External Ac Field Research Articles

Effects of bias on dynamics of an AC-driven two-electron quantum-dot molecule

The effects of bias on the dynamical localization of two interacting electrons in a pair of coupled quantum dots driven by external AC fields have been numerically investigated. With an effective two-site model and Floquet formalism, the time-dependent Schrodinger equation is numerically solved and the Pmin, the minimum of the population evolution of the initial state within a certain time period, is used to quantify the degree of the dynamical localization. Results indicate that the bias can change the energy of the initial state and break the dynamical symmetry of the system with a pure AC field. And the amplitude of the AC field with dynamical localization phenomenon changes with bias. All the numerical results are explained by the perturbation theory and two-level approximation.

Double Sine-Gordon ratchet induced by excitation of an internal mode

In a recent paper [Phys. Rev. E 69, 056612 (2004)] we showed the symmetry analysis of Flach et al. [Phys. Rev. Lett. 88, 184101 (2002)] which predicted the appearance of directed energy current in homogeneously spatially extended systems described by nonlinear field equations coupled to a heat bath in the presence of a correct choice for the time dependence of an external ac field, E(t), was due to the excitation of an internal mode. Flach applied their analysis to the sine-Gordon (SG) equation and verified the symmetry breaking numerically. In the SG case we showed the internal mode coupled to the center of the mass variable, X(t), that caused the symmetry breaking was Gamma(t) the slope of the kink. We also found that the phonon dressing of the SG kink by the ac driver, chi(t), was necessary for the occurrence of a directed energy current in the SG equation. We show in the case of the double sine-Gordon (DSG) equation that the excitation of the internal mode, R(t) (where R(t) is the separation of the two subkinks that make up the DSG soliton), combined with the phonon dressing of the DSG soliton also causes a directed energy current.

Mechanism of decay of trapped magnetic field in HTS bulk caused by application of AC magnetic field

In our previous work, it was observed that trapped magnetic field in an high-temperature superconductor (HTS) bulk was decayed and even erased by application of AC external field whose amplitude was much smaller than the peak value of the trapped magnetic field. Therefore, knowledge on the mechanism of the decay of the trapped magnetic field is important to design the machines and to develop a method to suppress the decay. This work studies a mechanism of the decay due to application of AC magnetic field by a numerical analysis and experiments. An analytical model is proposed to explain the mechanism based on the thermal effect due to AC losses in the bulk. In the model it is assumed that the AC loss characteristic follows the Bean model, which was experimentally demonstrated also in our previous work. We conducted experiments in which temperature rise of the bulk was measured by thermocouples and the decay of the trapped magnetic was observed by a hall probe. Results numerically calculated from the analytical model well agreed with those obtained from the experiments and we consider it was clarified that the decay of the trapped field was caused by the thermal effect due to AC loss. Once the mechanism has been clarified, methods to suppress the decay can be easily developed.

Soliton ratchets induced by excitation of internal modes.

Recently Phys. Rev. Lett. 88, 184101 (2002)]] used a symmetry analysis to predict the appearance of directed energy current in homogeneously spatially extended systems coupled to a heat bath in the presence of an external ac field E (t). Their symmetry analysis allowed them to make the right choice of E (t) so as to obtain symmetry breaking which causes directed energy transport for systems with a nonzero topological charge. Their numerical simulations verified the existence of the directed energy current. They argued that the origin of their strong rectification in the underdamped limit is due to the excitation of internal modes and their interaction with the translational kink motion. The internal mode mechanism as a cause of current rectification was also proposed by Salerno and Zolotaryuk [Phys. Rev. E. 65, 056603 (2002)]]. We use a rigorous collective variable for nonlinear Klein-Gordon equations to prove that the rectification of the current is due to the excitation of an internal mode Gamma (t), which describes the oscillation of the slope of the kink, and due to a dressing of the bare kink by the ac driver. The internal mode Gamma (t) is excited by its interaction with the center of mass of the kink, X (t), which is accelerated by E (t). The external field E (t) also causes the kink to be dressed. We derive the expressions for the dressing and numerically solve the equations of motion for Gamma (t), X (t), and the momentum P (t), which enable us to obtain the explicit expressions for the directed energy current and the ac driven kink profile. We then show that the directed energy current vanishes unless the slope Gamma (t) is a dynamical variable and the kink is dressed by the ac driver.

