Time-dependent Current Research Articles

Polarization and depolarization currents in kaolin

Ceramic materials are considered dielectrics with ionic conductivity where polarization and depolarization effects play an important role. These effects take place when an external electric field is applied or removed. The measurement was performed on the kaolin samples, which contain 1.5mass % of the physically bound water, at room temperature. Since the polarization current did not reach zero value after a long period of time, it was deduced that both the ions and electrons are the charge carriers. At room temperature, H+ and OH− ions are the main charge carriers, a role of Na+ and K+ is negligible. Power functions, which describe the time dependent currents, suggest several polarization mechanisms. The best fit of the experimental relationships between the current and time showed a sum of three exponentials, i.e. three polarization and depolarization mechanisms should be considered. We ascribed them to 1) hopping of H+ ions, 2) migration of originally trapped and then released H+ ions, and 3) migration of OH− ions.

Oscillating spin-orbit interaction in two-dimensional superlattices: Sharp transmission resonances and time-dependent spin-polarized currents

We consider ballistic transport through a lateral, two-dimensional superlattice with experimentally realizable, sinusoidally oscillating, Rashba-type spin-orbit interaction (SOI). The periodic structure of the rectangular lattice produces a spin-dependent miniband structure for static SOI. Using Floquet theory, transmission peaks are shown to appear in the mini-bandgaps as a consequence of the additional, time-dependent SOI. A detailed analysis shows that this effect is due to the generation of harmonics of the driving frequency, via which, e.g., resonances that cannot be excited in the case of static SOI become available. Additionally, the transmitted current shows space- and time-dependent partial spin polarization, in other words, polarization waves propagate through the superlattice.

High-harmonic generation from Bloch electrons in solids

We study the generation of high harmonic radiation by Bloch electrons in a model transparent solid driven by a strong mid-infrared laser field. We solve the single-electron time-dependent Schr\"odinger equation (TDSE) using a velocity-gauge method [New J. Phys. 15, 013006 (2013)] that is numerically stable as the laser intensity and number of energy bands are increased. The resulting harmonic spectrum exhibits a primary plateau due to the coupling of the valence band to the first conduction band, with a cutoff energy that scales linearly with field strength and laser wavelength. We also find a weaker second plateau due to coupling to higher-lying conduction bands, with a cutoff that is also approximately linear in the field strength. To facilitate the analysis of the time-frequency characteristics of the emitted harmonics, we also solve the TDSE in a time-dependent basis set, the Houston states [Phys. Rev. B 33, 5494 (1986)], which allows us to separate inter-band and intra-band contributions to the time-dependent current. We find that the inter-band and intra-band contributions display very different time-frequency characteristics. We show that solutions in these two bases are equivalent under an unitary transformation but that, unlike the velocity gauge method, the Houston state treatment is numerically unstable when more than a few low lying energy bands are used.

Progress towards Understanding Anomalous Heat Effect in Metal Deuterides

The present article summarizes anomalous heat events which were observed in a large number of electrolysis experiments using heavy water and palladium-based cathodes. The amount of excess heat produced by some of these experiments is too large to be accounted for by any known chemical processes. It was found that events of the anomalous heat effect (AHE) are accompanied by increased cell voltage during electrolysis and that there are characteristic cathode surface morphologies which are associated with excess heat events. AHE has been observed during electrolysis following dynamic stimulation of the cell by time-dependent electrolytic currents (SuperWaves) and ultrasonic excitation. Past experiments have increased our understanding of the anomalous heat effect in the palladium-deuterium systems, but there is much left to be learned.

