arXiv subjects

Andrei Velichko

Publications and source records attributed to Andrei Velichko.

52 records · Page 3Linked to original sources

arXiv2020

Higher Order and Long-Range Synchronization Effects for Classification and Computing in Oscillator-Based Spiking Neural Networks

In the circuit of two thermally coupled VO2 oscillators, we studied a higher order synchronization effect, which can be used in object classification techniques to increase the number of possible synchronous states of the oscillator system. We developed the phase-locking estimation method to determine the values of subharmonic ratio and synchronization effectiveness. In our experiment, the number of possible synchronous states of the oscillator system was twelve, and subharmonic ratio distributions were shaped as Arnold's tongues. In the model, the number of states may reach the maximum value of 150 at certain levels of coupling strength and noise. The long-range synchronization effect in a one-dimensional chain of oscillators occurs even at low values of synchronization effectiveness for intermediate links. We demonstrate a technique for storing and recognizing vector images, which can used for reservoir computing. In addition, we present the implementation of analog operation of multiplication, the synchronization based logic for binary computations, and the possibility to develop the interface between spike neural network and a computer. Based on the universal physical effects, the high order synchronization can be applied to any spiking oscillators with any coupling type, enhancing the practical value of the presented results to expand spike neural network capabilities.

cs.NE↗

arXiv2020

Novel hypostasis of old materials in oxide electronics: metal oxides for resistive random access memory applications

Transition-metal oxide films, demonstrating the effects of both threshold and nonvolatile memory resistive switching, have been recently proposed as candidate materials for storage-class memory. In this work we describe some experimental results on threshold switching in a number of various transition metal (V, Ti, Fe, Nb, Mo, W, Hf, Zr, Mn, Y, and Ta) oxide films obtained by anodic oxidation. Then, the results concerning the effects of bistable resistive switching in MOM and MOS structures on the basis of such oxides as V2O5, Nb2O5, and NiO are presented. It is shown that sandwich structures on the basis of the Au/V2O5/SiO2/Si, Nb/Nb2O5/Au, and Pt/NiO/Pt can be used as memory elements for ReRAM applications. Finally, model approximations are developed in order to describe theoretically the effect of nonvolatile unipolar switching in Pt NiO-Pt structures.

physics.app-ph↗

arXiv2020

UV-laser modification and selective ion-beam etching of amorphous vanadium pentoxide thin films

We present the results on excimer laser modification and patterning of amorphous vanadium pentoxide films. Wet positive resist-type and Ar ion-beam negative resist-type etching techniques were employed to develop UV-modified films. V2O5 films were found to possess sufficient resistivity compared to standard electronic materials thus to be promising masks for sub-micron lithog-raphy

physics.app-ph↗

arXiv2020

Electrical conductivity of vanadium dioxide switching channel

The electrical conductivity of the switching channel of vanadium dioxide thin-film sandwich structures is studied over a wide temperature range (15-300 K). It is shown that the electrical resistance of the channel varies with temperature as R~exp(aT-b/T) in the high-temperature region (above 70 K). The experimental results are discussed from the viewpoint of the small polaron hopping conduction theory which takes into account the influence of thermal lattice vibrations onto the resonance integral.

cond-mat.mtrl-sci↗

arXiv2020

Effect of memory electrical switching in metal/vanadium oxide/silicon structures with VO2 films obtained by the sol-gel method

Electrical switching and rectifying properties of the metal-VO2-Si structures, on both p-type and n-type silicon, with vanadium dioxide films obtained by an acetylacetonate sol-gel method, are studied. The switching effect is shown to be due to the semiconductor-to-metal phase transition (SMPT) in vanadium dioxide. The shift of the switching threshold voltage, accompanied by the memory effect, in forward bias of the p-Si-VO2 anisotype heterojunction is observed. To explain this effect, a model is proposed which suggests the existence of an additional series resistance associated with a channel at the VO2/Si interface, where a SiOx layer forms during the VO2 deposition process. This resistance is responsible for both threshold switching characteristics, and the memory effect, and the oxygen ion electromigration process is shown to underlie this effect. Potential applications of the observed phenomena, combining the effects of ReRAM and SMPT, in oxide electronics are discussed.