Photon-assisted mesoscopic transport through a toroidal carbon nanotube coupled to normal metal leads

The mesoscopic transport through a toroidal carbon nanotube (TCN) system applied with ac fields to the electrodes has been investigated by employing the nonequilibrium Green's function (NGF) technique. The Landauer–Büttiker-like formula is presented for numerical calculations of differential conductance and tunneling current. The conductance resonance takes place due to the electrons resonating in the quantum levels of TCN and side-band caused by the external ac fields. The photon-assisted transport can be observed both in conductance oscillation and current oscillation with respect to the magnetic flux. The side peaks and current suppressions are the main effect of photon absorption and emission in the transport procedure. The stair-like current–voltage characteristics are resulted from the quantum nature of TCN and applied microwave fields. The photon–electron pumping effect can be obtained by applying the microwave fields to the leads.

Time-dependent electron transport through a quantum wire

The influence of the external time-dependent field on electron transport through a quantum wire is studied using the model Hamiltonian and evolution operator method. The conductance, transmission coefficient, and current flowing through the wire connected to leads are obtained. It is shown that the average current can be expressed by the corresponding matrix elements of the evolution operator and it satisfies the Landauer-like formula. The sideband peaks in the average current and transmittance are investigated. In contrast to the even-odd conductance oscillation the six-atom period was found in the case where ac external field is applied to the wire.

Control of electron localization in a coupled quantum dot structure

We systematically investigate the dynamical behaviour of an electron in a double quantum dot system under the influence of an external AC field. It is assumed that the quantum dot confined structure exhibits a non-negligible Coulomb charging energy, inversely proportional to its small capacitance. The dynamic evolution of the system is obtained by numerically solving the coupled, nonlinear, equations derived from the time-dependent Schrödinger equation. We find cases where the electron is localized in the initially placed dot when both effects of the Coulomb charging energy and the external field are present, even though if either effect is absent the electron will tunnel between dots. We also show that we can pre-select the shape and rise time of a semi-infinite, pulsed, AC field in order to transfer an electron from the initially placed dot to the other dot and localize it there.

Effective dielectric response of nonlinear composites with external ac and dc electric field

The perturbation method is developed to investigate the effective nonlinear dielectric response of Kerr composites when the external ac and dc electric field is applied. Under the external ac and dc electric field Eapp=Ea(1+sin ωt), the effective coupling nonlinear response can be induced by the cubic nonlinearity of Kerr nonlinear materials at the zero frequency, the finite basic frequency ω, the second and the third harmonics, 2ω and 3ω, and so on. As an example, we have investigated the cylindrical inclusions randomly embedded in a host and derived the formulas of the effective nonlinear dielectric response at harmonics in dilute limit. For a higher concentration of inclusions, we have proposed a nonlinear effective-medium approximation by introducing the general effective nonlinear response. With the relationships between the effective nonlinear response at harmonics and the general effective nonlinear response, we have derived a set of formulas of the effective nonlinear dielectric responses at harmonics for a larger volume fraction.

High-Frequency Generation in Low-Mobility Superlattices

ABSTRACTWe study the transport properties of miniband electrons in an electromagnetic field as related to the high-frequency generation in a semiconductor superlattice (SL). We study the regimes where the external ac-field is in resonance and out of resonance with the two lowest SL minibands.In the diffusive quasi-static regime Bloch oscillations and electrical domains do not develop and high-frequency generation may occur as a result of the negative high-harmonic nonlinear conductivity. This regime is different from the frequency conversion in a stable nonlinear media. We point out that AlGaN/GaN material system has a great potential for the high-power SL terahertz (THz) source.