Insulation Resistance Degradation in Ni–BaTiO3Multilayer Ceramic Capacitors

Insulation resistance (IR) degradation in Ni-BaTiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> multilayer ceramic capacitors has been characterized by the measurement of both time to failure (TTF) and direct current leakage as a function of stress time under highly accelerated life test conditions. The measured leakage currenttime dependence data fit well to an exponential form, and a characteristic growth time τSD can be determined. A greater value of τSD represents a slower IR degradation process. Oxygen vacancy migration and localization at the grain boundary region results in the reduction of the Schottky barrier height and has been found to be the main reason for IR degradation in Ni-BaTiO3 capacitors. The reduction of barrier height as a function of time follows an exponential relation of φ(t) = φ(0)e <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2Kt</sup> , where the degradation rate constant K = K <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> e(-E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</sub> /kT) is inversely proportional to the mean TTF (MTTF) and can be determined using an Arrhenius plot. For oxygen vacancy electromigration, a lower barrier height φ(0) will favor a slow IR degradation process, but a lower φ(0) will also promote electronic carrier conduction across the barrier and decrease the IR. As a result, a moderate barrier height φ(0) (and therefore a moderate IR value) with a longer MTTF (smaller degradation rate constant K) will result in a minimized IR degradation process and the most improved reliability in Ni-BaTiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> multilayer ceramic capacitors.

Inertia, diffusion, and dynamics of a driven skyrmion

Skyrmions recently discovered in chiral magnets are a promising candidate for magnetic storage devices because of their topological stability, small size ($\sim 3-100$nm), and ultra-low threshold current density ($\sim 10^{6}$A/m$^2$) to drive their motion. However, the time-dependent dynamics has hitherto been largely unexplored. Here we show, by combining the numerical solution of the Landau-Lifshitz-Gilbert equation and the analysis of a generalized Thiele's equation, that inertial effects are almost completely absent in skyrmion dynamics driven by a time-dependent current. In contrast, the response to time-dependent magnetic forces and thermal fluctuations depends strongly on frequency and is described by a large effective mass and a (anti-) damping depending on the acceleration of the skyrmion. Thermal diffusion is strongly suppressed by the cyclotron motion and is proportional to the Gilbert damping coefficient $\alpha$. This indicates that the skyrmion position is stable, and its motion responds to the time-dependent current without delay or retardation even if it is fast. These findings demonstrate the advantages of skyrmions as information carriers.

Characterizing Battery Behavior for Time-Dependent Currents

Batteries connected to the dc grid are exposed to a variety of currents. We explore in this paper the response of the battery to commonly seen current waveforms to understand the impact of the converter on the battery. At the frequencies considered, the battery's small-signal model is shown through impedance spectroscopy and ripple tests to be well represented by a pure resistor, and at dc currents over 2 C, Joule heating is shown to have a nonnegligible effect on the impedance. Furthermore, the amplitude of pulses that can be applied is shown to be governed by diffusion limits and the battery's state of charge.

Quantum tomography of an electron.

The complete knowledge of a quantum state allows the prediction of the probability of all possible measurement outcomes, a crucial step in quantum mechanics. It can be provided by tomographic methods which have been applied to atomic, molecular, spin and photonic states. For optical or microwave photons, standard tomography is obtained by mixing the unknown state with a large-amplitude coherent photon field. However, for fermions such as electrons in condensed matter, this approach is not applicable because fermionic fields are limited to small amplitudes (at most one particle per state), and so far no determination of an electron wavefunction has been made. Recent proposals involving quantum conductors suggest that the wavefunction can be obtained by measuring the time-dependent current of electronic wave interferometers or the current noise of electronic Hanbury-Brown/Twiss interferometers. Here we show that such measurements are possible despite the extreme noise sensitivity required, and present the reconstructed wavefunction quasi-probability, or Wigner distribution function, of single electrons injected into a ballistic conductor. Many identical electrons are prepared in well-controlled quantum states called levitons by repeatedly applying Lorentzian voltage pulses to a contact on the conductor. After passing through an electron beam splitter, the levitons are mixed with a weak-amplitude fermionic field formed by a coherent superposition of electron-hole pairs generated by a small alternating current with a frequency that is a multiple of the voltage pulse frequency. Antibunching of the electrons and holes with the levitons at the beam splitter changes the leviton partition statistics, and the noise variations provide the energy density matrix elements of the levitons. This demonstration of quantum tomography makes the developing field of electron quantum optics with ballistic conductors a new test-bed for quantum information with fermions. These results may find direct application in probing the entanglement of electron flying quantum bits, electron decoherence and electron interactions. They could also be applied to cold fermionic (or spin-1/2) atoms.

