Time-dependent Current Research Articles

Room-temperature transient carrier transport in germanium single-hole/electron transistors

We report the experimental observation of transient carrier transports at room temperature in a Ge quantum-dot (QD) single-hole transistor (SHT) and single-electron transistor (SET). In addition to room-temperature Coulomb oscillations, hysteresis effects have been observed in the steady-state tunneling current of a Ge-SHT and a Ge-SET as gate voltage is swept in a loop. Time-dependent tunneling current of a Ge-SHT and a Ge-SET displays clear oscillatory or staircase behavior at a constant voltage stress condition, which indicates transient charging/discharging of electrons and holes via a Ge QD due to substantial quantum mechanics effect.

Space charge limited transient currents and oxygen vacancy mobility in amorphous BaTiO3 thin films

Time-dependent transient currents were studied at elevated temperatures (200–220°C) in amorphous barium titanate thin film capacitors. Current transients display a peak whose time position varies with applied voltage and temperature. The response is analyzed through space charge limited current transient theories. Extracted drift mobilities are in the 10−11–10−12cm2V−1s−1 range and show an activation energy of 1eV. The phenomena are associated with oxygen vacancies migration in BaTiO3.

Vertical structure of time-dependent currents in a mid-ocean ridge axial valley

Velocity measurements with high vertical resolution, and a two-dimensional linear quasianalytic model for subinertial oscillatory flows, are used to analyze the vertical structure of flow in the axial valley at Endeavour Segment, Juan de Fuca Ridge. At a site away from hydrothermal vents, observed semidiurnal flows are independent of depth, rectilinear and parallel to the valley axis, while subinertial flows are intensified and re-aligned along-valley toward the bottom. This behavior is consistent with solutions from the model, which show attenuation of subinertial across-valley flow with depth. This cross-flow attenuation is most pronounced for valleys with widths less than the internal Rossby radius of deformation. Reduction of across-valley flow with depth results in a weakened Coriolis force that cannot fully balance the along-valley pressure-gradient force. The resulting force imbalance yields a directly accelerated bottom-intensified along-valley flow. The importance of this physical process in other submarine valleys depends on their geometry, stratification and latitude. If active, this mechanism provides a dynamic background environment for the axial valley to which hydrothermal venting would add complexity. The strong vertical shears and spiraling flows observed within the axial valley for diurnal tidal and lower-frequency flows have important implications in the transport of hydrothermal vent fluid and the dispersal of larvae of vent organisms by bottom currents.

Transient response of photoexcited electrons: Negative and oscillating current

Time-dependent current of the electrons excited in the conduction band after ultrafast interband photogeneration is studied theoretically. The transient photocurrent is calculated for the nonlinear regime of response to a stationary electric field. The response demonstrates transient absolute negative conductivity when the electrons are excited slightly below the optical phonon energy, while the periodic oscillations of the electric current appear after formation of the streaming distribution. The quenching of these peculiarities by the elastic scattering of electrons is also considered.

Numerical analysis of coherent many-body currents in a single atom transistor

We study the dynamics of many atoms in the recently proposed Single Atom Transistor setup [A. Micheli, A. J. Daley, D. Jaksch, and P. Zoller, Phys. Rev. Lett. 93, 140408 (2004)] using recently developed numerical methods. In this setup, a localised spin 1/2 impurity is used to switch the transport of atoms in a 1D optical lattice: in one state the impurity is transparent to probe atoms, but in the other acts as a single atom mirror. We calculate time-dependent currents for bosons passing the impurity atom, and find interesting many body effects. These include substantially different transport properties for bosons in the strongly interacting (Tonks) regime when compared with fermions, and an unexpected decrease in the current when weakly interacting probe atoms are initially accelerated to a non-zero mean momentum. We also provide more insight into the application of our numerical methods to this system, and discuss open questions about the currents approached by the system on long timescales.

