Self-sustained Current Oscillations Research Articles

Hysteresis and self-oscillations in an artificial memristive quantum neuron

We theoretically study an artificial neuron circuit containing a quantum memristor in the presence of relaxation and dephasing. The charge transport in the quantum element is realized via tunneling of a charge through a quantum particle which shuttles between two terminals—a functionality reminiscent of classical diffusive memristors. We demonstrate that this physical principle enables hysteretic behavior in the current-voltage characteristics of the quantum device. In addition, being used in an artificial neural circuit, the quantum switcher is able to generate self-sustained current oscillations. Our analysis reveals that these self-oscillations are triggered only in quantum regimes with a moderate rate of relaxation, and cannot exist either in a purely coherent regime or at a very high decoherence. We investigate the hysteresis and instability leading to the onset of current self-oscillations and analyze their properties depending on the circuit parameters. Our results provide a generic approach to the use of quantum regimes for controlling hysteresis and generating self-oscillations. Published by the American Physical Society 2024

Tunable High-Frequency Acoustoelectric Current Oscillations in Fluorine-Doped Single-Walled Carbon Nanotubes

Herein, we report on a fluorine-doped single-walled carbon nanotube (FSWCNT) phenomenon, that yields tunable high-frequency self-sustained acoustoelectric direct current (ADC) oscillations. A tractable analytical method was used in the hypersound domain, to base the calculations on carriers in the lowest miniband. Hypothetically, the energy of interaction between the carriers and the acoustic phonons is less than the energy of the typical carriers. High-order harmonics of the acoustic phonons’ effective field could be disregarded under this supposition. The ADC was observed to exhibit a nonlinearity, that resulted from the carrier distribution function’s distortion as a result of interaction with the acoustic phonons, which had strong nonlinear effects. Theoretically, we demonstrated that the dynamics of space charge instabilities, due to Bragg reflection of Bloch oscillating carriers in the FSWCNT’s miniband, were the only factors which contributed to the creation of radiation in the terahertz (THz) frequency range. The study also investigated the influence of various FSWCNT parameters such as the overlapping integrals (Δs and Δz), ac-field E1, and carrier concentration noon the behaviour of the ADC. The results showed that the intensity of the ADC oscillation Jzzae/Joae could be tuned by adjusting Δs, Δz, E1, and no.This tunability suggests that FSWCNTs could be used as an active device operating at very high frequencies, potentially reaching the submillimeter wavelength range. The study also suggests the possibility of domain suppression and acoustic Bloch gain through dynamic ADC stabilisation.

Transition from noise-induced to self-sustained current spiking generated by a NbOx thin film threshold switch

A transition to self-sustained current oscillations was investigated in a circuit with a NbOx thin film device, acting as an artificial neuron. Above a threshold of the applied voltage, the circuit begins to demonstrate persistent but rather random current spiking, which becomes more regular with increasing applied voltage. Experimental measurements reveal two scaling regions of the interspike interval dependence on source voltage: an initial logarithmic dependence, which transitions to a power law with an exponent of -1/2. Using a numerical model of the artificial neuron circuit, we observe good agreement with our experimental data. We show that the logarithmic scaling behavior is associated with noise-induced spiking, while the power law is attributed to noisy self-sustained oscillations, which would exist in the system even with the absence of noise. These findings stress the importance of noise in the dynamics of NbOx threshold switches and shed light on their spiking response to variation of the input signals. The reported approach to spiking control in NbOx artificial oscillation neurons can help to develop an optimal design for neuromorphic neural networks and oscillator-based computing systems.

Optogalvanic effect and laser-induced current oscillations in hollow-cathode lamps

This paper presents a study of two commercial hollow-cathode lamps (HCLs) with the intention of demonstrating different phenomena in gas discharges. The optogalvanic effect in both HCLs is produced by a laser diode radiated at the wavelength that corresponds to neon transition 1s2–2p2 at 659.89 nm. The voltage–current characteristics of the lamps are explained using a classical theory of hollow-cathode discharge, while the optogalvanic signal is treated as a small perturbation of the discharge current. For certain values of voltage self-sustained current oscillations are observed in one of the HCLs. In the same HCL laser-induced optogalvanic dumped oscillations are detected. A phenomenological model that includes the effective circuit parameters of the discharge is used to explain the oscillation characteristics.

