Time-averaged Current Research Articles

Current shear and turbulence during a near-inertial wave

Surface currents and turbulent mixing were observed during a near-inertial wave (NIW) using an accousting doppler current profiler (ADCP) and satellite-tracked drifters. Drifter trajectories sampled at three depth levels show characteristics of an Ekman solution superposed with the NIW. Velocity and dissipation estimates from the ADCP reveal strong shear with a distinct constant flux layer in between the roughness length and a critical depth at 4m. Below, a shear free slab layer performing an inertial oscillation is observed. Dissipation, as estimated from the vertical beam of the ADCP, peaks in the wave-enhanced friction layer when the current opposes the wind and wave direction. Below the constant flux layer, maximum turbulence is observed when the NIW is in a phase that is in opposite direction to the time-averaged current. During this phase, currents at various depths rapidly realign in the entire boundary layer.

On correlations and fluctuations of time-averaged densities and currents with general time-dependence

We present technical results required for the description and understanding of correlations and fluctuations of the empirical density and current as well as diverse time-integrated and time-averaged thermodynamic currents of diffusion processes with a general time dependence on all time scales. In particular, we generalize the results from Dieball and Godec (2022 Phys. Rev. Lett. 129 140601); Dieball and Godec (2022 Phys. Rev. Res. 4 033243); Dieball and Godec (2022 arXiv:2206.04034 [cond-mat.stat-mech]) to additive functionals with explicit time dependence and transient or non-ergodic overdamped diffusion. As an illustration we apply the results to two-dimensional harmonically confined overdamped diffusion in a rotational flow evolving from a non-stationary initial distribution.

Electron Shuttle as an Autonomous Single-Electron Source

The majority of experimental realizations of single-electron sources rely on the periodic manipulation of the tunnel junctions through their gate voltages, and thus require a high level of control over the system. To circumvent the necessity of external driving, we utilize the self-oscillatory behavior of the electron shuttle. By means of waiting time distributions, which had not been applied to this autonomous system before, we extensively assess the performance of the shuttle as a single-electron source. We unveil a smooth transition between three regimes, whereas previous studies at the same mean field level of description only predict a sharp bifurcation based on the time-averaged electron current. Over the parameter range of already existing experimental realizations the electron shuttle can perform as a single-electron source, albeit with moderate precision. We propose an alternative design of the position-dependent tunneling rates, which significantly decreases the relative error of charge transmission, and thus paves the way for the realization of autonomous single-electron sources.

Downconversion of quantum fluctuations of photonic heat current in a circuit

We discuss the non-zero frequency noise of heat current with the explicit example of energy carried by thermal photons in a circuit. Instead of the standard circuit modelling that gives a convenient way of predicting time-averaged heat current, we describe a setup composed of two resistors forming the heat baths by collections of bosonic oscillators. In terms of average heat transport this model leads to identical results with the conventional one, but besides this, it yields a convenient way of dealing with noise as well. The non-zero frequency heat current noise does not vanish in equilibrium even at zero temperature, the result that is known for, e.g., electron tunneling. We present a modulation method that can convert the difficult-to-measure high frequency quantum noise down to zero frequency.

Impact of ion implantation on stacked oxide cylindrical gate junctionless accumulation mode MOSFET: An electrical and circuit level analysis

This article analyses the effect of ion implantation on electrical and circuit-level characteristics of stacked oxide cylindrical gate (CG) junctionless accumulation mode (JAM) MOSFET. Ion implantation and subsequent annealing create a Gaussian type doping profile inside the channel. A detailed study of electrical characteristics such as: drain current, threshold voltage, Ion/Ioff, transconductance (gm), output conductance (gd), capacitances (Cgs, Cgd, Cgg), unit gain frequency (fT), transit time (τ), transconductance frequency product (TFP) and gain bandwidth (GBW) product have been performed. Further, an inverter has been designed with a complimentary p-MOSFET and a lookup table-based Verilog-A model has been generated for the same. Various dc and transient parameters such as noise margin, rise time, fall time propagation delay, voltage overshoot, average current, and power dispassion have been analyzed in detail. Henceforth, a 6 T SRAM cell has been implemented with ion-implanted stacked oxide CG-JAM MOSFET and read margin, write margin, access time and N-curve analysis have been performed. Various figures of merits (FOMs) have also been studied for the device at the circuit level. All the device simulations have been performed using a 3-D TCAD tool from COGENDA. Verilog-A model-based circuit simulations have been done in the Cadence-Virtuoso platform.

