Technical University of Sofia
Recent publications
We have in depth analyzed the refractive-index behavior and optical absorption of below-band-gap light, in order to calculate the basic parameters of the energy-band structure of thin layers of non-crystalline semiconductors. By carrying out a semi-empirical determination of the influence of the finite (non-zero) width of the valence and conduction electronic bands, we find the dependence of the index of refraction upon the photon energy, n(E), which goes just one order beyond the Wemple–DiDomenico two-level single-oscillator expression, and we simultaneously obtain the spectral dependence of the absorption coefficient, α(E). By model fitting the measured normal-incidence transmittance spectrum, we demonstrate that with a highly-sensitive double-beam spectrophotometer, it can be accurately determined the energy distance, EM,Sol, between the corresponding ‘centers of mass’ of the bonding and anti-bonding electronic bands, and also a reasonable estimate of the so-called effective width, Δeff, of both valence and conduction bands. We have used this devised optical approach with a series of uniform and non-uniform thin layers of unhydrogenated fully a-Si, grown by RF-magnetron-sputtering deposition, onto room-temperature transparent glass substrates. The advantages of our novel approach are mainly due to the additional attention paid to the roles of the weak-absorption Urbach tail and the thickness non-uniformity of the studied a-Si films. We have also used a universal normal-incidence transmission expression reported by the authors in an earlier paper, which can be applied even to strongly-wedge-shaped semiconductor layers. Together with the use of the improved Solomon formula for the normal optical dispersion of the refractive index, the complete approach with all its elements constitutes the main novelty of the present paper, in comparison with other existing works.
In this paper, the concept of Lipschitz stability is introduced to impulsive delayed reaction-diffusion neural network models of fractional order. Such networks are an appropriate modeling tool for studying various problems in engineering, biology, neuroscience and medicine. Fractional derivatives of Caputo type are considered in the model. The effects of impulsive perturbations and delays are also under consideration. Lipschitz stability analysis is performed and sufficient conditions for global uniform Lipschitz stability of the model are established. The Lyapunov function approach combined with the comparison principle are employed in the development of the main results. The proposed criteria extend some existing stability results for such models to the Lipschitz stability case. The introduced concept is also very useful in numerous inverse problems.
Critical infrastructures are assets of invaluable importance, essential for the whole world. Since they serve core functions of our societies, they often become targets of terrorists. Many critical infrastructures are vulnerable, due to their short distance from public roads and in the past years, several vehicle-bomb incidents have been recorded. This paper focuses on the case of truck-bombs, which can either be created from scratch, or terrorists can easily hijack truck cargos carrying dangerous goods and turn them into bombs. The latter are typically called ADR truck cargos, according to the respective agreement of the 30th of September 1957, concerning the international carriage of dangerous goods by road. The proposed scheme performs threat assessment of neighboring critical infrastructures, aiming at preventing explosions of truck-bombs. To do so, each crucial point of a critical infrastructure is initially associated with a level of importance. Next, three scenarios are analyzed: (a) single-attack single-infrastructure, (b) multiple-attack single-infrastructure, and (c) multiple-attack multiple-infrastructure. To reduce computational complexity, the third scenario is simplified to one of the two other scenarios, by introducing a novel fusion technique for the non-overlapping segments of the Voronoi tessellation. By this way, an area of threat assessment is estimated for each critical infrastructure. Then, the threat level is assessed in real time by an innovative algorithm, which: (a) estimates the impact of multiple consecutive explosions, (b) uses five adapted threat levels and (c) introduces multiple criteria and minimum classification conditions based on the number of crucial points and their levels of importance. Extensive real world experimental results and comparisons to other works, exhibit the pros and cons of the proposed scheme. In particular, compared to related work, the proposed scheme improves: (a) computational time by 74.5%, (b) threat notification time by 86.9% and (c) estimated surveillance cost by 98.6%.
The nickel (Ni) coatings without and with embedded (5–7 vol. %) cubic boron nitride (c-BN) nanoparticles (10 nm in diameter) were deposited on carbon steel substrate by an electroless plating process. Coatings were tested in as-deposited and heat-treated (heating at 300 °C for 6 h) conditions. Coating structure characterisation was performed, as well as hardness and roughness measurements. Friction properties were tested in dry and in water (seawater) lubricated contact conditions, with bronze as a counter-body material. Both static and kinetic coefficients of friction were measured for two different surface texture preparations (initial and working). The first surface texture simulated the running-in condition, and the second surface texture represented the steady-state conditions. The enhancement of the abrasive and erosive wear resistance of heat-treated electroless Ni coatings with embedded c-BN nanoparticles was already proved in our previous studies. This study aims to investigate those influences on friction properties of electroless Ni coatings in different sliding conditions. The results show that the coefficients of friction did not differ too much between the coatings and that the surface roughness and presence of seawater had a much stronger influence.
The need to create automated methods for extracting knowledge from data arises from the accumulation of a large amount of data. This paper presents a conceptual model for integrating and processing medical data in three layers, comprising a total of six phases: a model for integrating, filtering, sorting and aggregating Covid-19 data. A medical data integration workflow was designed, including steps of data integration, filtering and sorting. The workflow for Covid-19 medical data from clinical records of 20400 potential patients was employed.
