Browsing by Department "St. Petersburg National Research University of Information Technologies, Mechanics and Optics (ITMO)"
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- Bearing fault analysis of bldc motor for electric scooter application
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2020-12) Kudelina, Karolina; Asad, Bilal; Vaimann, Toomas; Belahcen, Anouar; Rassõlkin, Anton; Kallaste, Ants; Lukichev, Dmitry V.In this paper, the bearing faults analysis of the brushless DC motor is presented. The research method is based on the analysis of the vibration signal of healthy as well as faulty bearings by the identification of specific frequencies on the vibration spectrum. For the experiment, the most common faults were inflicted on the bearings. As the used motor is intended for electric scooter applications, seven different damages were chosen, which are highly likely to occur during the scooter operation. The main bearing faults and the possibility of fault monitoring are addressed. The vibration data are gathered by the acceleration sensors placed on the motor at different locations and the spectrum analysis is performed using the fast Fourier transform. The variation in the amplitude of the frequency harmonics particularly the fundamental component is presented as a fault indicator. - Characterization of silver nanowire layers in the terahertz frequency range
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2021-12-01) Przewłoka, Aleksandra; Smirnov, Serguei; Nefedova, Irina; Krajewska, Aleksandra; Nefedov, Igor S.; Demchenko, Petr S.; Zykov, Dmitry V.; Chebotarev, Valentin S.; But, Dmytro B.; Stelmaszczyk, Kamil; Dub, Maksym; Zasada, Dariusz; Lisauskas, Alvydas; Oberhammer, Joachim; Khodzitsky, Mikhail K.; Knap, Wojciech; Lioubtchenko, DmitriThin layers of silver nanowires are commonly studied for transparent electronics. However, reports of their terahertz (THz) properties are scarce. Here, we present the electrical and optical properties of thin silver nanowire layers with increasing densities at THz frequencies. We demonstrate that the absorbance, transmittance and reflectance of the metal nanowire layers in the frequency range of 0.2 THz to 1.3 THz is non-monotonic and depends on the nanowire dimensions and filling factor. We also present and validate a theoretical approach describing well the experimental results and allowing the fitting of the THz response of the nanowire layers by a Drude–Smith model of conductivity. Our results pave the way toward the application of silver nanowires as a prospective material for transparent and conductive coatings, and printable antennas operating in the terahertz range—significant for future wireless communication devices. - Classical to quantum mechanical tunneling mechanism crossover in thermal transitions between magnetic states
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2016) Vlasov, Sergei; Bessarab, Pavel F.; Uzdin, Valery M.; Jónsson, HannesTransitions between states of a magnetic system can occur by jumps over an energy barrier or by quantum mechanical tunneling through the energy barrier. The rate of such transitions is an important consideration when the stability of magnetic states is assessed for example for nanoscale candidates for data storage devices. The shift in transition mechanism from jumps to tunneling as the temperature is lowered is analyzed and a general expression derived for the crossover temperature. The jump rate is evaluated using a harmonic approximation to transition state theory. First, the minimum energy path for the transition is found with the geodesic nudged elastic band method. The activation energy for the jumps is obtained from the maximum along the path, a saddle point on the energy surface, and the eigenvalues of the Hessian matrix at that point as well as at the initial state minimum used to estimate the entropic pre-exponential factor. The crossover temperature for quantum mechanical tunneling is evaluated from the second derivatives of the energy with respect to orientation of the spin vector at the saddle point. The resulting expression is applied to test problems where analytical results have previously been derived, namely uniaxial and biaxial spin systems with two-fold anisotropy. The effect of adding four-fold anisotropy on the crossover temperature is demonstrated. Calculations of the jump rate and crossover temperature for tunneling are also made for a molecular magnet containing an Mn4 group. The results are in excellent agreement with previously reported experimental measurements on this system. - Crossover temperature for quantum tunnelling in spin systems
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2016-09-15) Vlasov, S. M.; Bessarab, P. F.; Uzdin, V. M.; Jónsson, H.We derive an analytical expression for the crossover temperature corresponding to the transition from classical activation mechanism to temperature assisted quantum tunnelling in spin systems. The crossover temperature depends on the magnetic configuration and Hessian at the first order saddle point on the energy surface of the system. The theory is applied to several single spin models, including a system with four-fold anisotropy. Good agreement is obtained with experimental results for a molecular magnet containing Mn4. - Direct measurement of elastic modulus of InP nanowires with Scanning Probe Microscopy in PeakForce QNM mode