Photoinduced anomalous Hall effect in two-dimensional transition metal dichalcogenides

A circularly polarized a.c. pump field illuminated near resonance on two-dimensional transition metal dichalcogenides (TMDs) produces an anomalous Hall effect in response to a d.c. bias field. In this work, we develop a theory for this photo-induced anomalous Hall effect in undoped TMDs irradiated by a strong coherent laser field. The strong field renormalizes the equilibrium bands and opens up a dynamical energy gap where single-photon resonance occurs. The resulting photon dressed states, or Floquet states, are treated within the rotating wave approximation. A quantum kinetic equation approach is developed to study the non-equilibrium density matrix and time-averaged transport currents under the simultaneous influence of the strong a.c. pump field and the weak d.c. probe field. Dissipative effects are taken into account in the kinetic equation that captures relaxation and dephasing. The photo-induced longitudinal and Hall conductivities display notable resonant signatures when the pump field frequency reaches the spin-split interband transition energies. Rather than valley polarization, we find that the anomalous Hall current is mainly driven by the intraband response of photon-dressed electron populations near the dynamical gap at both valleys, accompanied by a smaller contribution due to interband coherences. These findings highlight the importance of photon-dressed bands and non-equilibrium distribution functions in achieving a proper understanding of photo-induced anomalous Hall effect in a strong pump field.

Simulations of argon plasma decay in a thermionic converter

The dynamics of an argon plasma in the gap of a thermionic diode is investigated using particle-in-cell (PIC) simulations. The time-averaged diode current, as a function of the relative electrical potential between the electrodes, is studied while the plasma density depletes due to recombination on the electrode surfaces. Simulations were performed in both one and two dimensions, and significant differences were observed in the plasma decay between the two cases. Specifically, in two dimensions it was found that the electrostatic potential gradually changes as the plasma decays, while in one dimension fluctuations in the plasma led to large potential fluctuations which changed the plasma decay characteristics relative to the two-dimensional case. This creates significant differences in the time-averaged diode current. Furthermore, it was found that the maximum time-averaged current is collected when the diode voltage is set to the flat-band condition, where the cathode and anode vacuum biases are equal. This suggests a novel technique of measuring the difference in work functions between the cathode and anode in a thermionic converter.

On the generation of electronic ring currents under vibronic coupling effects.

We study the generation of electronic ring currents in the presence of nonadiabatic coupling using circularly polarized light. For this, we introduce a solvable model consisting of an electron and a nucleus rotating around a common center and subject to their mutual Coulomb interaction. The simplicity of the model brings to the forefront the non-trivial properties of electronic ring currents in the presence of coupling to the nuclear coordinates and enables the characterization of various limiting situations transparently. Employing this model, we show that vibronic coupling effects play a crucial role even when a single E degenerate eigenstate of the system supports the current. The maximum current of a degenerate eigenstate depends on the strength of the nonadiabatic interactions. In the limit of large nuclear to electronic masses, in which the Born-Oppenheimer approximation becomes exact, constant ring currents and time-averaged oscillatory currents necessarily vanish.

Spin Resonance in a Quantum Dot at the Edge of a Topological Insulator with the Inclusion of Continuum States

The dynamics of electronic states under the action of a periodic electric field applied to a quantum dot created by magnetic barriers at the one-dimensional edge of a two-dimensional topological insulator based on a HgTe/CdTe quantum well has been studied. A configuration with two discrete levels is considered and transitions to continuum states above barriers are taken into account. The frequency of oscillations of the discrete level populations has been calculated for various field amplitudes. It has been shown numerically and analytically that the inclusion of the continuous spectrum leads to a decrease in the total population of discrete levels in time. The characteristic times of this decrease corresponding to transitions to continuum have been determined. The dynamics of average values of the energy, spin projections, coordinates, local probability density, and local spin density has been calculated. Time-averaged local probability density current that describes the quantum dot escape at transitions to the continuous spectrum has been calculated. The results of this work can be useful for the design of new generations of nanoelectronics and spintronics devices based on topological insulators.

