Time-dependent Current Research Articles

Eliminate spiral wave and spatiotemporal chaos by using short-term cardiac memory

Cardiac memory effect, which exhibits the time-delayed coupling of membrane voltage, is introduced into Luo-Rudy phase I cardiac model. The effect of the cardiac memory on spiral waves is investigated. The numerical results show that the cardiac memory can lead to irregular meandering of spiral waves. When the delay-time is properly chosen, the increase of memory strength causes the decrease of spiral wave frequency. If memory strength exceeds the critical value, the cardiac memory results in the disappearances of both spiral waves and spatiotemporal chaos in system because the time-dependent potassium ion current is over suppressed by cardiac memory.

Temperature Effects on Liquid Electrophoretic Inks

In liquid electrophotograpy inks are colloidal solutions of toner particles suspended in a dielectric liquid together with typically micellar toner charging components. Toner particles and charging agents are in physicochemical equilibrium with toner acquiring charge that may be temperature dependent. Viscous dissipative forces are primarily governed by carrier liquid viscosity which is also temperature dependent. Measurements of time dependent currents at high fields indicate that viscosity effects dominate temperature behaviour of Indigo electroinks.

Rapid genetic algorithm optimization of a mouse computational model: benefits for anthropomorphization of neonatal mouse cardiomyocytes.

While the mouse presents an invaluable experimental model organism in biology, its usefulness in cardiac arrhythmia research is limited in some aspects due to major electrophysiological differences between murine and human action potentials (APs). As previously described, these species-specific traits can be partly overcome by application of a cell-type transforming clamp (CTC) to anthropomorphize the murine cardiac AP. CTC is a hybrid experimental-computational dynamic clamp technique, in which a computationally calculated time-dependent current is inserted into a cell in real-time, to compensate for the differences between sarcolemmal currents of that cell (e.g., murine) and the desired species (e.g., human). For effective CTC performance, mismatch between the measured cell and a mathematical model used to mimic the measured AP must be minimal. We have developed a genetic algorithm (GA) approach that rapidly tunes a mathematical model to reproduce the AP of the murine cardiac myocyte under study. Compared to a prior implementation that used a template-based model selection approach, we show that GA optimization to a cell-specific model results in a much better recapitulation of the desired AP morphology with CTC. This improvement was more pronounced when anthropomorphizing neonatal mouse cardiomyocytes to human-like APs than to guinea pig APs. CTC may be useful for a wide range of applications, from screening effects of pharmaceutical compounds on ion channel activity, to exploring variations in the mouse or human genome. Rapid GA optimization of a cell-specific mathematical model improves CTC performance and may therefore expand the applicability and usage of the CTC technique.

Perturbation theory for Maxwell’s equations with a time-dependent current source

Using a set of ideas discussed in the second volume of Feynman Lectures, we develop a perturbation-theoretic scheme for solving Maxwell’s equations for time-dependent currents which are switched on at t = 0 .

A Functional Role for the 'Fibroblast-Like Cells' in Gastrointestinal Smooth Muscles (J Physiol 2011;589[Pt 3]:697-710)

The physiologic role of fibroblast-like cells (FLCs) has not been well established. Recently Kurahashi et al1 suggested the involvement of FLCs in neurotransmission by Ca2+-activated K+ (SK3) channels. They demonstrated that a network of more extensively branching cells with platelet-derived growth factor receptor alpha (PDGFRα) existed between the circular and longitudinal muscles in the plane of the myenteric plexus. FLCs (PDGFRα+) were located in the same anatomical spaces and distributed in similar density as interstitial cells of Cajal (ICC), but PDGFRα+ cells were distinct from c-Kit+ ICC, as shown by double labeling. Other double labeling experiments with anti-SK3 antibody revealed that PDGFRα+ cells in both muscle layers and the region of the myenteric plexus expressed SK3. Some other experiments showed that PDGFRα+ cells labeled with enhanced green fluorescent protein (eGFP) produced robust expression of P2ry1 (P2Y1 receptors) and Kcnn3 (SK3 channels). Whole-cell patch clamp results suggested that PDGFRα+ cells express a high density of apamin-sensitive Ca2+-activated K+ channels, which is consistent with the expression of SK3 channels: small amplitude and time-dependent outward currents without voltage-dependent inward currents in step depolarization, and significant reduction of this currents by apamin 300 nM and charybdotoxin (block both BK and IK conductance Ca2+-activated K+ channels) caused a small but significant reduction in outward currents. Single channel patch recording (excised inside-out patch) to change Ca2+ at the intracellular surface produced the results consistent with previous reported data of Ca2+-activated K+ channels. Pharmacological data using purines (ATP, β-NAD and ADP) evoked significant outward currents in PDGFRα+ cells. Apamin and MRS2500 (P2Y1 antagonist) attenuated current amplitudes. The authors concluded that PDGFRα+ cells are a novel class of excitable cells with large current densities attributable to SK channels that are the molecular and ionic apparatus to mediate enteric inhibitory responses to purines in gastrointestinal (GI) muscles.