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2016-11-23) Geydt, P.; Dunaevskiy, M.; Alekseev, P.; Kakko, J. P.; Haggrén, T.; Lähderanta, E.; Lipsanen, H.In this manuscript, we present the study of elastic properties of InP nanowires with help of scanning probe microscope in advanced PeakForce Tapping® regime. The measuring method was developed in order to investigate the Young's modulus of these cone-shaped structures with significant accuracy. The difference in InP elasticity for wurtzite phase and zinc- blende phase was revealed. It was shown that elastic modulus of InP nanowires significantly increases from 60 GPa to more than 100 GPa when diameter of a nanowire is reduced below 50 nm. The core-shell model for InP nanowire was used for the explanation of this effect. - Duplication, Collapse, and Escape of Magnetic Skyrmions Revealed Using a Systematic Saddle Point Search Method
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2018-11-06) Müller, Gideon P.; Bessarab, Pavel F.; Vlasov, Sergei M.; Lux, Fabian; Kiselev, Nikolai S.; Blügel, Stefan; Uzdin, Valery M.; Jónsson, HannesVarious transitions that a magnetic Skyrmion can undergo are found in calculations using a method for climbing up the energy surface and converging onto first order saddle points. In addition to collapse and escape through a boundary, the method identifies a transition where the Skyrmion divides and forms two Skyrmions. The activation energy for this duplication process can be similar to that of collapse and escape. A tilting of the external magnetic field for a certain time interval is found to induce the duplication process in a dynamical simulation. Such a process could turn out to be an important avenue for the creation of Skyrmions in future magnetic devices. - The effect of temperature and external field on transitions in elements of kagome spin ice
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2017-11-01) Liashko, Sergei Y.; Jónsson, Hannes; Uzdin, Valery M.Transitions between magnetic states of one and two ring kagome spin ice elements consisting of 6 and 11 prolate magnetic islands are calculated and the lifetime of the ground states evaluated using harmonic transition state theory and the stationary state approximation. The calculated values are in close agreement with experimental lifetime measurements made by Farhan and co-workers (Farhan et al 2013 Nat. Phys. 9 375) when values of the parameters in the Hamiltonian are chosen to be best estimates for a single island, obtained from measurements and micromagnetic modeling. The effective pre-exponential factor in the Arrhenius rate law for the elementary steps turns out to be quite small, on the order of 109 s-1, three orders of magnitude smaller than has been assumed in previous analysis of the experimental data, while the effective activation energy is correspondingly lower than the previous estimate. The application of an external magnetic field is found to strongly affect the energy landscape of the system. Even a field of can eliminate states that correspond to ground states in the absence of a field. The theoretical approach presented here and the close agreement found with experimental data demonstrates that the properties of spin ice systems can be calculated using the tools of rate theory and a Hamiltonian parametrized only from the properties of a single island. - Energy surface and transition rates in a hexagonal element of spin ice
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2017-10-28) Liashko, S. Y.; Uzdin, V. M.; Jónsson, H.The energy surface of a hexagonal element of Kagome spin ice consisting of six prolate magnetic islands is investigated as a function of applied magnetic field. Minimum energy paths for magnetic reversals are determined to estimate energy barriers and the transition rates estimated using harmonic transition state theory for magnetic systems. The overall transition rate between equivalent ground states is calculated using the stationary state approximation including all possible transition paths. The calculated transition rates are in close agreement with reported experimental measurements taken in the absence of an applied field. Predictions are made for the change in the energy landscape and transition rates as a magnetic field is applied. - Homogenization of metasurfaces formed by random resonant particles in periodical lattices
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2016-05-16) Andryieuski, Andrei; Lavrinenko, Andrei V.; Petrov, Mihail; Tretyakov, Sergei A.In this paper we suggest a simple analytical method for description of electromagnetic properties of a geometrically regular two-dimensional subwavelength arrays (metasurfaces) formed by particles with randomly fluctuating polarizabilities. We propose an analytical homogenization method applicable for normal wave incidence on particle arrays with dominating electric dipole responses and validate it with numerical point-dipole modeling using the supercell approach. We demonstrate that fluctuations of particle polarizabilities lead to increased diffuse scattering despite the subwavelength lattice constant of the array. - Laser deposition of resonant silicon nanoparticles on perovskite for photoluminescence enhancement
