Periodic Electrostatic Field Research Articles

Electrical Manipulation of Spin-Dependent Anisotropy of a Dirac Cone in a Graphene Superlattice with Alternating Periodic Electrostatic and Exchange Fields

We studied the spin-dependent behavior of the electronic properties of alternating periodic potentials applied to finite and infinite graphene superlattices coupled with tunable electrostatic and exchange fields. The band structures were evaluated using the transfer matrix approach. The results of tuning the coupled electrostatic potential and exchange field showed that the spin-dependent anisotropy of a Dirac cone depends on the difference between the amplitude of periodically modulated coupling. Spin-dependent collimation occurs when the modulations become zero-average potentials with the ratio of both periodically modulated strengths equals one, in which one spin can be moved freely, but the other one is highly collimated. In addition, we find that the number of extra Dirac points in the infinite superlattice is spin-dependent. In terms of spin-ups, their number increases with an increase in the strength of both modulated fields. To ensure this calculation, we also compute the conductance of finite periodic modulation at zero energy. It is shown that the peaks of the conductance occur when the extra Dirac point emerges. This result may be utilized to design graphene-based devices with highly spin-polarized collimators.

Threshold effect on the femtosecond laser-induced periodic subwavelength structure: An analytical approach

For linearly polarized femtosecond laser-induced periodic nanostructures on a metal surface, the underlying physics of self-formation patterns is yet to be fully explained. We demonstrated here that the stabilized periodic interface electrostatic field (Coulomb field) arising from the surface phase-locking resonant dominates the self-formation of ripple structures. The kinetics of ultrafast laser ablation on solid surface were discussed. The threshold fluence and upper limit fluence for self-formation of ripple structures were calculated. The underlying mechanisms were clarified with both simulation and experimental results. Analytical formula for the interspace between ripples (i.e., wavelength of surface Langmuir wave) was developed. As illustrated by the experimental results, the competition of the two ablation mechanisms, namely surface Coulomb ablation and direct ultrafast laser ablations, was shown to result in distinct structuring patterns for a typical Gaussian pulse laser irradiation on the target surface at the appropriate laser energy fluence.

Epitaxial Electrocrystallization of Magnesium via Synergy of Magnesiophilic Interface, Lattice Matching, and Electrostatic Confinement.

Rechargeable magnesium batteries are particularly advantageous for renewable energy storage systems. However, the inhomogeneous Mg electrodeposits greatly shorten their cycle life under practical conditions. Herein, the epitaxial electrocrystallization of Mg on a three-dimensional magnesiophilic host is implemented via the synergy of a magnesiophilic interface, lattice matching, and electrostatic confinement effects. The vertically aligned nickel hydroxide nanosheet arrays grown on carbon cloth (abbreviated as "Ni(OH)2@CC") have been delicately designed, which satisfy the essential prerequisite of a low lattice geometrical misfit with Mg (about 2.8%) to realize epitaxial electrocrystallization. Simultaneously, the ionic crystal nature of Ni(OH)2 displays a periodic and hillock-like electrostatic potential field over its exposed facets, which can precisely capture and confine the reduced Mg0 species onto the local electron-enriched sites at the atomic level. The Ni(OH)2@CC substrate undergoes sequential Mg-ion intercalation, underpotential deposition, and electrocrystallization processes, during which the uniform, lamellar Mg electrodeposits with a locked crystallographic orientation are formed. Under practical conditions (10 mA cm-2 and 10 mAh cm-2), the Ni(OH)2@CC substrate exhibits stable Mg stripping/plating cycle performances over 600 h, 2 orders of magnitude longer than those of the pristine copper foil and carbon cloth substrates.

Terahertz emission by multiple resonances under external periodic electrostatic field.

In the presence of electron neutral collisions of a frequency (ν) of the order of ν≥0.5ω_{p}(ω_{p} is the plasma frequency), the collisional effects play an adverse role in the mechanism of terahertz (THz) radiation. The present work describes an approach for the efficient emission of THz radiation with the application of an external periodic electric field in the density rippled collisional plasma wherein an additional transverse component of the current is generated that adds to the THz radiation mechanism. The THz field obtained by coupling of the lasers' field with the external field is termed as an external field induced THz (EFIT). Here, the periodic wave number of the external field enables tuning of the THz radiation and helps achieve multiple resonance conditions for the excitation of large amplitude nonlinear plasma currents.