Visualizing nonlinear resonance in nanomechanical systems via single-electron tunneling

Numerous reports have elucidated the importance of mechanical resonators comprising quantum-dot-embedded carbon nanotubes (CNTs) for studying the effects of single-electron transport. However, there is a need to investigate the single-electron transport that drives a large amplitude into a nonlinear regime. Herein, a CNT hybrid device has been investigated, which comprises a gate-defined quantum dot that is embedded into a mechanical resonator under strong actuation conditions. The Coulomb peak positions synchronously oscillate with the mechanical vibrations, enabling a single-electron “chopper” mode. Conversely, the vibration amplitude of the CNT versus its frequency can be directly visualized via detecting the time-averaged single-electron tunneling current. To understand this phenomenon, a general formula is derived for this time-averaged single-electron tunneling current, which agrees well with the experimental results. By using this visualization method, a variety of nonlinear motions of a CNT mechanical oscillator have been directly recorded, such as Duffing nonlinearity, parametric resonance, and double-, fractional-, mixed- frequency excitations. This approach opens up burgeoning opportunities for investigating and understanding the nonlinear motion of a nanomechanical system and its interactions with electron transport in quantum regimes.

Electron-Tunneling-Assisted Non-Abelian Braiding of Rotating Majorana Bound States.

It has been argued that fluctuations of fermion parity are harmful for the demonstration of non-Abelian anyonic statistics. Here, we demonstrate a striking exception in which such fluctuations are actively used. We present a theory of coherent electron transport from a tunneling tip into a Corbino geometry Josephson junction where four Majorana bound states (MBSs) rotate. While the MBSs rotate, electron tunneling happens from the tip to one of the MBSs thereby changing the fermion parity of the MBSs. The tunneling events in combination with the rotation allow us to identify a novel braiding operator that does not commute with the braiding cycles in the absence of tunneling, revealing the non-Abelian nature of MBSs. The time-averaged tunneling current exhibits resonances as a function of the tip voltage with a period that is a direct consequence of the interference between the noncommuting braiding operations. Our work opens up a possibility for utilizing parity nonconserving processes to control non-Abelian states.

3D-modelling of Lake Kivu: Horizontal and vertical flow and temperature structure under spatially variable atmospheric forcing

With the increasing extraction of methane from Lake Kivu, there is a growing need to evaluate the effect of such operations on the lake’s permanent density stratification. This requires understanding of the spatial structure and variability of flow velocities and constituents in Lake Kivu. In this study, we develop a 3D hydrodynamic model of Lake Kivu, set-up within DELFT3D at a 750 m grid spacing and forced by COSMO-CLM atmosphere model results at a 2.8 km grid spacing. Validation shows that the model correctly reproduces the generation and breakdown of the temperature stratification in the upper mixed layer and predicts flow velocity magnitudes and directions similar to measurements both at the surface and at greater depth. Analysis of currents reveals a surface current pattern with two clockwise circulations, one around the whole lake and a smaller one in the northern part, with velocities around 0.1 m/s. This pattern is consistently present over an (ensemble-)averaged day, both in the wet and in the dry season, while day-by-day variations are large. Time-averaged deep currents are found to be a few mm/s at maximum. However, the variations can be substantial, with standard deviations up to 2 cm/s for the currents at 220 m depth, attributed to internal seiches. The temperature stratification, present during the entire wet season, is found to first break down in the dry season in the southern part of the lake. This is explained by the spatial differences in the wind stress and the evaporation heat fluxes during the dry season.