Comparative study of many-body perturbation theory and time-dependent density functional theory in the out-of-equilibrium Anderson model

We study time-dependent electron transport through an Anderson model. The electronic interactions on the impurity site are included via the self-energy approximations at Hartree-Fock (HF), second Born (2B), GW, and T-Matrix level as well as within a time-dependent density functional (TDDFT) scheme based on the adiabatic Bethe-Ansatz local density approximation (ABALDA) for the exchange correlation potential. The Anderson model is driven out of equilibrium by applying a bias to the leads and its nonequilibrium dynamics is determined by real-time propagation. The time-dependent currents and densities are compared to benchmark results obtained with the time-dependent density matrix renormalization group (tDMRG) method. Many-body perturbation theory beyond HF gives results in close agreement with tDMRG especially within the 2B approximation. We find that the TDDFT approach with the ABALDA approximation produces accurate results for the densities on the impurity site but overestimates the currents. This problem is found to have its origin in an overestimation of the lead densities which indicates that the exchange correlation potential must attain nonzero values in the leads.

Near-field: A finite-difference time-dependent method for simulation of electrodynamics on small scales

We develop near-field (NF), a very efficient finite-difference time-dependent (FDTD) approach for simulating electromagnetic systems in the near-field regime. NF is essentially a time-dependent version of the quasistatic frequency-dependent Poisson algorithm. We assume that the electric field is longitudinal, and hence propagates only a set of time-dependent polarizations and currents. For near-field scales, the time step (dt) is much larger than in the usual Maxwell FDTD approach, as it is not related to the velocity of light; rather, it is determined by the rate of damping and plasma oscillations in the material, so dt = 2.5 a.u. was well converged in our simulations. The propagation in time is done via a leapfrog algorithm much like Yee's method, and only a single spatial convolution is needed per time step. In conjunction, we also develop a new and very accurate 8 and 9 Drude-oscillators fit to the permittivity of gold and silver, desired here because we use a large time step. We show that NF agrees with Mie-theory in the limit of small spheres and that it also accurately describes the evolution of the spectral shape as a function of the separation between two gold or silver spheres. The NF algorithm is especially efficient for systems with small scale dynamics and makes it very simple to introduce additional effects such as embedding.

Leakage behaviors of ferroelectric (Bi3.15Nd0.85)Ti3O12 thin film derived from RF sputtering

The leakage behaviors of ferroelectric layered-perovskite (Bi3.15Nd0.85)Ti3O12 (BNdT) thin films deposited on Pt/TiO2/Ti/Si substrates by RF sputtering have been carefully investigated. Their I–V curves exhibit a rich diversity of conducting behaviors at different temperature and voltage ranges, which may be accounted for by various conducting mechanisms, including Ohmic behavior, Schottky emission, space-charge-limited current (SCLC), and Fowler-Nordheim tunneling effects. Their time-dependent leakage currents show a nearly linear relationship with respect to logarithmic relaxation time at initial stage ( 400 s) showing characteristics of ionic space-charge limit current relaxation.

