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Pavel Borisov

Publications and source records attributed to Pavel Borisov.

11 recordsLinked to original sources

Scalable platform enabling reservoir computing with nanoporous oxide memristors for image recognition and time series prediction

Typical mammal brains have some form of random connectivity between neurons. Reservoir computing, a neural network approach, uses random weights within its processing layer along with built-in recurrent connections and short-term, fading memory, and is shown to be time and training efficient in processing spatiotemporal signals. Here we prepared a niobium oxide-based thin film memristor device with intrinsic structural in-homogeneity in the form of random nanopores and performed computational tasks of XOR operations, image recognition, and time series prediction and reconstruction. For the latter task we chose a complex three-dimensional chaotic Lorenz-63 time series. By applying three temporal voltage waveforms individually across the device and training the readout layer with electrical current signals from a three-output physical reservoir, we achieved satisfactory prediction and reconstruction accuracy in comparison to the case of no reservoir. This work highlights the potential for scalable, on-chip devices using all-oxide reservoir systems, paving the way for energy-efficient neuromorphic electronics dealing with time signals.

cond-mat.dis-nn

Stochastic Dynamics of Diffusive Memristor Blocks for Neuromorphic Computing

Biological systems use neural circuits to integrate input information and produce outputs. Synaptic convergence, where multiple neurons converge their inputs onto a single downstream neuron, is common in natural neural circuits. However, understanding specific computations performed by such neural blocks and implementating them in hardware requires further research. This work focuses on synaptic convergence in a simplified circuit of three spiking artificial neurons based on diffusive memristors. Numerical modelling and experiments reveal input voltage combinations that enable targeted activation of spiking for specific neuron configurations. We analyse the statistical characteristics of spiking patterns and interpret them from a computational perspective. The numerical simulations match experimental measurements. Our findings contribute to development of universal functional blocks for neuromorphic systems.

cond-mat.mes-hall

Stress-induced Artificial neuron spiking in Diffusive memristors

Diffusive memristors owing to their ability to produce current spiking when a constant or slowly changing voltage is applied are competitive candidates for the development of artificial electronic neurons. These artificial neurons can be integrated into various prospective autonomous and robotic systems as sensors, e.g. ones implementing object grasping and classification. We report here Ag nanoparticle-based diffusive memristor prepared on a flexible polyethylene terephthalate (PET) substrate in which the electric spiking behaviour was induced by the electric voltage under an additional stimulus of external mechanical impact. By changing the magnitude and frequency of the mechanical impact, we are able to manipulate the spiking response of our artificial neuron. This functionality to control the spiking characterstics paves a pathway for the development of touch-perception sensors that can convert local pressure into electrical spikes for further processing in neural networks. We have proposed a mathematical model which captures the operation principle of the fabricated memristive sensors and qualitatively describes the measured spiking behaviour.

physics.app-ph

Stochastic Instabilities of the Diffusive Memristor

Recently created diffusive memristors have garnered significant research interest owing to their distinctive capability to generate a diverse array of spike dynamics which are similar in nature to those found in biological cells. This gives the memristor an opportunity to be used in a wide range of applications, specifically within neuromorphic systems. The diffusive memristor is known to produce regular, chaotic and stochastic behaviors which leads to interesting phenomena resulting from the interactions between the behavioral properties. The interactions along with the instabilities that lead to the unique spiking phenomena are not fully understood due to the complexities associated with examining the stochastic properties within the diffusive memristor. In this work, we analyze both the classical and the noise induced bifurcations that a set of stochastic differential equations, justified through a Fokker-Planck equation used to model the diffusive memristor, can produce. Finally, we replicate the results of the numerical stochastic threshold phenomena with experimentally measured spiking.

physics.app-ph

Memristive behavior of functionalized graphene quantum dot and polyaniline nanocomposites

Zero-dimensional graphene quantum dots (GQD) dispersed in conducting polymer matrix display a striking range of optical, mechanical, and thermoelectric properties which can be utilized to design next-generation sensors and low-cost thermoelectric. This exotic electrical property in GQDs is achieved by exploiting the concentration of the GQDs and by tailoring the functionalization of the GQDs. However, despite extensive investigation, the nonlinear resistivity behavior leading to memristive like characteristic has not been explored much. Here, we report electrical characterisation of nitrogen functionalized GQD (NGQD) embedded in a polyaniline (PANI) matrix. We observe a strong dependence of the resistance on current and voltage history, the magnitude of which depends on the NGQD concentration and temperature. We explain this memristive property using a phenomenological model of the alignment of PANI rods with a corresponding charge accumulation arising from the NGQD on its surface. The NGQD-PANI system is unique in its ability to matrix offers a unique pathway to design neuromorphic logic and synaptic architectures with crucial advantages over existing systems.

cond-mat.mes-hall

4D visualization of the photoexcited coherent magnon by an X-ray free electron laser