Real-time dynamics of spin-dependent transport through a double-quantum-dot Aharonov-Bohm interferometer with spin-orbit interaction

The spin-resolved nonequilibrium real-time electron transport through a double-quantum-dot (DQD) Aharonov-Bohm (AB) interferometer with spin-orbit interaction (SOI) is explored. The SOI and AB interference in the real-time dynamics of spin transport is expressed by effective magnetic fluxes. Analytical formulas for the time-dependent currents, for initially unpolarized spins, are presented. In many cases, there appear spin currents in the electrodes, for which the spins in each electrode are polarized along characteristic directions, predetermined by the SOI parameters and by the geometry of the system. Special choices of the system parameters yield steady-state currents in which the spins are fully polarized along these characteristic directions. The time required to reach this steady state depends on the couplings of the DQD to the leads. The magnitudes of the currents depend strongly on the SOI-induced effective fluxes. Without the magnetic flux, the spin-polarized current cannot be sustained to the steady states, due to the phase rigidity for this system. For a nondegenerate DQD, transient spin transport can be produced by the sole effects of SOI. We also show that one can extract the spin-resolved currents from measurements of the total charge current.

Quantum diffusion: A simple, exactly solvable model

We propose a simple quantum mechanical model describing the time dependent diffusion current between two fermion reservoirs that were initially disconnected and characterized by different densities or chemical potentials. The exact, analytical solution of the model yields the transient behavior of the coupled fermion systems evolving to a final steady state, whereas the long-time behavior is determined by a power law rather than by exponential decay. Similar results are obtained for the entropy production which is proportional to the diffusion current.

Ab initio study of the photocurrent at the Au/Si metal–semiconductor nanointerface

Photo-induced charge transfer at the interface of two materials is a fundamental process in photovoltaic applications. In this study, we have considered a model of a simplified photovoltaic cell composed of a Si nanocrystal co-doped with Al and P, interfacing with Au electrodes. The photo-induced time-dependent electric currents were computed from a combination of ab initio electronic structure and time-dependent density matrix methodology, and using the continuity equation for electronic currents. A dissipative equation of motion for the reduced density matrix for electronic degrees of freedom is used to study the phonon-induced relaxation of hot electrons in the simulated system. Equations are solved in a basis set of orbitals generated ab initio from a density functional. Non-adiabatic couplings between electronic orbitals are computed on-the-fly along nuclear trajectories. Charge carrier dynamics induced by selected photoexcitations show that hole relaxation in energy and in space is much faster than electron relaxation. The overall net charge transfer across the slab is small; however, local currents at the Si/Au interfaces are substantial. It is also shown that the relaxation of the induced current can be used to parameterise the dynamical conductivity by means of a fluctuation–dissipation relation.

The transport mechanism of the human sodium/myo-inositol transporter 2 (SMIT2/SGLT6), a member of the LeuT structural family.

The sodium/myo-inositol transporter 2 (SMIT2) is a member of the SLC5A gene family, which is believed to share the five-transmembrane segment inverted repeat of the LeuT structural family. The two-electrode voltage-clamp (TEVC) technique was used to measure the steady-state and the pre-steady-state currents mediated by human SMIT2 after expression in Xenopus laevis oocytes. Phlorizin is first shown to be a poor inhibitor of pre-steady-state currents for depolarizing voltage pulse. From an up to threefold difference between the apparent ON and OFF transferred charges during a voltage pulse, we also show that a fraction of the transient current recorded for very negative potentials is not a true pre-steady-state current coming from the cotransporter conformational changes. We suggest that this transient current comes from a time-dependent leak current that can reach large amplitudes when external Na(+) concentration is reduced. A kinetic model was generated through a simulated annealing algorithm. This algorithm was used to identify the optimal connectivity among 19 different kinetic models and obtain the numerical values of the associated parameters. The proposed 5-state model includes cooperative binding of Na(+) ions, strong apparent asymmetry of the energy barriers, a rate-limiting step that is likely associated with the translocation of the empty transporter, and a turnover rate of 21 s(-1). The proposed model is a proof of concept for a novel approach to kinetic modeling of electrogenic transporters and allows insight into the transport mechanism of members of the LeuT structural family at the millisecond timescale.