In order to meet the requirements of practical applications, a model of deforming manifold in the embedded space is proposed. The deforming vector and deforming field are presented to precisely describe the deforming process, which have clear physical meanings. The proposed model is a modification of the general differential dynamic model, with constraints of spatial and temporal continuity on the deforming field. The deformation integral and derivative are presented as compact expressions of manifold deforming process. Moreover, a specific autonomous deforming field with flattening effect is defined, which provides a novel geometric viewpoint on data dimension reduction. The effectiveness of this autonomous deforming field is proved by numerical computation simulations, which indicate the promising potential of the proposed model in practical dimension reduction tasks.
The aim of this study is to compare effective dose (E) estimations based on different methods for patients with recurrent computed tomography (CT) examinations. Seventeen methods were used to determine the E of each phase as well as the total E of the CT examination. These included three groups of estimations: based on the use of published E, calculated from typical or patient-specific values of volume computed tomography dose index (CTDIvol) and dose-length product (DLP) multiplied by conversion coefficients, and based on patient-specific calculations with use of software. The E from a single phase of the examination varied with a ratio from 1.3 to 6.8 for small size patients, from 1.2 to 6.5 for normal size patients, and from 1.7 up to 18.1 for large size patients, depending on the calculation method used. The cumulative effective dose (CED) ratio per patient for the different size groups varied as follows: from 1.4 to 2.5 (small), from 1.7 to 4.3 (normal), and from 2.2 up to 6.3 (large). The minimum CED across patients varied from 38 up to 200 mSv, while the variation of maximum CED was from 122 up to 538 mSv. Although E is recommended for population estimations, it is sometimes needed and used for individual patients in clinical practice. Its value is highly dependent on the method applied. Individual estimations of E can vary up to 18.1 times and CED estimations can differ up to 6 times. The related large uncertainties should always be taken into account.
An approach for texturing of gas-sensitive nanocoatings by using surface acoustic waves (SAW) is presented in this article. The objective of the work is to enhance the performance of precise SAW-based gas sensors due to the increased specific area of the sensitive nanocoating, induced during its growth and to replace the expensive lithographic techniques for nanopatterning, typically used for this purpose. The technique can be used for tuneable alignment of nanoparticles or nanowires and it is scale-independent. To control the texture of the sensitive nanocoating, a specific electrode topology was used to generate waves with a specific space distribution, which in turn caused assembling of the nanoparticles increasing the adsorption capacity. In this way, a broader dynamic range of 7,000 ppm was achieved (three times extended as compared to the non-textured sensing film), measurement error of 0.6% against 4% for the non-patterned, faster response time in the sub-seconds range (970 ms vs 1.1 s), negligible hysteresis of 10 mV (against >100 mV), and very good sensitivity of 5 µV per ppm, which are in line with the current standards for ethanol sensors. The enhanced sensor parameters were achieved by implementation of conventional patterning technologies without the need for nanolithographic techniques for the texturing the nanocoating. The method is low-cost, and applicable in a variety of sensing structures despite the sensing coating (optical, biological, etc.).
This paper presents a theoretical study of the excitonic spectra of a hexagonal two-dimensional lattice of trimers of bacteriorhodopsin (BR). The investigations follow the treating of the electronic excitations in molecular and organic solids. Formalism of dielectric permittivity and of gyration tensor allows to calculate their components near the frequency of electronic excitation of chromophore of retinal with λ≈ 570 nm. The excitonic spectra of noninteracting trimers and their demonstration in the linear absorption and circular dichroism (CD) spectra represent the model of the spectra of hexagonal 2D lattice with excitonic inter-trimer transfers between the nearest neighbors. The two cases of oblique and perpendicular incidence of left- and right-circularly polarized electromagnetic waves have been considered and corresponding CD signals have been calculated. In accordance with the experimental data a negative CD couplet appears (at oblique incidence) connected with the chiral excitonic coupling whereas at perpendicular incidence a single CD band of Lorentz-type is caused by the intrinsic gyrotropy of the molecules.
We prove the decay in the energy space for the solution to the defocusing biharmonic Hartree-Fock equations with mass-supercritical and energy-subcritical Choquard-type nonlinearity in space dimension d⩾3. We treat both the free and the perturbed by a potential case. As a direct consequence, we obtain large-data scattering in H2(Rd)N,N⩾1.
Future Railway Mobile Communication System which is critical for European Train Control Systems is expected to provide highly reliable, safe, and secure services and applications. The paper studies how mobile edge cloud computing may support the requirements of railway applications, discussing virtualization of rail functions and co-existence with mobile edge services. A new mobile edge service is designed that provides core functionality for many railways use cases enabling application-initiated and manipulated data sessions. Using the service interfaces, on-board and trackside applications can establish data sessions e.g. in case of emergency. The service is described by typical use cases, data types, resources and supported methods, and some implementation issues are discussed.
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1,597 members
Malinka Ivanova
  • Faculty of Applied Mathematics and Informatics
Vassil Galabov
  • Department of Industrial Automation
Mariya Aleksandrova
  • Department of Microelectronics
Zahari Zarkov
  • Department of Electrical Machines
8, Kliment Ohridski Blvd., BG-1000, Sofia, Bulgaria
Head of institution
Prof. Ivan Kralov, PhD, DSc