Wavefunction of plasmon excitations with space charge effects

The one dimensional (1D) driven quantum coupled pseudoforce system governing the dynamics of collective Langmuir electron oscillations is used in order to investigate the effects of variety of space charge distributions on plasmon excitations of a nearly free electron gas with an arbitrary degree of degeneracy and electron fluid temperature. A generalized closed form analytic expression for the grand wavefunction of collective excitations in the presence of an arbitrary space charge distribution is presented based on the stationary solutions of the driven coupled pseudoforce system which has been derived from the Schrödinger-Poisson model. The wavefunction and electrostatic potential profiles for some special cases such as the Heaviside charge distribution, Dirac charge sheet, impurity charge sheet in the 1D plasmonic lattice, and the Kroning-Penney Dirac charge distributions with particular applications in plasmonics and condensed matter physics are investigated in this paper. It is remarkably found that two parallel Dirac charged sheets completely shield all interior plasmon excitations with any given energy value from outside electrostatic fields and charge densities. It is also found that the presence of even a weakly charged impurity layer within a perfect 1D plasmonic crystal profoundly alters the periodic electrostatic field of the crystal lattice, and hence, the Bloch character of the wavefunction is considered in the bandgap theory of solids. The current investigation of electron excitations in arbitrary degenerate electron gas in the presence of static charge distributions may be used to develop analytic models for a variety of real physical situations. It also helps in further developments of the rapidly growing fields of nanotechnology and plasmonics.

Stability analysis using multiple scales homotopy approach of coupled cylindrical interfaces under the influence of periodic electrostatic fields

The influence of axial periodic electric fields on streaming flows through three coaxial infinitely vertical cylinders is considered. The three fluid layers are assumed to be incompressible, dielectric, viscous and saturated through porous media. To relax the mathematical manipulation of the problem, the viscous potential theory is considered. Through the current work, the stability analysis of the coupled Mathieu equations is developed in the analogy of the multiple scales homotopy technique. Away from the symmetric and anti-symmetric modes, the present study investigates a general case of the surface waves deflections. To overcome the lengthy of the algebraic calculations, the matrices concept is utilized. The stability analysis reveals the resonance as well as non-resonance cases. A set of graphs are depicted to indicate some resonance cases for a chosen sample through a dimensionless system. Therefore, the influence of some physical parameters on the stability picture is indicated. In addition, the perturbed solutions of the governed Mathieu equations are graphed.

Drifting Plasma Confinement With a Spatially Periodic Field

Some possible plasma confinement configurations are identified in which a spatially periodic electrostatic or magnetostatic field may provide or enhance ion confinement. The plasma drifts at a speed much faster than the ion thermal speed. The periodic field has a spatial period that is much smaller than the plasma size, and the field has a nonnegligible strength only at the plasma edge. The periodic field could be produced, for example, by a sequence of electrodes with alternating applied voltages or a sequence of wires with alternating current directions. Classical trajectory Monte Carlo simulation results are used to develop expressions for predicting the conditions under which ions are reflected away from an artificially structured boundary that produces a spatially periodic field, without the ions reaching the artificially structured boundary.

INVESTIGATION AND OPTIMIZATION OF LOW-ENERGY HEAVY-ION BEAM DYNAMICS IN PERIODIC AXISYMMETRICAL STRUCTURES WITH DC FOCUSING

It is well-known that nonsynchronous harmonics of a RF field can focus particles. In order to focus ultra-low energy beam particles, a spatially periodic electrostatic field can be used, too. In this case the transverse focusing is provided by a periodic array of electrostatic lenses. This article is dedicated to questions of two high-current low-energy beam focusing types used in an injector-buncher of an ion linear accelerator, an application of a spatially periodic electrostatic focusing being examined for the first time. A two-dimensional nonlinear motion equation is derived in the Hamiltonian form in a smooth approximation. It allows us to study a correlation between the longitudinal and transverse ion beam dynamics at low energies. An investigation of the phase and transverse beam dynamics stability conditions in the initial part of the linac are given consideration. Conditions of the beam dynamics stability are found and analyzed. An approach to the field optimization problem is described. A method of the geometric parameter choice is worked out for a realization of the required field distributions in each structure. To verify the results of the beam dynamics stability analysis and optimized structure parameter choice, numerical simulations of low-energy heavy-ion beams are carried out.