Space-charge limited current with a finite injection velocity revisited

The problem of space-charge limited current flow between two electrodes with a finite particle injection velocity is revisited theoretically, and numerically with particle-in-cell (PIC) simulations. Using an Eulerian approach, we obtain a complete analytical solution for both the space-charge limited (SCL) current (which agrees with previously derived expressions), as well as the spatial profiles of all physical quantities. These analytical solutions are unique in that they apply to both fluid, and kinetic particle descriptions of the system, and we demonstrate how they can be used to verify the accuracy and correctness of certain aspects of numerical simulation codes. For injection currents above the SCL limit, the PIC simulations show that the system becomes unstable, and self-excited space charge oscillations are generated with a period of the order of the particle transit time between the electrodes. In this oscillatory regime, the maximum time-averaged current that can be transmitted depends on the ratio of the particle injection energy to the applied potential difference across the electrodes, and is always less than the SCL current. By considering a finite particle injection temperature, these oscillations can be damped for sufficiently high temperatures, and we show that a new non-oscillatory steady-state solution can be obtained.

Fast-switching magnet serving a spallation-driven ultracold neutron source

A fast-switching, high-repetition-rate magnet and power supply have been developed for and operated at TRIUMF, to deliver a proton beam to the new ultracold neutron (UCN) facility. The facility possesses unique operational requirements: a time-averaged beam current of 40~$\mu$A with the ability to switch the beam on or off for several minutes. These requirements are in conflict with the typical operation mode of the TRIUMF cyclotron which delivers nearly continuous beam to multiple users. To enable the creation of the UCN facility, a beam-sharing arrangement with another facility was made. The beam sharing is accomplished by the fast-switching (kicker) magnet which is ramped in 50~$\mu$s to a current of 193~A, held there for approximately 1~ms, then ramped down in the same short period of time. This achieves a 12~mrad deflection which is sufficient to switch the proton beam between the two facilities. The kicker magnet relies on a high-current, low-inductance coil connected to a fast-switching power supply that is based on insulated-gate bipolar transistors (IGBTs). The design and performance of the kicker magnet system and initial beam delivery results are reported.

Topological phases and pumps in the Su–Schrieffer–Heeger model periodically modulated in time

By the Floquet theory, we transform the Su–Schrieffer–Heeger model with the periodically modulated nearest-neighbor (NN) and next-nearest-neighbor (NNN) interactions into an effective 2D model, which holds the total Chern number of ±1 modulated by the parameter θ. Under a staggered electric potential, the topological phase diagrams of the effective 2D model are reshaped and similar to the well-known Haldane model. While under a staggered Zeeman field, the topological phase diagram has the same shape as the former case, but with different Chern numbers, such as the spin and valley Chern numbers. With the combination of the staggered Zeeman field and the electric field, the effective 2D model holds even richer topological phases. At last, we find some types of topological pump, which can generate the time-averaged current without any bias voltage. The current depends on their different Chern numbers. In other words, we can modulate the parameters to obtain various Chern numbers to control the topological pump.

Large deviations and dynamical phase transitions in stochastic chemical networks

Chemical reaction networks offer a natural nonlinear generalization of linear Markov jump processes on a finite state-space. In this paper, we analyze the dynamical large deviations of such models, starting from their microscopic version, the chemical master equation. By taking a large-volume limit, we show that those systems can be described by a path integral formalism over a Lagrangian functional of concentrations and chemical fluxes. This Lagrangian is dual to a Hamiltonian, whose trajectories correspond to the most likely evolution of the system given its boundary conditions. The same can be done for a system biased on time-averaged concentrations and currents, yielding a biased Hamiltonian whose trajectories are optimal paths conditioned on those observables. The appropriate boundary conditions turn out to be mixed so that, in the long time limit, those trajectories converge to well-defined attractors. We are then able to identify the largest value that the Hamiltonian takes over those attractors with the scaled cumulant generating function of our observables, providing a nonlinear equivalent to the well-known Donsker-Varadhan formula for jump processes. On that basis, we prove that chemical reaction networks that are deterministically multistable generically undergo first-order dynamical phase transitions in the vicinity of zero bias. We illustrate that fact through a simple bistable model called the Schlögl model, as well as multistable and unstable generalizations of it, and we make a few surprising observations regarding the stability of deterministic fixed points and the breaking of ergodicity in the large-volume limit.