Charge Qubit Purification by an Electronic Feedback Loop

We propose the manipulation of an isolated qubit by a simple instantaneous closed-loop feedback scheme in which a time-dependent electronic detector current is directly back-coupled into qubit parameters. As a specific detector model, we employ a capacitively coupled single-electron transistor. We demonstrate the stabilization of pure delocalized qubit states above a critical detector-qubit coupling. This electronic purification is independent of the initial qubit state and is accomplished after few electron jumps through the detector. Our simple scheme can be used for the efficient and robust initialization of solid-state qubits in quantum computational algorithms at arbitrary temperatures.

Phase response curves of subthalamic neurons: experimental measurement and theoretical prediction

The subthalamic nucleus (STN) is an autonomously active population of glutamatergic neurons that occupies a pivotal location within the basal ganglia, receiving direct cortical input and innervating the globus pallidus and substantia nigra. As intrinsically oscillating neurons, STN cells may be suitable candidates for reduction to phase models, where the state of each neuron is represented by a single number (phase) and their response to synaptic input is given by infinitesimal phase response curves (iPRCs). Phase models are analytically tractable and assist the study of large neural populations, but whether they can describe the behavior of STN cells with sufficient accuracy is an open question. We addressed this issue by measuring the iPRCs of STN cells in brain slices, using both glutamatergic synaptic input and brief current injection. We found that iPRCs measured synaptically were relatively insensitive to the size of the EPSP used to measure them, and that iPRCs measured with current pulses were the same regardless of the polarity of the current, at least during the first 60% of the oscillation cycle; both observations suggest that iPRCs might accurately describe the response of STN cells to finite inputs. However, we also found that iPRCs measured synaptically and by current injection were dramatically different, with current pulse iPRCs giving much less phase shift for a given stimulus amplitude. We developed and analyzed a biophysical model of STN cells, to better understand the nature of subthalamic iPRCs. This model produced an iPRC that closely resembled those we measured experimentally. Observing that this model consisted mainly of kinetically fast voltagedependent currents and AHP currents that can be treated as a function of time since the last spike, we explored the iPRCs of cells whose conductances are direct functions of voltage. Ignoring the spike itself by assuming that it is triggered at a certain threshold potential followed by return (2 ms later) to resetting potential, we found that the iPRC of such cells is a simple function of their IV curves and that their iPRC is valid for arbitrarily large currents. The contribution of AHP currents could therefore be covered by treating them as applied currents acting through this IV curve-based iPRC. Following this approach, we developed a method for deriving the iPRC of any cell well described by a combination of fast voltage-dependent currents and time-dependent AHP currents, covering a wide range of cell types. We were also able to estimate experimentally the IV curves and AHP currents of STN cells in which we had empirically determined the synaptic and current pulse iPRCs, allowing us to compare theoretically predicted iPRCs to experimentally measured iPRCs. We found that the theoretically predicted PRC approximated the synaptic–but not the current pulse–iPRC. We hypothesize that somatically injected charge slowly trickles into distal dendrites, reducing its effectiveness at advancing the phase, whereas synaptically delivered charge comes from precharged dendrites. We conclude that current injection is not an effective method for measuring iPRCs in dendritic neurons.

Quantum transient currents in molecular systems weakly coupled with electrodes

The transient current dynamics of multilevel nanoscale systems weakly coupled with two electrodes has been theoretically investigated by the nonequilibrium Green’s function method. We have proposed a technique for decomposing the total time-dependent current into almost independent current components. This is a powerful technique for gaining insight into transient current behavior because the decomposed currents exhibit simple behaviors similar to those of single-level systems. We have clarified the transient current behaviors of a hydrogen molecule (two-level system) and an octatetraene molecule (eight-level system) connected to two electrodes as typical examples of multilevel systems. The transient current of the molecular hydrogen system is characterized by two current components with the same relaxation time and different oscillation periods, whereas the current of the octatetraene molecular system is decomposed into eight components, a few of which dominate the total current behavior during the initial stage.