The role of defects in the electrical properties of NbO2 thin film vertical devices

Epitaxial NbO2 thin films were grown on Si:GaN layers deposited on Al2O3 substrates using pulsed laser deposition. Pulsed current-voltage (IV) curves and self-sustained current oscillations were measured across a 31 nm NbO2 film and compared with a similar device made from polycrystalline NbO2 film grown on TiN-coated SiO2/Si substrate. Crystal quality of the as grown films was determined from x-ray diffractometry, x-ray photoelectron spectroscopy and atomic force microscopy data. The epitaxial film device was found to be more stable than the defect-rich polycrystalline sample in terms of current switching and oscillation behaviors.

Self-sustained current oscillations in spin-blockaded quantum dots

Self-sustained current oscillation observed in spin-blockaded double quantum dots is explained as a consequence of periodic motion of dynamically polarized nuclear spins (along a limit cycle) in the spin-blockaded regime under an external magnetic field and a spin-transfer torque. It is shown, based on the Landau-Lifshtz-Gilbert equation, that a sequence of semistable limit cycle, Hopf and homoclinic bifurcations occurs as the external field is tuned. The divergent period near the homoclinic bifurcation explains well why the period in the experiment is so long and varies by many orders of magnitudes.

Chaotic dynamics dependence on doping density in weakly coupled GaAs/AlAs superlattices

A discrete sequential tunneling model is used for studying the influence of the doping density on the dynamical behaviors in weakly coupled GaAs/AlAs superlattices. Driven by the DC bias, the system exhibits self-sustained current oscillations induced by the period motion of the unstable electric field domain, and an electrical hysteresis in the loop of current density voltage curve is deduced. It is found that the hysteresis range strongly depends on the doping density, and the width of the hysteresis loop increases with increasing the doping density. By adding an external driving ac voltage, more complicated nonlinear behaviors are observed including quasiperiodicity, period-3, and the route of an inverse period-doubling to chaos when the driving frequency changes.

Physics of self-sustained oscillations in the positive glow corona

The physics of self-sustained oscillations in the phenomenon of positive glow corona is presented. The dynamics of charged-particle oscillation under static electric field has been briefly outlined; and, the resulting self-sustained current oscillations in the electrodes have been compared with the measurements from the positive glow corona experiments. The profile of self-sustained electrode current oscillations predicted by the presented theory qualitatively agrees with the experimental measurements. For instance, the experimentally observed saw-tooth shaped electrode current pulses are reproduced by the presented theory. Further, the theory correctly predicts the pulses of radiation accompanying the abrupt rises in the saw-tooth shaped current oscillations, as verified from the various glow corona experiments.

Experimental observation of spontaneous chaotic current oscillations in GaAs/Al0.45Ga0.55As superlattices at room temperature

Spontaneous self-sustained chaotic current oscillations are observed experimentally in lightly-doped weakly-coupled GaAs/Al0.45Ga0.55As superlattices at room temperature for the first time. The mole fraction of Aluminum in the barrier is chosen to be 0.45 to suppress the thermal carrier leakage through the X-band valley. The effective nonlinearity induced by the sequential well-to-well resonant tunneling can still be strong enough to induce spontaneous chaotic current oscillations even at room temperature. The frequency spectrum of the chaotic current oscillations is ranged from DC to 4 GHz, which can be used as ultra-wide-band noise sources with a bandwidth of several Giga Hertz.

Effect of temperature on resonant electron transport through stochastic conduction channels in superlattices

We show that resonant electron transport in semiconductor superlattices with an applied electric and tilted magnetic field can, surprisingly, become more pronounced as the lattice and conduction electron temperature increases from 4.2 K to room temperature and beyond. It has previously been demonstrated that at certain critical field parameters, the semiclassical trajectories of electrons in the lowest miniband of the superlattice change abruptly from fully localised to completely unbounded. The unbounded electron orbits propagate through intricate web patterns, known as stochastic webs, in phase space, which act as conduction channels for the electrons and produce a series of resonant peaks in the electron drift velocity versus electric field curves. Here, we show that increasing the lattice temperature strengthens these resonant peaks due to a subtle interplay between thermal population of the conduction channels and transport along them. This enhances both the electron drift velocity and the influence of the stochastic webs on the current-voltage characteristics, which we calculate by making self-consistent solutions of the coupled electron transport and Poisson equations throughout the superlattice. These solutions reveal that increasing the temperature also transforms the collective electron dynamics by changing both the threshold voltage required for the onset of self-sustained current oscillations, produced by propagating charge domains, and the oscillation frequency.