Photon-assisted electron transport through a three-terminal quantum dot system with nonresonant tunneling channels

We have studied the electron transport through a quantum dot coupled to three leads in the presence of external microwave fields supplied to different parts of the considered mesoscopic system. Additionally, we introduced a possible nonresonant tunneling channels between leads. The quantum dot charge and currents were determined in terms of the appropriate evolution operator matrix elements and under the wide band limit the analytical formulas for time-averaged currents and differential conductance were obtained. We have also examined the response of the considered system on the rectangular-pulse modulation imposed on different quantum dot-leads barriers as well as the time-dependence of currents flowing in response to suddenly removed (or included) connection of a quantum dot with one of the leads.

A microchip ring trap for cold atoms

We describe a method to create a circular magnetic waveguide for deBroglie waves on a microchip. The guide is a two-dimensional magnetic minimum for trapping weak-field seeking states of atoms or molecules with a magnetic dipole moment. It is created completely by electric currents in wires that are lithographically patterned on a multi-level chip. We describe the geometry and time-dependent currents of the wires and show that it is possible to wrap the waveguide in a complete circle with minimal perturbations due to the leads or wire crossings. This maximal area geometry is suited for rotation sensing with atom interferometry via the Sagnac effect using either cold thermal atoms and molecules or Bose-condensed systems.

Radio frequency glow discharge source with integrated voltage and current probes used for sputtering rate and emission yield measurements at insulating samples

Radio frequency glow discharge optical emission spectroscopy (RF-GD-OES) is routinely used for the chemical analysis of solid samples. Two independent electrical signals from the discharge are required for quantification. When sputtering insulating samples, the voltage over the discharge is not directly measurable. The coupling capacity of the sample is required in order to calculate the discharge voltage. A procedure is outlined where the coupling capacity is determined using an electrical measurement without discharge. The calculated time-dependent discharge voltage and current are evaluated using a plasma equivalent circuit. An insulating sample is sputtered at constant cathode voltage and current. The emission yield for an aluminium line is comparable to that of conducting reference material.

Non-equilibrium behavior of HCN channels: Insights into the role of HCN channels in native and engineered pacemakers

I(f), encoded by the hyperpolarization-activated, cyclic nucleotide-modulated (HCN) channel gene family, modulates cardiac pacing. During cardiac pacing, changes in membrane potential are rapid, preventing the very slow HCN channels from reaching equilibrium. Here, we examined the properties of HCN channels under non-equilibrium conditions to shed insight into how different HCN isoforms contribute to cardiac pacing. HCN1, 2 and 4 channels were heterologously expressed in Xenopus laevis oocytes or mammalian Cos7 cells and subjected to voltage clamp. We found that HCN1 channel activation (V1/2) depended strongly on the holding potential (V(H)) for short (100 ms; V1/2=-118 mV, -78 mV and -19 mV for V(H)= +70, -75 and -140 mV, respectively, in Xenopus oocytes) but not long (300-ms) test-pulses, hinting that shifts of V1/2 under non-equilibrium conditions may alter the impact of I(f) in different phases of the cardiac circle. Consistent with this notion, when a train of SA nodal-like action potentials was applied in voltage-clamp experiments, HCN1 exhibited pronounced current-voltage (IV)-hysteresis. Using computational modeling, we demonstrate that the intrinsically sluggish HCN1 activation kinetics underlie their IV-hysteretic behavior and do not hinder the ability to modulate cardiac pacing. By contrast, HCN4 did not exhibit IV-hysteresis. This difference can be attributed to the relatively large activation time constant and markedly delayed onsets of time-dependent HCN4 currents. Indeed, HCN2 channels, which have intermediate activation time constants and delays, displayed and intermediate hysteretic phenotype. We conclude that non-equilibrium properties of HCN channels contribute to cardiac pacing. These results provide insight for tuning the firing rate of endogenous and induced pacemakers using engineered HCN constructs with distinct gating phenotypes.