Photorefractive recording in ac-biased cadmium telluride

We report effect of external ac-field on photorefractive properties of CdTe:Ge at 1.06μm. Phenomena related to excitation of the space-charge waves (SCW) of traps recharging, such as spatial subharmonics generation and resonant two-beam coupling, are studied. Dramatic enhancement of the photorefractive response is achieved in ac-biased crystal by adjustment of recording conditions (field and grating spacing) to eigen resonance related to excitation of the space charge waves.

Ac field-induced quantum rectification effect in tunnel junctions

We study the appearance of directed current in tunnel junctions (quantum ratchet effect), in the presence of an external ac field $f(t).$ The current is established in a one-dimensional discrete inhomogeneous ``tight-binding model.'' By making use of a symmetry analysis we predict the right choice of $f(t)$ and obtain the directed current as a difference between electron transmission coefficients in opposite directions, $\ensuremath{\Delta}{T=T}^{\mathrm{LR}}\ensuremath{-}{T}^{\mathrm{RL}}.$ Numerical simulations confirm the predictions of the symmetry analysis and moreover, show that the directed current can be drastically increased by a proper choice of frequency and amplitudes of the ac field $f(t).$

Decay of trapped magnetic field in HTS bulk caused by application of AC magnetic field

In our previous work it was observed that trapped magnetic field in a YBCO bulk was decayed and even erased by application of AC external field whose amplitude was much smaller than the peak value of the trapped magnetic field. To study a reason for decay we investigated temperature rise of a YBCO bulk subject to the AC external magnetic field by use of thermocouples. A case was observed that the temperature rise of the bulk reached almost the critical temperature. This result shows that the decay of the trapped magnetic field was related to the temperature rise due to AC losses in the bulk.

Trapped field characteristic of HTS bulk in AC external magnetic field

Although immutable trapped field is required in superconducting bulk applications as a quasipermanent magnet, the trapped field is influenced and changed by time-varying external magnetic field in a realistic operational environment of electrical devices. This means that shielding current distribution within bulk is changed by the time-varying magnetic field and the transient magnetic flux movement results in temperature rise and finally reduction of the trapped field. In this paper, we experimentally investigated the transition of trapped field while applying external AC magnetic field with various amplitude and frequency to a disk-shaped YBCO bulk. And we also numerically investigate the relationship between characteristic of trapped-field attenuation and shielding current distribution within the bulk using a newly developed simulation program. This program is based on the finite element method (FEM) considering the voltage-current (E-J) characteristics. It is found that the shielding current distribution is dependent on frequency of AC external field and trapped field attenuation is closely related to the shielding current.

Effect of magnetic flux distribution on GMR in Ag/Co multilayers

Since the discovery of giant magnetoresistance (GMR) in multilayers (MLs) various applications of this effect are studied. One practical feature of ML GMR sensing elements is the magnetic flux distribution in the direction perpendicular to the surface of ML in the current-in-plane geometry. It results from the fact that top and bottom ferromagnetic layers concentrate more magnetic flux than central layers of the stack. This effect is studied in this work in evaporated Ag/Co MLs deposited onto Si substrates. The thickness of Co was 1.2–1.4 nm, i.e. Co layers were continuous. Ag layers were 5.4–5.8 nm thick. Number of Ag/Co periods was only ≤5 to expose the relative influence of top and bottom parts of ML. To make the ML structure symmetrical, an additional Co layer was deposited as first on the substrate. Ag/Co MLs were analyzed by X-ray diffraction (XRD) and X-ray reflectivity (XRR). The magnetic flux distribution was simulated. GMR measurements were performed in dynamic conditions in the external ac field (50 Hz) with increasing swing. By simulations of XRR spectra the effective roughness of ML interfaces 0.4–0.8 nm was obtained. The GMR signal at RT increased with number of periods and the external field ≤16 kA/m up to approximately 2%. The influence of distribution of the magnetic flux density on GMR was inferred from the behavior of the positions of maxima of GMR on the magnetic field axis vs. number of periods. It can be explained by the statistical distribution of coercivity of different domains in Co layers.