physics.app-ph↗

arXiv2020

Vanadium oxide thin films and fibers obtained by acetylacetonate sol-gel method

Vanadium oxide films and fibers have been fabricated by the acetylacetonate sol-gel method followed by annealing in wet nitrogen. The samples are characterized by X-ray diffraction and electrical conductivity measurements. The effects of a sol aging, the precursor decomposition and the gas atmosphere composition on the annealing process, structure and properties of the films are discussed. The two-stage temperature regime of annealing of amorphous films in wet nitrogen for formation of the well crystallized VO2 phase is chosen: 1) 25-550 C and 2) 550-600 C. The obtained films demonstrate the metal-insulator transition and electrical switching. Also, the effect of the polyvinylpyrrolidone additive concentration and electrospinning parameters on qualitative (absence of defects and gel drops) and quantitative (length and diameter) characteristics of vanadium oxide fibers is studied.

physics.app-ph↗

arXiv2020

Electrical and optical properties of hydrated amorphous vanadium oxide

Electrical and optical properties of amorphous vanadium oxide thin films obtained by electrochemical anodic oxidation are studied. It is shown that under cathodic polarization the hydrogen insertion into vanadium oxide from an electrolyte occurs. Metal-insulator transition in amorphous HxVO2 is found to be preserved up to high concentration (x ~ 1.5) of hydrogen. Memory switching with the N-type negative differential resistance, associated with the H+ ionic transfer, is observed in "V/hydrated amorphous vanadium oxide/Au" sandwich structures.

physics.app-ph↗

arXiv2020

Influence of doping on the properties of vanadium oxide gel films

Effect of doping with H and W on the properties of V2O5 and VO2 derived from V2O5 gel has been studied. It is shown that the treatment of V2O5 in low-temperature RF hydrogen plasma for 1 to 10 min. leads to either hydration of vanadium pentoxide or its reduction (depending on the treatment conditions) to lower vanadium oxides. For some samples, which are subject to plasma treatment in the discharge active zone, a non-ordinary temperature dependence of resistance, with a maxi-mum at T ~ 100 K, is observed. For W-doped VO2 films, it is shown that substitution of V4+ with W6+ results in a decrease of the temperature of metal-insulator transition. Also, it has been shown that the doping of the initial films with ~3 at.% of W reduces the statistical scatter in the thresh-old parameters of the switching devices with S-shaped I-V characteristics on the basis of V2O5 gel films.

cond-mat.mtrl-sci↗

arXiv2020

Electrical switching and oscillations in vanadium dioxide

We have studied electrical switching with S-shaped I-V characteristics in two-terminal MOM devices based on vanadium dioxide thin films. The switching effect is associated with the metal-insulator phase transition. Relaxation oscillations are observed in circuits with VO2-based switches. Dependences of the oscillator critical frequency Fmax, threshold power and voltage, as well as the time of current rise, on the switching structure size are obtained by numerical simulation. The empirical dependence of the threshold voltage on the switching region dimensions and film thickness is found. It is shown that, for the VO2 channel sizes of 10*10 nm, Fmax can reach the value of 300 MHz at a film thickness of ~20 nm. Next, it is shown that oscillatory neural networks can be implemented on the basis of coupled VO2 oscillators. For the weak capacitive coupling, we revealed the dependence of the phase difference upon synchronization on the coupling capacitance value. When the switches are scaled down, the limiting time of synchronization is reduced to Ts ~13 μs, and the number of oscillation periods for the entering to the synchronization mode remains constant, Ns ~ 17. In the case of weak thermal coupling in the synchronization mode, we observe in-phase behavior of oscillators, and there is a certain range of parameters of the supply current, in which the synchronization effect becomes possible. With a decrease in dimensions, a decrease in the thermal coupling action radius is observed, which can vary in the range from 0.5 to 50 μm for structures with characteristic dimensions of 0.1 to 5 μm, respectively. Thermal coupling may have a promising effect for realization of a 3D integrated oscillatory neural network.