Electrical reliability aspects of HfO 2 high- k gate dielectrics with TaN metal gate electrodes under constant voltage stress

The dielectric breakdown property of ultrathin 2.5 and 5.0 nm hafnium oxide (HfO 2) gate dielectric layers with metal nitride (TaN) gate electrodes for metal oxide semiconductor (MOS) structure has been investigated. Reliability studies were performed with constant voltage stressing to verify the processing condition effects (film thicknesses and post metal annealing temperatures) on times to breakdown. The leakage current characteristics are improved with post metal annealing temperatures (PMA) for both 2.5 and 5.0 nm HfO 2 physical thicknesses. However, it is more prominent (∼2 orders of magnitudes) for 2.5 nm HfO 2 film thickness. The values of oxide-trapped charge density and interface-state density are also improved for 2.5 nm HfO 2 film. The different stages of charge-trapping behaviors, i.e., stress-induced leakage current, soft and hard breakdown mechanisms have been detected. During constant voltage stress of the MOS capacitors, an increase in the time-dependent gate current is observed, followed by the occurrence of several fluctuations. The amplitude of the fluctuations is much larger in the 5.0 nm HfO 2 gate dielectric layer compared to the 2.5 nm HfO 2 layer. After the occurrence of such fluctuations, the current–voltage characteristics exhibited an increased in gate current compared to the fresh (unstressed) devices.

The Mechanism of Intrinsic Amplification of Hyperpolarizations and Spontaneous Bursting in Striatal Cholinergic Interneurons

Striatal cholinergic interneurons pause their ongoing firing in response to sensory stimuli that have acquired meaning as a signal for learned behavior. In slices, these cells exhibit both spontaneous activity patterns and spontaneous pauses very similar to those seen in vivo. The mechanisms responsible for ongoing firing and spontaneous pauses were studied in striatal slices using perforated patch recordings. All hyperpolarizations, whether spontaneous or generated by current injection, were amplified and shaped by two hyperpolarization-activated currents. Hyperpolarization onsets were regeneratively amplified by a potassium current (KIR) whose activation promoted further hyperpolarization. The termination of hyperpolarizations was controlled by a time-dependent nonspecific cation current (HCN). The duration and even the sizes of spontaneous and driven hyperpolarizations and pauses in spontaneous activity in cholinergic interneurons are largely autonomous properties of the neuron, rather than reflections of characteristics of the input eliciting the response.

Time-dependent current-voltage relation in electron guns

We studied mathematical models for degradation behaviors of current and voltage in electron guns especially using oxide cathodes. We found that the current and the voltage, I(t) and V(t), follow the stretched exponential decay in oxide cathodes. On this basis, we derived a general expression for the time-dependent current–voltage relation as I(t)=p(t)V(t)δ(t), where δ(t) is a time-dependent exponent and the perveance, p(t), is a function of δ(t). The exponent δ(t) indicates the deviation of the classical Child–Langmuir relation (I=pV3∕2). This deviation is attributed to the gradual change of the electron gun geometry over time.

Antagonistic regulation of swelling-activated Cl−current in rabbit ventricle by Src and EGFR protein tyrosine kinases