Domain wall depinning in random media by ac fields.

The viscous motion of an interface driven by an ac external field of frequency omega(0) in a random medium is considered here in the nonadiabatic regime. The velocity exhibits a smeared depinning transition showing a double hysteresis which is absent in the adiabatic case omega(0)-->0. Using scaling arguments and an approximate renormalization group calculation we explain the main characteristics of the hysteresis loop. In the low frequency limit these can be expressed in terms of the depinning threshold and the critical exponents of the adiabatic case.

Circuit coupling method applied to bulk superconductors

This paper presents a new finite-element method for including a bulk superconductor in an electrical circuit. The method solves simultaneously the finite-element equations of A/spl psi/ formulation and the circuit equations. At the same time, a power law defines the superconducting properties. The method then compares the classical homogeneous model of a Bi-2223 tape to a multifilament model. The results of a simulation of conductors carrying alternative current while subjected to an external ac field are presented.

Domain walls in random media driven by AC fields

The viscous motion of an interface driven by an ac external field of frequency $\omega_0$ in a random medium is considered here for the first time. The velocity exhibits a smeared depinning transition showing a double hysteresis which is absent in the adiabatic case $\omega_0 \rightarrow 0$. Using scaling arguments and an approximate renormalization group calculation we explain the main characteristics of the hysteresis loop. In the low frequency limit these can be expressed in terms of the depinning threshold and the critical exponents of the adiabatic case.

Targeted transfer of solitons in continua and lattices.

We propose a robust mechanism of targeted energy transfer along a line, as well as on a surface, in the form of transport of coherent solitary-wave structures, driven by a moving, spatially localized external ac field ("arm") in a lossy medium. The efficiency and robustness of the mechanism are demonstrated analytically and numerically in terms of the nonlinear Schrödinger (NLS) equation, and broad regions of stable operation are identified in the model's parameter space. Direct simulations show that the driving arm can manipulate solitons equally well in a lattice NLS model. A salient feature, which is revealed by simulations and explained analytically, is a resonant character of the operation of the driving arm in the lattice medium, both integer and fractional resonances being identified. Numerical experiments also demonstrate that the same solitary-wave-transport mechanism works well in two-dimensional lattice media.

Broken symmetries and directed collective energy transport in spatially extended systems.

We study the appearance of directed energy current in homogeneous spatially extended systems coupled to a heat bath in the presence of an external ac field E(t). The systems are described by nonlinear field equations. By making use of a symmetry analysis, we predict the right choice of E(t) and obtain directed energy transport for systems with a nonzero topological charge Q. We demonstrate that the symmetry properties of motion of topological solitons (kinks and antikinks) are equivalent to the ones for the energy current. Numerical simulations confirm the predictions of the symmetry analysis and, moreover, show that the directed energy current drastically increases as the dissipation parameter alpha reduces.

Frequency dependence of AC magnetization in stacked Bi-2223 Ag-sheathed tapes

The frequency dependence of AC magnetization in stacked Bi-2223 Ag-sheathed tapes is experimentally and numerically evaluated in an external magnetic field perpendicular to their wide faces. The sample consists of 10 pieces of non-twisted Bi-2223 multifilamentary tapes, which are vertically stacked. The magnetization in the external AC field, whose frequency is varied from 1 to 60 Hz, is measured with a saddle-shaped pickup coil. The measured AC magnetization is compared not only with the theoretical curves based on the critical state model but also with the numerically calculated losses which are derived using the voltage–current characteristics represented by the n-value model, where the local electric field is proportional to the power n of local current density. Furthermore, it is shown that the eddy-current losses in a silver matrix cannot be ignored in the perpendicular-field losses at a commercial frequency.