Magnetic bloch states and carrier transport in two-dimensional semiconductor lattice structures with spin-orbit interaction

Quantization rules have been obtained for the Hall conductance of fully occupied Landau subbands of the two-dimensional electron gas with the Dresselhaus spin-orbit interaction in a periodic electrostatic field of a superlattice and a transverse magnetic field. The spin-orbit interaction mixes states of different magnetic subbands and changes the quantization rule for the Hall conductance compared to spinless particles. The calculations have been performed for the two-dimensional electrons in the structures with both a weak (AlGaAs/GaAs) and sufficiently strong (GaAs/In0.23Ga0.77As) spin-orbit interaction and Zeeman splitting. It has been found that the distribution of the Hall conductance among the magnetic subbands depends on the geometric parameters of the superlattices and promptly changes upon the touching of the adjacent subbands in the spectrum. The quantization rule for the Hall conductance in real semiconductor structures with relatively strong spin-orbit interaction has been shown to differ from that calculated by Thouless et al. [Phys. Rev. Lett. 49, 405 (1982)] for the systems without the spin-orbit interaction and Zeeman effect.

Measurement of submilliwatt, picosecond terahertz emission from a femtosecond-laser-pumped solid-state dc to ac radiation converter based on a ZnSe crystal

We measured the terahertz pulse emission from a femtosecond-laser-pumped solid-state dc to ac radiation converter using a 150fs Ti:sapphire laser pulse for dense plasma diagnostics. The laser-produced ionization front was directly modulated from a periodic electrostatic field to pulsed emission. The central frequency of the emission was measured to be 0.13THz having a bandwidth of 0.1THz and a peak power of 0.2mW. This emission source is suitable for use in various novel diagnostic techniques, such as dense plasma diagnostics.

Photorefractive effect on all optical polling of glass

The appearance of the photorefractive effect on all optical polling (OP) of a bulk glass is investigated. During OP, a refractive-index grating accumulates in the glass and radiation effectively diffracts through it. The theory of the self-diffraction of light has been developed and the resultant expression for the amplitude of the diffracted light has been deduced, considering the geometry of formation of the refractive-index grating in glass upon interaction of the intersecting Gaussian beams. The results of the experimental investigation of the diffraction of light on the photorefractive gratings induced in some oxide glasses have been presented. The experimental results obtained are in agreement with the theory, based on a model of formation of a spatially periodic electrostatic field in glass via the coherent photogalvanic effect. The phenomenon of light diffraction can be used to investigate the physical properties of all OP in different glass media and to search for new, effective photorefractive materials.

Generation of Electromagnetic Wave Pulse Used by Discharged Plasmas

Generation of the electromagnetic wave pulse from a periodic electrostatic field via a laser-produced ionization front is reviewed. Experimental results of the generation of electromagnetic wave pulse from a DC to AC radiation converter (DARC) are described.

Experimental observation of further frequency upshift from dc to ac radiation converter with perpendicular dc magnetic field

A frequency upshift of a short microwave pulse is generated by the interaction between a relativistic underdense ionization front and a periodic electrostatic field with a perpendicular dc magnetic field. When the dc magnetic field is applied, further frequency upshift of 3 GHz is observed with respect to an unmagnetized case which has typically a GHz range. The radiation frequency depends on both the plasma density and the strength of the dc magnetic field, i.e., the plasma frequency and the cyclotron frequency. The frequency of the emitted radiation is in reasonable agreement with the theoretical values.

Approximate solution of the Dirac equation in the transverse focusing and longitudinal periodic fields of a crystal

The solution of the Dirac equation is obtained for a relativistic electron in the focusing and periodic electrostatic fields modeling the field at the axis or plane of a crystal.