Regulation of swelling-activated Cl(-) current (I(Cl,swell)) is complex, and multiple signaling cascades are implicated. To determine whether protein tyrosine kinase (PTK) modulates I(Cl,swell) and to identify the PTK involved, we studied the effects of a broad-spectrum PTK inhibitor (genistein), selective inhibitors of Src (PP2, a pyrazolopyrimidine) and epidermal growth factor receptor (EGFR) kinase (PD-153035), and a protein tyrosine phosphatase (PTP) inhibitor (orthovanadate). I(Cl,swell) evoked by hyposmotic swelling was increased 181 +/- 17% by 100 microM genistein, and the genistein-induced current was blocked by the selective I(Cl,swell) blocker tamoxifen (10 microM). Block of Src with PP2 (10 microM) stimulated tamoxifen-sensitive I(Cl,swell) by 234 +/- 27%, mimicking genistein, whereas the inactive analog of PP2, PP3 (10 microM), had no effect. Moreover, block of PTP by orthovanadate (1 mM) inhibited I(Cl,swell) and prevented its stimulation by PP2. In contrast with block of Src, block of EGFR kinase with PD-153035 (20 nM) inhibited I(Cl,swell). Several lines of evidence argue that the PP2-stimulated current was I(Cl,swell): 1) the stimulation was volume dependent, 2) the current was blocked by tamoxifen, 3) the current outwardly rectified with both symmetrical and physiological Cl(-) gradients, and 4) the current reversed near the Cl(-) equilibrium potential. To rule out contributions of other currents, Cd(2+) (0.2 mM) and Ba(2+) (1 mM) were added to the bath. Surprisingly, Cd(2+) suppressed the decay of I(Cl,swell), and Cd(2+) plus Ba(2+) eliminated time-dependent currents between -100 and +100 mV. Nevertheless, these divalent ions did not eliminate I(Cl,swell) or prevent its stimulation by PP2. The results indicate that tyrosine phosphorylation controls I(Cl,swell), and regulation of I(Cl,swell) by the Src and EGFR kinase families of PTK is antagonistic.

Theory of Switching of Multiaxial Ferroelectrics (the Main Stages)

The kinetics of switching of multiaxial ferroelectric crystals with 180° and 90° domains under conditions of normal and layer-by-layer domain growth is studied using the multidimensional theory of first-order phase transitions. The main stages of the process of switching of a multiaxial ferroelectric are considered under the assumption that repolarized cylinder-shaped domains grow three-dimensionally. A closed set of equations describing the kinetics of switching is derived with allowance for a change in repolarization in the course of the phase transition. Equations for the time-dependent switching current are derived. The main characteristics of switching are compared qualitatively with the experimental data on barium titanate.

General theory for the interference of two-photon and one-photon processes in semiconductor heterostructures

We develop a general theory for the dynamics of multi-photon processes in semiconductor heterostructures. The resulting effective multi-band Bloch equations describe the dynamics of electrons in the reduced set of bands between which the optical pulses induce quasi-resonant transitions. The model is specialized to the case of interfering one- and two-photon transitions across the band gap. The withdrawn bands are included as intermediate states for an effective interaction that is quadratic in the electromagnetic fields. The benefit of this perturbative approach is to lead to equations of motion for slowly varying quantities only, in the spirit of the rotating wave approximation. Coulomb interaction and relaxation can also easily be included. Finally, a general expression for the time dependent polarization current that is consistent with the approximations involved by the effective multi-band Bloch equations is derived.

Shot-noise control in ac-driven nanoscale conductors

We derive within a time-dependent scattering formalism expressions for both the current through ac-driven nanoscale conductors and its fluctuations. The results for the time-dependent current, its time average, and, above all, the driven shot noise properties assume an explicit and serviceable form by relating the propagator to a non-Hermitian Floquet theory. The driven noise cannot be expressed in terms of transmission probabilities. The results are valid for a driving of arbitrary strength and frequency. The connection with commonly known approximation schemes such as the Tien-Gordon approach or a high-frequency approximation is elucidated together with a discussion of the corresponding validity regimes. Within this formalism, we study the coherent suppression of current and noise caused by properly chosen electromagnetic fields.

Contribution of a time-dependent and hyperpolarization-activated chloride conductance to currents of resting and hypotonically shocked rat hepatocytes.