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2017-11-27) Tiguntseva, E. Y.; Zalogina, A. S.; Milichko, V. A.; Zuev, D. A.; Omelyanovich, M. M.; Ishteev, A.; Cerdan Pasaran, A.; Haroldson, R.; Makarov, S. V.; Zakhidov, A. A.Hybrid lead halide perovskite based optoelectronics is a promising area of modern technologies yielding excellent characteristics of light emitting diodes and lasers as well as high efficiencies of photovoltaic devices. However, the efficiency of perovskite based devices hold a potential of further improvement. Here we demonstrate high photoluminescence efficiency of perovskites thin films via deposition of resonant silicon nanoparticles on their surface. The deposited nanoparticles have a number of advances over their plasmonic counterparts, which were applied in previous studies. We show experimentally the increase of photoluminescence of perovskite film with the silicon nanoparticles by 150 % as compared to the film without the nanoparticles. The results are supported by numerical calculations. Our results pave the way to high throughput implementation of low loss resonant nanoparticles in order to create highly effective perovskite based optoelectronic devices. - Potential Reduction of Peripheral Local SAR for a Birdcage Body Coil at 3 Tesla Using a Magnetic Shield
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2021-10-08) van Leeuwen, C. C.; Steensma, B. R.; Glybovski, S. B.; Lunenburg, M. F.J.; Simovski, C.; Klomp, D. W.J.; van den Berg, C. A.T.; Raaijmakers, A. J.E.The birdcage body coil, the standard transmit coil in clinical MRI systems, is typically a shielded coil. The shield avoids interaction with other system components, but Eddy Currents induced in the shield have an opposite direction with respect to the currents in the birdcage coil. Therefore, the fields are partly counteracted by the Eddy currents, and large coil currents are required to reach the desired B1+ level in the subject. These large currents can create SAR hotspots in body regions close to the coil. Complex periodic structures known as metamaterials enable the realization of a magnetic shield with magnetic rather than electric conductivity. A magnetic shield will have Eddy currents in the same direction as the coil currents. It will allow generating the same B1+ with lower current amplitude, which is expected to reduce SAR hotspots and improve homogeneity. This work explores the feasibility of a birdcage body coil at 3 T with a magnetic shield. Initially, we investigate the feasibility by designinga scale model of a birdcage coil with an anisotropic implementation of a magnetic shield at 7 T using flattened split ring resonators. It is shown that the magnetic shield destroys the desired resonance mode because of increased coil loading. To enforce the right mode, a design is investigated where each birdcage rung is driven individually. This design is implemented in a custom built birdcage at 7 T, successfully demonstrating the feasibility of the proposed concept. Finally, we investigate the potential improvements of a 3 T birdcage body coil through simulations using an idealized magnetic shield consisting of a perfect magnetic conductor (PMC). The PMC shield is shown to eliminate the peripheral regions of high local SAR, increasing the B1+ per unit maximum local SAR by 27% in a scenario where tissue is present close to the coil. However, the magnetic shield increases the longitudinal field of view, which reduces the transmit efficiency by 25%. - Qualitative insight and quantitative analysis of the effect of temperature on the coercivity of a magnetic system
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2016-02-01) Moskalenko, Mariia; Bessarab, Pavel F.; Uzdin, Valery M.; Jónsson, HannesThe temperature dependence of the response of a magnetic system to an applied field can be understood qualitatively by considering variations in the energy surface characterizing the system and estimated quantitatively with rate theory. In the system analysed here, Fe/Sm-Co spring magnet, the width of the hysteresis loop is reduced to a half when temperature is raised from 25 K to 300 K. This narrowing can be explained and reproduced quantitatively without invoking temperature dependence of model parameters as has typically been done in previous data analysis. The applied magnetic field lowers the energy barrier for reorientation of the magnetization but thermal activation brings the system over the barrier. A 2-dimensional representation of the energy surface is developed and used to gain insight into the transition mechanism and to demonstrate how the applied field alters the transition path. Our results show the importance of explicitly including the effect of thermal activation when interpreting experiments involving the manipulation of magnetic systems at finite temperature. - Rate of thermal transitions in kagome spin ice