Non-equilibrium linear-response transport through quantum dot beyond time homogeneity at Hartree-Fock level

The expression for current, induced by finite bias with additional time-dependent (TD) external perturbation, through a molecule/dot is derived using the non-equilibrium Green's function (GF) formalism in the standard two-probe geometry. The GFs as well as self energies (SEs) are split into time-homogeneous (TH) and time-inhomogeneous (TIH) contributions, where the former are obtained as a result of zeroth-order expansion of the full, two-time corresponding quantities and the latter we find as linear corrections. The TD charge in the dot consists of a charge that is injected from the electrodes and the charge that is induced in the dot. The TD potential, induced in the dot due to dot TD charge, was treated self consistently at Hartree–Fock (HF) level as a TIH part of Coulomb interaction related SE. It is assumed that TH quantities are solved either exactly or approximately, i.e., using density functional theory (DFT). The theory is charge conserving and its gauge invariance is explicitly shown. The contribution of TD HF potential to the total Coulomb interaction energy vanishes in the case of one-electron existence, i.e., the self-interaction error (SIE), beyond the one associated with the DFT, was not introduced. Known results in a special case of time homogeneity are recovered and extended to TIH transport. The issues of current partitioning and the displacement current are resolved naturally, without any additional assumptions about any of quantities, due to explicit inclusion of dot potential. The special cases of wide-band limit, zero bias, and zero-bias wide-band limit are also considered and in each case the corresponding expression for the TD current is derived. The theory is particularly suitable for use in connection with DFT when it provides a first-principle microscopic linear-response description of the non-equilibrium TIH quantum transport useful for calculation of TD current through quantum dots, molecules, junctions, or devices at the nano-scale.

The effect of atomic hydrogen on the kinetics of iron passivation in neutral solutions

Current-time dependences are recorded on iron in borate buffer (pH 7.4 and 6.7) and its mixture with NS4 solution (pH 6.7) at the potentials of passivity, active-passive transition, and prepassivation of iron. Hydrogenation of the metal is found to accelerate the dissolution of iron in steady passive state, produce no effect on the growth of a barrier layer, and prevent the formation of a primary passivating film. Atomic hydrogen decelerates the active dissolution of iron, which determines the anodic current at the initial stage of the metal passivation.

Wave-packet representation of leads for efficient simulations of time-dependent electronic transport

We present theoretical foundations and numerical demonstration of an efficient method for performing time-dependent many-electron simulations for electronic transport. The method employs the concept of stroboscopic wavepacket basis for the description of electrons' dynamics in the semi-infinite leads.The rest of the system can be treated using common propagation schemes for finite electronic systems. We use the implementation of our method to study the time-dependent current response in armchair graphene nano-ribbons (AGNR) with sizes up to 800 atoms described within tight-binding approximation. The character of the time-dependent current is studied for different magnitudes of the bias voltage, variable width and length of AGNRs, different positions of the current measurement, and for full and reduced coupling of the AGNRs to the electrodes.

Vibrational effects on UV/Vis laser-driven π-electron ring currents in aromatic ring molecules

We present the results of a theoretical study of vibrational effects on UV/Vis laser-driven π-electron ring currents in aromatic ring molecules. We consider vibrational effects on both coherent and non-coherent (single quantum state) ring currents. The coherent ring current originates from an excitation of a pair of quasi-degenerate electronic states by an ultrashort linearly polarized UV/Vis laser pulse, while the non-coherent ring current originates from by an excitation of a degenerated electronic state of an aromatic ring molecule with high symmetry by a circularly polarized electric field of a UV/Vis laser pulse. The magnitude of a generated ring current can be expressed as an average of those of the bond currents for both the coherent and non-coherent cases. We derive an analytical expression for the magnitude of the bond currents in the adiabatic approximation. Using the expression, we performed calculations of a non-coherent ring current generated in the optically allowed excited state (1E1U) of benzene and the time evolution of coherent ring current of (P)-2,2-biphenol. Vibrational effects on the non-coherent ring current of benzene were found to be negligibly small. We paid particular attention to the vibrational effects induced by the torsion mode on time evolution of the coherent ring current along the bond bridging between the two aromatic rings of (P)-2,2-biphenol. By comparing the time evolution of the coherent ring current with that in the frozen-nuclear approximation, we found that inclusion of the low-frequency torsion mode brings about modulations in the beating in the ring current. The modulations in the time evolution of the coherent ring current were brought about by contribution of several pairs of the coherently excited vibronic states. Coherent vibronic ring currents generated from pairs of the coherently excited vibronic states interfere each other. The existence of the pairs originates from relatively large potential displacement of the torsion mode between the two excited states for (P)-2,2-biphenol. Temperature effects on the time-dependent coherent ring current were also investigated.