Deriving currents on the one-dimensional lattice in the regime of dynamic localization

In this paper one deals with a transparent derivation of dynamic localization conditions for the electron on the 1D lattice proceeding irrespective of the concrete form of the periodic modulation of the time dependent electric field. This amounts to account for periodic zero minima generated in the time dependence of the mean square displacement (MSD). Related DL conditions have also been established by looking for the extremal points of the MSD. Interestingly enough, the conditions established in this manner are similar to the ones written down before for pure ac fields but exhibiting a rather unusual modulation. Related time dependent currents can be readily established, now by resorting to discrete derivatives. Our typical example is the dc-ac electric field. We found that in the regime of dynamic localization the MSD and the current get characterized by the same period, i.e by the period of the time dependent modulation. Moreover, the current crosses the time axis just in the maxima-locations of the MSD.

Nanoparticle Transport in Conical-Shaped Nanopores

This report presents a fundamental study of nanoparticle transport phenomena in conical-shaped pores contained within glass membranes. The electrophoretic translocation of charged polystyrene (PS) nanoparticles (80- and 160-nm-radius) was investigated using the Coulter counter principle (or "resistive-pulse" method) in which the time-dependent nanopore current is recorded as the nanoparticle is driven across the membrane. Particle translocation through the conical-shaped nanopore results in a direction-dependent and asymmetric triangular-shaped resistive pulse. Because the sensing zone of conical-shaped nanopores is localized at the orifice, the translocation of nanoparticles through this zone is very rapid, resulting in pulse widths of ~200 μs for the nanopores used in this study. A linear dependence between translocation rate and nanoparticle concentration was observed from 10(7) to 10(11) particles/mL for both 80- and 160-nm-radius particles, and the magnitude of the resistive pulse scaled approximately in proportion to the particle volume. A finite-element simulation based on continuum theory to compute ion fluxes was combined with a dynamic electric force-based nanoparticle trajectory calculation to compute the position- and time-dependent nanoparticle velocity as the nanoparticle translocates through the conical-shaped nanopore. The computational results were used to compute the resistive pulse current-time response for conical-shaped pores, allowing comparison between experimental and simulated pulse heights and translocation times. The simulation and experimental results indicate that nanoparticle size can be differentiated based on pulse height, and to a lesser extent based on translocation time.

Enhanced thermopower under a time-dependent gate voltage

We derive formal expressions of time-dependent energy and heat currents through a nanoscopic device using the Keldysh nonequilibrium Green function technique. Numerical results are reported for a metal-dot-metal junction where the dot level energy is abruptly changed by a step-shaped voltage pulse. Analytical linear responses are obtained for the time-dependent thermoelectric coefficients. We show that in the transient regime the Seebeck coefficient can be enhanced by an amount (as much as $40%$) controlled by both the dot energy and the height of the voltage step.

Nonadiabatic generation of a pure spin current in a one-dimensional quantum ring with spin-orbit interaction

We demonstrate the theoretical possibility of obtaining a pure spin current in a 1D ring with spin-orbit interaction by irradiation with a nonadiabatic, two-component terahertz laser pulse, whose spatial asymmetry is reflected by an internal phase difference $\ensuremath{\phi}$. The solutions of the equation of motion for the density operator are obtained for a spin-orbit coupling linear in the electron momentum (Rashba) and they are used to calculate the time-dependent charge and spin currents. We find that there are critical values of $\ensuremath{\phi}$ at which the charge current disappears, while the spin current reaches a maximum or a minimum value.

Effect of gas flow on electronic transport in a DNA-decorated carbon nanotube

We calculate the two-time current correlation function using the experimental data of thecurrent–time characteristics of the Gas–DNA-decorated carbon nanotube field effecttransistor. The pattern of the correlation function is a measure of the sensitivity andselectivity of the sensors and suggest that these gas flow sensors may also be used as DNAsequence detectors. The system is modelled by a one-dimensional tight-bindingHamiltonian and we present analytical calculations of quantum electronic transport for thesystem using the time-dependent nonequilibrium Green’s function formalism andthe adiabatic expansion. The zeroth and first order contributions to the current and are calculated, where is the Landauer formula. The formula for the time-dependent current is then used tocompare the theoretical results with the experiment.