Electrical control of ferromagnetism and bias anomaly in Mn-doped semiconductor heterostructures

The interplay of tunneling transport and carrier-mediated ferromagnetism in narrow semiconductor multiquantum well structures containing layers of GaMnAs is investigated within a self-consistent Green's function approach, accounting for disorder in the Mn-doped regions and unwanted spin flips at heterointerfaces on phenomenological ground. We find that the magnetization in GaMnAs layers can be controlled by an external electric bias. The underlying mechanism is identified as spin-selective hole tunneling in and out of the Mn-doped quantum wells, whereby the applied bias determines both hole population and spin polarization in these layers. In particular, we predict that, near resonance, ferromagnetic order in the Mn-doped quantum wells is destroyed. The interplay of both magnetic and transport properties combined with structural design potentially leads to several interrelated physical phenomena, such as dynamic spin filtering, tunneling-induced bias anomaly, electrical control of magnetization in individual magnetic layers, and, under specific bias conditions, to self-sustained current and magnetization oscillations (magnetic multistability). Relevance to recent experimental results is discussed.

Enhanced performance of CO poisoned proton exchange membrane fuel cells via triode operation

The effect of triode operation on the performance of CO poisoned PEM fuel cells was investigated. In this mode of operation a third, auxiliary, electrode is introduced in addition to the anode and the cathode. Application of electrolytic current in the auxiliary circuit, comprising the cathode and the auxiliary electrode was found to significantly enhance the time-averaged power output of a state-of-the-art PEMFC unit operating with a 70 ppm CO in H 2 atmospheric pressure mixture. Both normal and triode operation were found to lead to self-sustained current and potential oscillations in the fuel cell circuit over wide ranges of external resistive load. The time averaged increase in power output was found to be typically a factor of three higher than the power output in conventional fuel cell operation and up to a factor of 1.32 larger than the power sacrificed in the auxiliary circuit. The mechanism of the enhanced anodic electrocatalysis was investigated via the use of two reference electrodes and the results are discussed together with a possible design for application of the triode concept in stacks.

Discrete self-oscillation period branches observed in semiconductor superlattices

We investigate the self-sustained current oscillation of a weakly coupled semiconductor superlattice in the dynamical voltage band using a microscopic sequential tunneling model. With the voltage as a control parameter, two types of branches of current oscillation period versus voltage have been observed, which correspond to various oscillation scenarios. The first branch type consists of a series of period branches in accordance with how many charge dipoles need to be created at the emitter side to trigger a dipole-tripole oscillation scenario. For the second branch type, charge dipoles are generated periodically at the emitter, but all of them fail to develop completely and die out, thereby leading to a low-period oscillation scenario without the dipole-tripole process. The bistability between different branches is also observed by voltage up-sweeping and down-sweeping.

Proposal for a ferromagnetic multiwell spin oscillator

The highly nonlinear coupling of transport and magnetic properties in a multiwell heterostructure, which comprises ferromagnetic quantum wells made of diluted magnetic semiconductors, is theoretically investigated. The interplay of resonant tunneling and carrier-mediated ferromagnetism in the magnetic wells induces very robust, self-sustained current and magnetization oscillations. Over a large window of steady bias voltages the spin polarization of the collector current is oscillating between positive and negative values, realizing a spin oscillator device.

Two miniband model for self-sustained oscillations of the current through resonant-tunneling semiconductor superlattices

A two miniband model for electron transport in semiconductor superlattices that includes scattering and interminiband tunnelling is proposed. The model equations for Wigner functions in a basis spanned by Pauli matrices include electron-electron scattering in the Hartree approximation and modified Bhatnagar-Gross-Krook collision terms. For strong applied fields, balance equations for the electric field and the miniband populations are derived using a Chapman-Enskog perturbation technique. These equations are then solved numerically for a dc voltage biased superlattice. Results include self-sustained current oscillations due to repeated nucleation of electric field pulses at the injecting contact region and their motion toward the collector. Numerical reconstruction of the Wigner functions shows that the miniband with higher energy is empty during most of the oscillation period: it becomes populated only when the local electric field (corresponding to the passing pulse) is sufficiently large to trigger resonant tunneling.