physics.app-ph↗

arXiv2020

Switching dynamics of single and coupled VO2-based oscillators as elements of neural networks

In the present paper, we report on the switching dynamics of both single and coupled VO2-based oscillators, with resistive and capacitive coupling, and explore the capability of their application in oscillatory neural networks. Based on these results, we further select an adequate SPICE model to describe the modes of operation of coupled oscillator circuits. Physical mechanisms influencing the time of forward and reverse electrical switching, that determine the applicability limits of the proposed model, are identified. For the resistive coupling, it is shown that synchronization takes place at a certain value of the coupling resistance, though it is unstable and a synchronization failure occurs periodically. For the capacitive coupling, two synchronization modes, with weak and strong coupling, are found. The transition between these modes is accompanied by chaotic oscillations. A decrease in the width of the spectrum harmonics in the weak-coupling mode, and its increase in the strong-coupling one, is detected. The dependences of frequencies and phase differences of the coupled oscillatory circuits on the coupling capacitance are found. Examples of operation of coupled VO2 oscillators as a central pattern generator are demonstrated.

cs.NE↗

arXiv2020

Thermal coupling and effect of subharmonic synchronization in a system of two VO2 based oscillators

We explore a prototype of an oscillatory neural network (ONN) based on vanadium dioxide switching devices. The model system under study represents two oscillators based on thermally coupled VO2 switches. Numerical simulation shows that the effective action radius RTC of coupling depends both on the total energy released during switching and on the average power. It is experimentally and numerically proved that the temperature change dT commences almost synchronously with the released power peak and T-coupling reveals itself up to a frequency of about 10 kHz. For the studied switching structure configuration, the RTC value varies over a wide range from 4 to 45 mkm, depending on the external circuit capacitance C and resistance Ri, but the variation of Ri is more promising from the practical viewpoint. In the case of a "weak" coupling, synchronization is accompanied by attraction effect and decrease of the main spectra harmonics width. In the case of a "strong" coupling, the number of effects increases, synchronization can occur on subharmonics resulting in multilevel stable synchronization of two oscillators. An advanced algorithm for synchronization efficiency and subharmonic ratio calculation is proposed. It is shown that of the two oscillators the leading one is that with a higher main frequency, and, in addition, the frequency stabilization effect is observed. Also, in the case of a strong thermal coupling, the limit of the supply current parameters, for which the oscillations exist, expands by ~ 10 %. The obtained results have a universal character and open up a new kind of coupling in ONNs, namely, T-coupling, which allows for easy transition from 2D to 3D integration. The effect of subharmonic synchronization hold promise for application in classification and pattern recognition.

cs.NE↗

arXiv2019

Oscillator Circuit for Spike Neural Network with Sigmoid Like Activation Function and Firing Rate Coding

The study presents an oscillator circuit for a spike neural network with the possibility of firing rate coding and sigmoid-like activation function. The circuit contains a switching element with an S-shaped current-voltage characteristic and two capacitors; one of the capacitors is shunted by a control resistor. The circuit is characterised by a strong dependence of the frequency of relaxation oscillations on the magnitude of the control resistor. The dependence has a sigmoid-like form and we present an analytical method for dependence calculation. Finally, we describe the concept of the spike neural network architecture with firing rate coding based on the presented circuit for creating neuromorphic devices and artificial intelligence.

cs.ET↗

arXiv2019

A Model of an Oscillatory Neural Network with Multilevel Neurons for Pattern Recognition and Computing