Non-Markovian dynamics of a nanomechanical resonator measured by a quantum point contact

We study the dynamics of a nanomechanical resonator (NMR) subject to a measurement by a low-transparency quantum point contact (QPC) or tunnel junction in the non-Markovian domain. We derive the non-Markovian number-resolved (conditional) and unconditional master equations valid to second order in the tunneling Hamiltonian without making the rotating-wave approximation and the Markovian approximation, generally made for systems in quantum optics. Our non-Markovian master equation reduces, in appropriate limits, to various Markovian versions of master equations in the literature. We find considerable difference in dynamics between the non-Markovian case and its Markovian counterparts. We also calculate the time-dependent transport current through the QPC, which contains information about the measured NMR system. We find an extra transient current term proportional to the expectation value of the symmetrized product of the position and momentum operators of the NMR. This extra current term, with a coefficient coming from the combination of the imaginary parts of the QPC reservoir correlation functions, has a substantial contribution to the total transient current in the non-Markovian case, but was generally ignored in the studies of the same problem in the literature. Considering the contribution of this extra term, we show that a significant qualitative and quantitative difference in the total transient current between the non-Markovian and the Markovian wideband-limit cases can be observed. Thus, it may serve as a witness or signature of the non-Markovian features in the coupled NMR-QPC system.

Implantation of Sinoatrial Node Cells Into Canine Right Ventricle: Biological Pacing Appears Limited by the Substrate

Biological pacing has been proposed as a physiologic counterpart to electronic pacing, and the sinoatrial node (SAN) is the general standard for biological pacemakers. We tested the expression of SAN pacemaker cell activity when implanted autologously in the right ventricle (RV). We induced complete heart block and implanted electronic pacemakers in the RV of adult mongrel dogs. Autologous SAN cells isolated enzymatically were studied by patch clamp to confirm SAN identity. SAN cells (400,000) were injected into the RV subepicardial free wall and dogs were monitored for 2 weeks. Pacemaker function was assessed by overdrive pacing and IV epinephrine challenge. SAN cells expressed a time-dependent inward current (I(f)) activating on hyperpolarization: density = 4.3 ± 0.6 pA/pF at -105 mV. Four of the six dogs demonstrated >50% of beats originating from the implant site at 24 h. Biological pacemaker rates on days 7-14 = 45-55 bpm and post-overdrive escape times = 1.5-2.5 s. Brisk catecholamine responsiveness occurred. Dogs implanted with autologous SAN cells manifest biological pacing properties dissimilar from those of the anatomic SAN. This highlights the importance of cell and substrate interaction in generating biological pacemaker function.

Design of effective kernels for spectroscopy and molecular transport: Time-dependent current–density-functional theory

Time-dependent current-density-functional theory (TDCDFT) provides an, in principle, exact scheme to calculate efficiently response functions for a very broad range of applications. However, the lack of approximations valid for a range of parameters met in experimental conditions has so far delayed its extensive use in inhomogeneous systems. On the other side, in many-body perturbation theory accurate approximations are available, but at a price of a higher computational cost. In the present work, the possibility of combining the advantages of both approaches is exploited. In this way, an exact equation for the exchange-correlation kernel of TDCDFT is obtained, which opens the way for a systematic improvement of the approximations adopted in practical applications. Finally, an approximate kernel for an efficient calculation of spectra of solids and molecular conductances is suggested and its validity is discussed.

Numerical Computation of the Nearshore Current Considering Wave-Current Interactions at Gangjeong Coastal Area, Jeju Island, Korea

Nearshore current phenomena have considerable effects on sediment transport, erosion and accumulation as the water circulation system within nearshore regions is generated and changed by wave-current interactions. Therefore, predicting such nearshore current is very important. In this study, by considering the wave-current interactions and vertical variation of currents, nearshore current phenomena can be more accurately computed. A nonlinear parabolic mild slope equation and a quasi-3D equation are used in wave deformation and circulation analyses, respectively. Velocities caused by tide are also applied to boundaries using the volume flux, so that more exact numerical analyses can be performed. The numerical model is verified through two test cases, and good agreements are obtained. To investigate the actual flow tendency, time dependent nearshore currents are computed at the Gangjeong coastal area, Jeju, Korea. From the numerical results, important phenomena for the nearshore current such as the wave-current interactions, longshore current, and rip-current are well captured.