Systematic diagnostics of the electrical, optical, and physicochemical characteristics of low-temperature atmospheric-pressure helium plasma sources

The performance characteristics of two plasma sources, based on different types of discharge, were assessed. Three primary aspects of these sources were examined. These were electrical parameters (voltage and current flowing through a target), plasma parameters (gas temperature and electron density, determined using optical emission spectroscopy), and levels of gas-phase oxygen atoms (measured by vacuum ultraviolet absorption spectroscopy) and liquid-phase ▪OH radicals (generated by plasma treatment of water and detected using an electron spin resonance spin-trapping technique). As a result, there were few significant differences such as electron density, oxygen atom density, and gas temperature between the two plasma sources. However, the time-averaged electrical currents flowing to the target and the amount of liquid-phase ▪OH radicals showed a large difference, especially when the plume made contact with the target. Thus, many points of similarity but some differences in the two devices were found by the benchmarking study. These shall contribute to our understanding of the mechanisms for each consequence in medical applications.

Quasiprobability for the arrival-time problem with links to backflow and the Leggett-Garg inequalities

The arrival time problem for the free particle in one dimension may be formulated as the problem of determining a joint probability for the particle being found on opposite sides of the $x$-axis at two different times. We explore this problem using a two-time quasi-probability linear in the projection operators, a natural counterpart of the corresponding classical problem. We show that it can be measured either indirectly, by measuring its moments in different experiments, or directly, in a single experiment using a pair of sequential measurements in which the first measurement is weak (or more generally, ambiguous). We argue that when positive, it corresponds to a measurement-independent arrival time probability. For small time intervals it coincides approximately with the time-averaged current, in agreement with semiclassical expectations. The quasi-probability can be negative and we exhibit a number of situations in which this is the case. We interpret these situations as the presence of `quantumness', in which the arrival time probability is not properly defined in a measurement-independent manner. Backflow states, in which the current flows in the direction opposite to the momentum, are shown to provide an interesting class of examples such situations. We also show that the quasi-probability is closely linked to a set of two-time Leggett-Garg inequalities, which test for macroscopic realism.

Current reversal in interacting colloids under time-periodic drive

Using molecular dynamics simulations, we study particle-transport in a system of interacting colloidal particles on a ring, where the system is driven by a time-dependent external potential, moving along the ring. We consider two driving protocols: (i) the external potential barrier moves with a uniform velocity $v$ along the ring, and (ii) it moves in discrete jumps with jump-length $l$ and waiting time $\tau$ with an effective velocity $v=l/\tau$. The time-averaged (dc) particle current, which always remains positive in case (i), interestingly reverses its direction in case (ii) upon tuning the particle-number density $\rho_0$ and the effective barrier velocity $v$. We also find a scaling form for the current in terms of number density, barrier velocity, barrier height and temperature of the system.

Magnetic moment generation in small gold nanoparticles via the plasmonic inverse Faraday effect

We theoretically investigate the creation of a magnetic moment in gold nanoparticles by circularly polarized laser light. To this end, we describe the collective electron dynamics in gold nanoparticles using a semiclassical approach based on a quantum hydrodynamic model that incorporates the prin- cipal quantum many-body and nonlocal effects, such as the electron spill-out, the Hartree potential, and the exchange and correlation effects. We use a variational approach to investigate the breathing and the dipole dynamics induced by an external electric field. We show that gold nanoparticles can build up a static magnetic moment through the interaction with a circularly polarized laser field. We analyze that the responsible physical mechanism is a plasmonic, orbital inverse Faraday effect, which can be understood from the time-averaged electron current that contains currents rotating on the nanoparticles surface. The computed laser-induced magnetic moments are sizeable, of about 0.35 muB/atom for a laser intensity of 450 GW/cm2 at plasmon resonance.