Liquid scintillator time projection chamber — time dependence in organic liquids

Results are presented from a small-scale experiment to investigate the use of room temperature organic liquid scintillator as the active medium for a time projection chamber (TPC). A time dependent background current has been observed in these organic liquids under high electric fields, analogous to those observed in other earlier experiments. Historically, the optical properties of liquid scintillators have been well characterised, but their charge transport properties have not yet conclusively been demonstrated. The observation of a dark current in the liquids presents a further challenge to consider when using this class of liquids. The target of this research is to offer a cost effective alternative to liquid noble gas detectors in neutrino physics.

A Coarse-Mesh Method for the Time-Dependent Transport Equation

A new solution technique is derived for the time-dependent transport equation. This approach extends the steady-state coarse-mesh transport method that is based on global-local decompositions of large (i.e., full-core) neutron transport problems. The new method is based on polynomial expansions of the space, angle, and time variables in a response-based formulation of the transport equation. The local problem (coarse-mesh) solutions, which are entirely decoupled from each other, are characterized by space-, angle-, and time-dependent response functions. These response functions are, in turn, used to couple an arbitrary sequence of local problems to form the solution of a much larger global problem. In the current work, the local problem (response function) computations are performed using the Monte Carlo method, while the global (coupling) problem is solved deterministically. The spatial coupling is performed by orthogonal polynomial expansions of the partial currents on the local problem surfaces, and similarly, the time-dependent response of the system (i.e., the time-varying flux) is computed by convolving the time-dependent surface partial currents and time-dependent volumetric sources against precomputed time-dependent response kernels.

Interface State Effects on Resistive Switching Behaviors of Pt/Nb-Doped SrTiO3 Single-Crystal Schottky Junctions

In this study, we investigated the resistive switching (RS) behavior of Pt/Nb-doped SrTiO3 (Pt/Nb:STO) single-crystal junctions in air and vacuum. We performed steady-state electrical characterizations: the direct current (DC) current–voltage relationship and relaxation current–time dependence under an applied voltage step. The ideality factor of the junction suggested the existence of interface states and tunneling current. We observed that the relaxation current followed the Curie–von Schweidler law; electrical conduction was dominated by a space-charge-limited current based on charge recombination at the interface states. The dynamic electric response was obtained using an alternating current (AC) conductance technique. The carrier lifetime at the interface traps was largely dependent on the resistance state and the ambient environment. Thus, surface potential modification by charge capture/release at the interface traps played a crucial role in the RS of our junctions. Additionally, the ambient effect showed that oxygen desorption (adsorption) at the Nb:STO surface increased (decreased) the interface state density. Finally, an RS model based on interface states in Pt/Nb:STO was proposed.

Proposal for an on-demand source of polarized electrons into the edges of a topological insulator

We propose a device that allows for the emission of pairs of spin-polarized electrons into the edge-states of a two dimensional topological insulator. Charge and spin emission is achieved using a periodically driven quantum dot weakly coupled to the edge states of the host topological insulator. We present calculations of the emitted time-dependent charge and spin currents of such a dynamical scatterer using the Floquet scattering matrix approach. Experimental signatures of spin-polarized two-particle emission can be found in noise measurements. Here a new form of noise suppression, named $\mathbb{Z}_2$--antibunching, is introduced. Additionally, we propose a set-up in which entanglement of the emitted electrons is generated. This entanglement is based on a post-selection procedure and becomes manifest in a violation of a Clauser-Horne-Shimony-Holt inequality.