Transport properties of quantum wires in spatially periodic magnetic and electrostatic fields

Electron transport properties of quantum wires in the presence of modulated magnetic and electrostatic fields are investigated. We find dips in conductance just below each mode threshold in the presence of a periodic magnetic modulation. The combination of both electrostatic and magnetic modulations makes the conductance dips more pronounced. The effects of temperature and background magnetic field on the conductance dips are also studied. Increasing the number of periods of magnetic modulation can lead to magnetic blocking and reopening of the channel, as the field strength is increased. For higher incoming energy, a larger value of the magnetic modulation amplitude is needed to block the channel and to reopen it. For thicker magnetic barriers, the range of field strength for magnetic reopening narrows. Electrostatic modulation suppresses the magnetic reopening.

Quasiclassical calculation of magnetoresistance oscillations of a two-dimensional electron gas in spatially periodic magnetic and electrostatic fields.

A classical approach, relating magnetoresistance oscillations of a two-dimensional electron gas (2DEG) in a weak lateral superlattice to the guiding center drift of cyclotron orbits, is extended to superlattices defined by spatially periodic electrostatic and magnetic fields of arbitrary shape but equal lattice constants. The results are applied to the experimentally relevant situation of modulation fields produced by periodic arrays of magnetized strips or dots on the surface of a heterostructure containing a 2DEG. Magnetic modulation fields of different symmetries, tuned by the orientation of the magnetization, are superimposed on the stress-induced electrostatic modulation and lead to characteristic interference effects on the Weiss oscillations of the magnetoresistance. \textcopyright{} 1996 The American Physical Society.

Hofstadter-type energy spectra in lateral superlattices defined by periodic magnetic and electrostatic fields.

We calculate the energy spectrum of an electron moving in a two-dimensional lattice which is defined by an electric potential and an applied perpendicular magnetic field modulated by a periodic surface magnetization. The spatial direction of this magnetization introduces complex phases into the Fourier coefficients of the magnetic field. We investigate the effect of the relative phases between electric and magnetic modulation on band width and internal structure of the Landau levels.

Approximate solution of the Dirac equation in an arbitrary, periodic electrostatic field

The Dirac equation for a relativistic electron in a periodic electric field, simulating the field of a crystal axis, is solved.

Conceptual design of a plasma undulator

A micropitched plasma undulator is proposed for the first time. It is composed of an array of hundreds of discharged and pinched slender columns of plasma. Two kinds of undulating forces on the electron beam can be considered: one is a periodic magnetic field generated by the electric currents flowing through the array of the small radius plasma columns and the other is a periodic electrostatic field generated by the interaction of the relativistic electron beam with the rippled density of the plasma. Methods for both producing and stabilizing the array of the plasma columns are discussed. The effects of collisions between the plasma particles and the electron beam on the orbit and the momentum of the electrons are also discussed, leading to the conclusion that they can be negligibly small, as far as the gas pressure in the chamber leaking from the plasma region does not disturb the performance of the electron accelerator.

Spontaneous free-electron two-quantum Stark emission in an arbitrary direction from a zero-temperature electron beam

Emissions from relativistic electrons travelling in periodic electrostatic fields were observed by Smith and Purcell (1953) and Doucas et at. (1992) as extraordinarily (e.g. 1018 times) stronger than any emission that can be conceived with classical electrodynamics under any equivalent condition. The mechanism is identified as the free-electron two-quantum Stark (FETQS) emission generated by the axial uniform motion, which cannot be radiated in the axial direction. From the excellent agreement between the theoretical result for FETQS emission driven by the axial uniform motion, and the experimental observations and many emission phenomena in plasmas, it is concluded that a high-energy electron follows a quantum-mechanically derived formula (without taking the classical limit ←0), although this diverges in the classical limit. From the extraordinarily large FETQS emission due to macroscopic motion, we speculate that even the FETQS emission generated by electron spin can be macroscopically observable. The spin-generated FETQS emission in an arbitrary direction is calculated using relativistic quantum mechanics. It is found that the total power of this emission scales as γ2 times the emission power in the equivalent magnetic wiggler, where γ is the Lorentz factor of the electron, and the emission is practically confined in a cone of angle 1/γ about the axial direction.