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2016-09-15) Liashko, S. Y.; Uzdin, V. M.; Jónsson, H.The rate of thermal transitions in a kagome spin ice element is calculated using harmonic transition state theory for magnetic systems. Each element consists of six prolate magnetic islands. Minimum energy paths on the multidimensional energy surface are found to estimate activation energy. Vibrational frequencies are also calculated to estimate the rate of the various transitions. An overall transition rate between equivalent ground states is calculated by using the stationary state approximation including all possible transition paths. The resulting transition rate is in a good agreement with experimentally measured lifetime. - Realistic text replacement with non-uniform style conditioning
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2021) Nerinovsky, Arseny; Buzhinsky, Igor; Filchenkov, AndreyIn this work, we study the possibility of realistic text replacement. The goal of realistic text replacement is to replace text present in the image with user-supplied text. The replacement should be performed in a way that will not allow distinguishing the resulting image from the original one. We achieve this goal by developing a novel non-uniform style conditioning layer and apply it to an encoder-decoder ResNet based architecture. The resulting model is a single-stage model, with no post-processing. We train the model with a combination of adversarial, style, content and L-1 losses. Qualitative and quantitative evaluations show that the model achieves realistic text replacement and outperforms existing approaches in multilingual and challenging scenarios. Quantitative evaluation is performed with direct metrics, like SSIM and PSNR, and proxy metrics based on the performance of a text recognition model. The proposed model has several potential applications in augmented reality. - Self-damping of the relaxation oscillations in miniature pulsed transmitter for sub-nanosecond-precision, long-distance LIDAR
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2020-12) Vainshtein, Sergey; Duan, Guoyong; Rahkonen, Timo; Taylor, Zachary; Zemlyakov, Valery; Egorkin, Vladimir; Smolyanskaya, Olga; Skotnicki, Thomas; Knap, WojciechPeak power is a critical factor for sub-nanosecond-pulsed transmitters utilizing laser diodes (LD) and applied to long distance LIDARs (light detection and ranging) for drones and automotive applications. Receiver speed is not anymore a limiting factor thanks to replacing linear (typically avalanche) detectors and a broad-band amplifier with a single photon avalanche detector (SPAD). Consequently the transmitters become the bottle neck in the resolution and ranging. The simplest and lowest-possible-cost transmitter consists of a switch, an LD, a storage capacitor C, and unavoidable parasitic loop inductance L. In the resulting resonant circuit, the principal problem consists of suppressing relaxation oscillations. Traditional way of oscillation damping reduce peak current and increase the pulse width. Here we show that specific transient properties of a Si avalanche switch solves the problem automatically provided the inductance is sufficiently low. This finding advances the state-of-the-art by reaching 90 W/1ns/200 kHz pulses from a miniature low-cost transmitter based on Si avalanching bipolar junction transistor (ABJT). Besides, the same self-damping effect may be realized in other switches maintaining significant residual voltage despite of fast current reduction. - Time dependency of current harmonics for switch-mode power supplies
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2020-11-01) Iqbal, Muhammad Naveed; Kütt, Lauri; Asad, Bilal; Vaimann, Toomas; Rassõlkin, Anton; Demidova, Galina L.This paper presents the time-dependent variance in the current harmonics emission by power supplies during power quality measurements. Power quality problems are becoming more significant with the adoption of power electronic-based circuits such as power supplies. The switch-mode power supplies are widespread as industrial, commercial, and domestic electrical loads. They draw non-sinusoidal current from the utility and inject current harmonics. Therefore, they are the reason for poor power quality and reduction in the power factor. The current harmonics emission from these power supplies depends on the circuit topology, operating conditions, and filter inside them. The harmonic emission estimations are critical for network operators; however, various uncertainties have made it a complicated task. The time-dependent stability affects the magnitude and phase angle of the harmonic current measurements and estimation of power quality indices. This paper investigates the variation in current harmonics emitted by the power supply during the initial unstable period under constant load and operating conditions. - Tip-surface interaction and rate of magnetic transitions
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2016-09-15) Ivanov, A. V.; Bessarab, P. F.; Uzdin, V. M.; Jónsson, H.Calculations of the interaction between a magnetic tip of an atomic force microscope with a magnetic surface using the non-collinear extension of the Alexander-Anderson model are described. The mechanism and rate of thermally activated magnetic transitions in a cluster of atoms at the tip is investigated. The results are compared with experimental data and found to be in good agreement with measured lifetimes [R. Schmidt et. al., Phys. Rev. B 86, 174402 (2012).]. The results are also compared with previously reported density functional theory calculations.