Using acoustic waves to induce high-frequency current oscillations in superlattices

We show that GHz acoustic waves in semiconductor superlattices can induce THz electron dynamics that depend critically on the wave amplitude. Below a threshold amplitude, the acoustic wave drags electrons through the superlattice with a peak drift velocity overshooting that produced by a static electric field. In this regime, single electrons perform drifting orbits with THz frequency components. When the wave amplitude exceeds the critical threshold, an abrupt onset of Bloch-like oscillations causes negative differential velocity. The acoustic wave also affects the collective behavior of the electrons by causing the formation of localised electron accumulation and depletion regions, which propagate through the superlattice, thereby producing self-sustained current oscillations even for very small wave amplitudes. We show that the underlying single-electron dynamics, in particular the transition between the acoustic wave dragging and Bloch oscillation regimes, strongly influence the spatial distribution of the electrons and the form of the current oscillations. In particular, the amplitude of the current oscillations depends non-monotonically on the strength of the acoustic wave, reflecting the variation of the single-electron drift velocity.

Magnetoswitching of current oscillations in dilute magnetic semiconductor nanostructures

Strongly nonlinear transport through dilute magnetic semiconductor multiquantum wells occurs due to the interplay between confinement, Coulomb, and exchange interaction. Nonlinear effects include the appearance of spin-polarized stationary states and self-sustained current oscillations as possible stable states of the nanostructure, depending on its configuration and control parameters such as voltage bias and level splitting due to an external magnetic field. Oscillatory regions grow in size with well number and level splitting. A systematic analysis of the charge and spin response to voltage and magnetic field switching of II-VI dilute magnetic semiconductor multiquantum wells is carried out. The description of stationary and time-periodic spin-polarized states, the transitions between them and the responses to voltage or magnetic field switching have great importance due to the potential implementation of spintronic devices based on these nanostructures.

Terahertz generation and chaotic dynamics in single-walled zigzag carbon nanotubes

We study self-sustained terahertz current oscillation and chaotic dynamics in semiconducting single-walled zigzag carbon nanotubes using the time-dependent drift diffusion equations. The current oscillation under a dc voltage bias originates from the negative differential velocity of carbon nanotube which induces the motion and recycling of unstable domain. Numerical simulation indicates that different nonlinear oscillatory modes appear when an external high-frequency ac voltage is superimposed to the dc voltage bias and its driving amplitude varies. The appearance of different nonlinear oscillating modes, including periodic and chaotic, is attributed to the competition between the natural oscillation and the external driving oscillation. The transitions between periodic and chaotic states are carefully studied using chaos-detecting methods, such as bifurcation diagram, phase portraits, first return map, and Fourier spectrum. The resulting bifurcation diagram displays an interesting and complex transition picture with the driving amplitudes as the control parameter.

Self-sustained current oscillations in the kinetic theory of semiconductor superlattices

We present the first numerical solutions of a kinetic theory description of self-sustained current oscillations in n-doped semiconductor superlattices. The governing equation is a single-miniband Boltzmann–Poisson transport equation with a BGK (Bhatnagar–Gross–Krook) collision term. Appropriate boundary conditions for the distribution function describe electron injection in the contact regions. These conditions seamlessly become Ohm’s law at the injecting contact and the zero charge boundary condition at the receiving contact when integrated over the wave vector. The time-dependent model is numerically solved for the distribution function by using the deterministic Weighted Particle Method. Numerical simulations are used to ascertain the convergence of the method. The numerical results confirm the validity of the Chapman–Enskog perturbation method used previously to derive generalized drift-diffusion equations for high electric fields because they agree very well with numerical solutions thereof.

Self-sustained spin-polarized current oscillations in multiquantum well structures

Nonlinear transport through diluted magnetic semiconductor nanostructures is investigated. We have considered a II–VI multiquantum well nanostructure whose wells are selectively doped with Mn. The response to a dc voltage bias may be either a stationary or an oscillatory current. We have studied the transition from stationary to time-dependent current as a function of the doping density and the number of quantum wells. Analysis and numerical solution of a nonlinear spin transport model shows that the current in a structure without magnetic impurities is stationary, whereas current oscillations may appear if at least one well contains magnetic impurities. For long structures having two wells with magnetic impurities, a detailed analysis of nucleation of charge dipole domains shows that self-sustained current oscillations are caused by repeated triggering of dipole domains at the magnetic wells and motion towards the collector. Depending on the location of the magnetic wells and the voltage, dipole domains may be triggered at both wells or at only one. In the latter case, the well closer to the collector may inhibit domain motion between the first and the second well inside the structure. Our study could allow design of oscillatory spin-polarized current injectors.