The current study uses a novel method of multilevel neurons and high order synchronization effects described by a family of special metrics, for pattern recognition in an oscillatory neural network (ONN). The output oscillator (neuron) of the network has multilevel variations in its synchronization value with the reference oscillator, and allows classification of an input pattern into a set of classes. The ONN model is implemented on thermally-coupled vanadium dioxide oscillators. The ONN is trained by the simulated annealing algorithm for selection of the network parameters. The results demonstrate that ONN is capable of classifying 512 visual patterns (as a cell array 3 * 3, distributed by symmetry into 102 classes) into a set of classes with a maximum number of elements up to fourteen. The classification capability of the network depends on the interior noise level and synchronization effectiveness parameter. The model allows for designing multilevel output cascades of neural networks with high net data throughput. The presented method can be applied in ONNs with various coupling mechanisms and oscillator topology.

cs.ET↗

arXiv2019

Switch Elements with S-Shaped Current-Voltage Characteristic in Models of Neural Oscillators

In this paper, we present circuit solutions based on a switch element with the S-type I-V characteristic implemented using the classic FitzHugh-Nagumo and FitzHugh-Rinzel models. Using the proposed simplified electrical circuits allows the modeling of the integrate-and-fire neuron and burst oscillation modes with the emulation of the mammalian cold receptor patterns. The circuits were studied using the experimental I-V characteristic of an NbO2 switch with a stable section of negative differential resistance (NDR) and a VO2 switch with an unstable NDR, considering the temperature dependences of the threshold characteristics. The results are relevant for modern neuroelectronics and have practical significance for the introduction of the neurodynamic models in circuit design and the brain-machine interface. The proposed systems of differential equations with the piecewise linear approximation of the S-type I-V characteristic may be of scientific interest for further analytical and numerical research and development of neural networks with artificial intelligence.

physics.app-ph↗

arXiv2019

A Method for Evaluating Chimeric Synchronization of Coupled Oscillators and Its Application for Creating a Neural Network Information Converter

This paper presents a new method for evaluating the synchronization of quasi-periodic oscillations of two oscillators, termed "chimeric synchronization". The family of metrics is proposed to create a neural network information converter based on a network of pulsed oscillators. In addition to transforming input information from digital to analogue, the converter can perform information processing after training the network by selecting control parameters. In the proposed neural network scheme, the data arrives at the input layer in the form of current levels of the oscillators and is converted into a set of non-repeating states of the chimeric synchronization of the output oscillator. By modelling a thermally coupled VO2-oscillator circuit, the network setup is demonstrated through the selection of coupling strength, power supply levels, and the synchronization efficiency parameter. The distribution of solutions depending on the operating mode of the oscillators, sub-threshold mode, or generation mode are revealed. Technological approaches for the implementation of a neural network information converter are proposed, and examples of its application for image filtering are demonstrated. The proposed method helps to significantly expand the capabilities of neuromorphic and logical devices based on synchronization effects.

nlin.AO↗

arXiv2018

Method of increasing the information capacity of associative memory of oscillator neural networks using high-order synchronization effect

Computational modelling of two- and three-oscillator schemes with thermally coupled $VO_2$-switches is used to demonstrate a novel method of pattern storage and recognition in an impulse oscillator neural network (ONN) based on the high-order synchronization effect. The method ensures high information capacity of associative memory, i.e. a large number of synchronous states $N_s$. Each state in the system is characterized by the synchronization order determined as the ratio of harmonics number at the common synchronization frequency. The modelling demonstrates attainment of $N_s$ of several orders both for a three-oscillator scheme $N_s$~650 and for a two-oscillator scheme $N_s$~260. A number of regularities are obtained, in particular, an optimal strength of oscillator coupling is revealed when $N_s$ has a maximum. A general tendency toward information capacity decrease is shown when the coupling strength and switch inner noise amplitude increase. An algorithm of pattern storage and test vector recognition is suggested. It is also shown that the coordinate number in each vector should be one less than the switch number to reduce recognition ambiguity. The demonstrated method of associative memory realization is a general one and it may be applied in ONNs with various mechanisms and oscillator coupling topology.

nlin.AO↗