Hepatocellular Cl- flux is integral to maintaining cell volume and electroneutrality in the face of the many transport and metabolic activities that describe the multifaceted functions of these cells. Although a significant volume-regulated Cl- current (VRAC) has been well described in hepatocytes, the Cl- channels underlying the large resting anion conductance have not been identified. We used a combination of electrophysiological and molecular approaches to describe potential candidates for this conductance. Anion currents in rat hepatocytes and WIF-B and HEK293T cells were measured under patch electrode-voltage clamp. With K+-free salts of Cl- comprising the major ions externally and internally, hyperpolarizing steps between -40 and -140 mV activated a time-dependent inward current in hepatocytes. Steady-state activation was half-maximal at -63 mV and 28-38% of maximum at -30 to -45 mV, previously reported hepatocellular resting potentials. Gating was dependent on cytosolic Cl-, shifting close to 58 mV/10-fold change in Cl- concentration. Time-dependent inward Cl- currents and a ClC-2-specific RT-PCR product were also observed in WIF-B cells but not HEK293T cells. All cell types exhibited typical VRAC in response to dialysis with hypertonic solutions. DIDS (0.1 mM) inhibited the hepatocellular VRAC but not the inward time-dependent current. Antibodies against the COOH terminus of ClC-2 reacted with a protein between 90 and 100 kDa in liver plasma membranes. The results demonstrate that rat hepatocytes express a time-dependent inward Cl- channel that could provide a significant depolarizing influence in the hepatocyte.

Low-frequency negative capacitance effect in systems of metallic nanoparticles embedded in dielectric matrix

A negative capacitance (NC) effect in a low-frequency range (4–8×105Hz), previously shown to take place mainly in semiconductor structures, is evidenced in a nanometric system constituted by metallic (Ga) nanoparticles embedded in an insulating (SiOxwithx~1) matrix. The dependence of the NC phenomenon on the time-dependent transient current through the system is evidenced. A remarkable enhancement of the effect with size reduction of the nanoparticles is manifested. The physical mechanism responsible for the current inertia appears to be related to the space charges located at the multiple insulator–metal interfaces.

Time-dependent partition-free approach in resonant tunneling systems

An extended Keldysh formalism, well suited to properly take into account the initial correlations, is used in order to deal with the time-dependent current response of a resonant tunneling system. We use a partition-free approach by Cini in which the whole system is in equilibrium before an external bias is switched on. No fictitious partitions are used. Despite a more involved formulation, this partition-free approach has many appealing features being much closer to what is experimentally done. In particular, besides the steady-state responses one can also calculate physical dynamical responses. In the noninteracting case we clarify under what circumstances a steady-state current develops and compare our result with the one obtained in the partitioned scheme. We prove a theorem of asymptotic equivalence between the two schemes for arbitrary time-dependent disturbances. We also show that the steady-state current is independent of the history of the external perturbation (memory-loss theorem). In the so-called wide-band limit an analytic result for the time-dependent current is obtained. In the interacting case we work out the lesser Green function in terms of the self-energy and we recover a well-known result in the long-time limit. In order to overcome the complications arising from a self-energy which is nonlocal in time we propose an exact nonequilibrium Green-function approach based on time-dependent density-functional theory. The equations are no more difficult than an ordinary mean-field treatment. We show how the scattering-state scheme by Lang follows from our formulation. An exact formula for the steady-state current of an arbitrary interacting resonant tunneling system is obtained. As an example the time-dependent current response is calculated in the random-phase approximation.

Evolution of the maximum ion impact energy at an electrode during sheath and presheath formation

The evolution of the system sheath/presheath has been investigated experimentally. The ions of maximum kinetic impact energy εmax collected at the surface of an orifice electrode are thereby used as test particles. The orifice electrode cylindrically surrounds an Ar plasma dc-excited in a parallel plate configuration, at a pressure of 10.8 × 10−2 mbar. A negative rectangular low-voltage pulse is applied to the orifice electrode. The resulting time dependent ion current is extracted and investigated. A combination of pulse length and retarding field variation allows one to determine εmax and its time dependence.The time evolution of εmax is measured during sheath and presheath formation. Sheath relaxation is investigated by applying a voltage pulse to an already existing sheath/presheath system in front of the orifice electrode. This is compared to the evolution of a new sheath starting from the sheath-free electrode. Probe measurements confirm that the presheath is formed within the last part of energy increase of εmax over an energy value of kTe/2.