X-ray free electron lasers (XFEL) create femtosecond X-ray pulses with high brightness and high longitudinal coherence allowing to extend X-ray spectroscopy and scattering techniques into the ultrafast time-domain. These X-rays are a powerful probe for studying coherent quasiparticle excitations in condensed matter triggered by an impulsive optical laser pump. However, unlike coherent phonons, other quasiparticles have been rarely observed due to small signal changes and lack of standards for the identification. Here, we exploit resonant magnetic X-ray diffraction using an XFEL to visualize a photoexcited coherent magnon in space and time. Large intensity oscillations in antiferromagnetic and ferromagnetic Bragg reflections from precessing moment are observed in a multiferroic Y-type hexaferrite. The precession trajectory reveals that a large, long-lived, photoinduced magnetic-field changes the net magnetization substantially through the large-amplitude of the magnon. This work demonstrates an efficient XFEL probe for the coherent magnon in the spotlight for opto-spintronics application.

cond-mat.str-el

Structural and electrical characterization of NbO2 thin film vertical devices grown on TiN-coated SiO2/Si substrates

We report on the electrical properties of polycrystalline NbO2 thin film vertical devices grown on TiN coated SiO2/Si substrates using pulsed laser deposition. First, we analyzed the thickness and contact size dependences of threshold switching of NbO2 films grown in 10 mTorr Ar/O2 mixed growth pressure, where 25.1%/74.9% of NbO2/Nb2O5 surface composition content was estimated by ex-situ x-ray photoelectron spectroscopy. The leakage current properties in the insulating state were dominated by trap-charge assisted Poole-Frankel conduction mechanism. The threshold switching and self-sustained current oscillatory behavior of films with different NbO2/Nb2O5 composition ratios was measured and analyzed. The second film was grown in lower (1 mTorr) growth pressure which resulted in a higher (34.2%/65.8%) NbO2/Nb2O5 film surface composition. The film grown in higher growth pressure demonstrated lower off-state leakage current, faster switching and self-sustained oscillations with higher frequency than the film grown in lower growth pressure.

physics.app-ph

Weak ferromagnetism and short range polar order in NaMnF$_3$ thin films

The orthorhombically distorted perovskite NaMnF$_{3}$ has been predicted to become ferroelectric if an $a=c$ distortion of the bulk $Pnma$ structure is imposed. In order to test this prediction, NaMnF$_{3}$ thin films were grown on SrTiO$_{3}$ (001) single crystal substrates via molecular beam epitaxy. The best films were smooth and single phase with four different twin domains. In-plane magnetization measurements revealed the presence of antiferromagnetic ordering with weak ferromagnetism below the Néel temperature $T_N=66$ K. For the dielectric studies, NaMnF$_{3}$ films were grown on a 30 nm SrRuO$_{3}$ (001) layer used as a bottom electrode grown via pulsed laser deposition. The complex permittivity as a function of frequency indicated a strong Debye-like relaxation contribution characterized by a distribution of relaxation times. A power-law divergence of the characteristic relaxation time revealed an order-disorder phase transition at 8 K. The slow relaxation dynamics indicated the formation of super-dipoles (superparaelectric moments) that extend over several unit cells, similar to polar nanoregions of relaxor ferroelectrics.

cond-mat.mtrl-sci

Antiferromagnetic spin Seebeck Effect

We report on the observation of the spin Seebeck effect in antiferromagnetic MnF$_2$. A device scale on-chip heater is deposited on a bilayer of Pt (4 nm)/MnF$_2$ (110) (30 nm) grown by molecular beam epitaxy on a MgF$_2$ (110) substrate. Using Pt as a spin detector layer it is possible to measure thermally generated spin current from MnF$_2$ through the inverse spin Hall effect. The low temperature (2 - 80 K) and high magnetic field (up to 140 kOe) regime is explored. A clear spin flop transition corresponding to the sudden rotation of antiferromagnetic spins out of the easy axis is observed in the spin Seebeck signal when large magnetic fields (>9 T) are applied parallel the easy axis of the MnF$_2$ thin film. When magnetic field is applied perpendicular to the easy axis, the spin flop transition is absent, as expected.

cond-mat.mtrl-sci

Preparation, characterization, and electrical properties of epitaxial NbO2 thin film lateral devices

Epitaxial NbO2 (110) films, 20 nm thick, were grown by pulsed laser deposition on Al2O3 (0001) substrates. The Ar/O2 total pressure during growth was varied to demonstrate the gradual transformation between NbO2 and Nb2O5 phases, which was verified using x-ray diffraction, x-ray photoelectron spectroscopy, and optical absorption measurements. Electric resistance threshold switching characteristics were studied in a lateral geometry using interdigitated Pt top electrodes in order to preserve the epitaxial crystalline quality of the films. Volatile and reversible transitions between high and low resistance states were observed in epitaxial NbO2 films, while irreversible transitions were found in case of Nb2O5 phase. Electric field pulsed current measurements confirmed thermally-induced threshold switching.

cond-mat.mtrl-sci

Phase diagram of a three-dimensional antiferromagnet with random magnetic anisotropy

Three-dimensional (3D) antiferromagnets with random magnetic anisotropy (RMA) experimentally studied to date do not have random single-ion anisotropies, but rather have competing two-dimensional and three-dimensional exchange interactions which can obscure the authentic effects of RMA. The magnetic phase diagram Fe$_{x}$Ni$_{1-x}$F$_{2}$ epitaxial thin films with true random single-ion anisotropy was deduced from magnetometry and neutron scattering measurements and analyzed using mean field theory. Regions with uniaxial, oblique and easy plane anisotropies were identified. A RMA-induced glass region was discovered where a Griffiths-like breakdown of long-range spin order occurs.

cond-mat.mtrl-sci