact gradients with small noises of nonrandom nature. In contrast to earlier results, all estimates in the present paper are obtained in model generality

In this paper we introduce stochastic parameters into the network game model with production and knowledge externalities. This model was proposed by V. Matveenko and A. Korolev as a generalization of the two-period Romer model. Agents differ in their productivities which have deterministic and stochastic (Wiener) components. We study the dynamics of a single agent and the dynamics of a dyad where two agents are aggregated. We derive explicit expressions for the dynamics of a single agent and dyad dynamics in the form of Brownian random processes, and qualitatively analyze the solutions of stochastic equations and systems of stochastic equations.

The 24th European Conference on Advances in Databases and Information Systems (ADBIS 2020) was set to be held in Lyon, France, during August 25–28, 2020, in conjunction with the 24th International Conference on Theory and Practice of Digital Libraries (TPDL 2020) and the 16th EDA days on Business Intelligence & Big Data (EDA 2020). However, because of the worldwide COVID-19 crisis, ADBIS, TPDL, and EDA had to take place online during August 25–27, 2020. Yet, the three con- ferences joined their forces to propose common keynotes, workshops, and a Doctoral Consortium.

In this paper, we consider the (n,3)-MAXSAT problem. The problem is a special case of the Maximum Satisfiability problem with an additional requirement that in the input formula each variable appears at most three times. Here, we improve previous upper bounds for (n,3)-MAXSAT in terms of n (number of variables) and in terms of k (number of clauses that we are required to satisfy). Moreover, we prove that satisfying more clauses than the simple all true assignment is an NP-hard problem.

We present an analysis of spectral and threshold characteristics of InGaAs/GaAs quantum well-dot microdisk laser operated under cw current injection at room temperature without external cooling. The experimental values of the threshold current for the disk and ring microlasers are compared. We observe that the threshold current can be significantly decreased in devices with large diameters (more than 30 m) by using the ring geometry

Barium fluoride (BaF2) crystals are considered as a candidate scintillator for scintillation detectors in high-energy physics because of high radiation hardness and short decay time (<; 1 ns) of the fast emission component. However, the high level of the slow emission component can deteriorate the detector time resolution. Special approaches are required to decrease this slow component. Using solar-blind photodetectors is one of the possible solutions. In this article, we describe a solar-blind photomultiplier with microchannel plates (MCP) that uses an aluminum gallium nitride (AlGaN) photocathode and a complete scintillation detector channel with a BaF 2 crystal and the photomultiplier. The results of the cosmic-ray tests are presented that show the efficiency of detection of the BaF 2 fast emission component and suppression of the slow one in the proposed scintillation detector.

The problem of approximation by entire functions of exponential type defined on a countable set E of continua Gn, E = ⋃n∈ZGn⋃n∈ZGn is considered in this paper. It is assumed that all Gn are pairwise disjoint and are situated near the real axis. It is also assumed that all Gn are commensurable in a sense and have uniformly smooth boundaries. A function f is defined independently on each Gn and is bounded on E and f (r) has a module of continuity ω which satisfies the condition (1). An entire function Fσ of exponential type ≤σ is then constructed so that the following estimate of approximation of the function f   by functions Fσ is valid

In the NeurIPS 2018 Artificial Intelligence for Prosthetics challenge, participants were tasked with building a controller for a musculoskeletal model with a goal of matching a given time-varying velocity vector. Top participants described their algorithms in this paper. Many solutions use similar relaxations and heuristics, such as reward shaping, frame skipping, discretization of the action space, symmetry, and policy blending. However, each team implemented different modifications of the known algorithms by, for example, dividing the task into subtasks, learning low-level control, or by incorporating expert knowledge and using imitation learning.

With an ever-increasing amount of open-source software, the popularity of services like GitHub that facilitate code reuse, and common misconceptions about the licensing of open-source software, the problem of license violations in the code is getting more and more prominent. In this study, we compile an extensive corpus of popular Java projects from GitHub, search it for code clones, and perform an original analysis of possible code borrowing and license violations on the level of code fragments. We chose Java as a language because of its popularity in industry, where the plagiarism problem is especially relevant because of possible legal action. We analyze and discuss distribution of 94 different discovered and manually evaluated licenses in files and projects, differences in the licensing of files, distribution of potential code borrowing between licenses, various types of possible license violations, most violated licenses, etc. Studying possible license violations in specific blocks of code, we have discovered that 29.6% of them might be involved in potential code borrowing and 9.4% of them could potentially violate original licenses.

In this paper, we present a systematic overview of different endogenous optimization-based characteristic functions and discuss their properties. Furthermore, we define and analyze in detail a new, η-characteristic function. This characteristic function has a substantial advantage over other characteristic functions in that it can be obtained with a minimal computational effort and has a reasonable economic interpretation. In particular, the new characteristic function can be seen as a reduced version of the classical Neumann--Morgenstern characteristic function, where the players both from the coalition and from the complementary coalition use their previously computed strategies instead of solving respective optimization problems. Our finding are illustrated by a pollution control game with n non-identical players. For the considered game, we compute all characteristic functions and compare their properties. Quite surprisingly, it turns out that both the characteristic functions and the resulting cooperative solutions satisfy some symmetry relations.