Space-dependent Electric Field Research Articles

Magnetic bipolarity and other electronic aspects of (4,4) silicon carbide nanotubes ((4,4)SiCNT) decorated by light noble metals; (Cu, Ag)

The changes in structural and electronic properties of (4,4)SiCNT, induced by decoration with copper or silver atoms, are investigated. To this end, the dispersion-corrected density functional theory and Quantum-Espresso computational package, in its spin-polarized mode, are employed. It is found that adsorption of Cu or Ag atoms onto (4,4)SiCNT is exothermic and chemical. From the corresponding geometrical and electronic characteristics, it is concluded that decoration of (4,4)SiCNT by Cu or Ag causes a bulging in the nanotube which breaks the structural symmetry of the compounds. It turns out that in both cases, Si atoms in the tube bulge more drastically in comparison with the C atoms. Consequently, space-dependent electric fields form in the vicinity of adsorption sites, causing the spin degeneracy removal and the two compounds become ferromagnetic bipolar semiconductors. The latter conclusion is further supported by the electronic energy band structures and densities of states. It is also shown that in both materials, the electronic effective masses are by far larger than that in pristine (4,4)SiCNT. The possibility of tuning the magnetic properties of the two compounds by applications of gate (side-wise) potentials is discussed too. The present results then demonstrate novel paths to generate electrically switchable spin-polarized currents.

Chirp effects on pair production in oscillating electric fields with spatial inhomogeneity

Chirp effects on the vacuum pair creation are studied with space and time dependent electric field models. For rapidly oscillating electric fields, the total particle number enhancement only occurs for larger chirp values and the very large chirp may render the effect from the finite spatial size trivial. For slowly oscillating electric fields, the chirp and carrier phase parameters have crucial effects on the particle momentum distribution. The quasihomogeneous results are analyzed using local density approximation where the consequence of the field strength dependence of the homogeneous results is noticed.

Chaotic dynamics of the hydrogen atom in a space-dependent electric field

We study the chaotic dynamics of the hydrogen atom in a space-dependent gradient electric field. We use the Poincaré surface of section (PSS) and the Lyapunov exponents to characterize chaotic dynamics. The dynamical character of this system is studied and the regular-chaos transition is found when the system passes from the non-integrable case to the integrable one. It is found that for a given gradient electric field, this system has a critical scaled energy When the scaled energy the structure of the whole phase space is nearly regular. However, as the scaled energy chaotic structures appear in the PSS. As the scaled energy is very large, the whole PSS is chaotic. The chaotic property of this system is further verified by the positive value of the maximum Lyapunov exponent. In addition, we show that with increasing electric field gradient the dynamics of this system becomes increasingly chaotic. Our work provides an example to study the chaotic dynamics of the hydrogen atom by using the space-dependent electric field. We hope that our results can guide the future experimental researches about the dynamical property of the atoms or molecules in the non-uniform external fields.

Asimptotik İterasyon Metodu İle Bir Dış Elektromanyetik Alan İçerisindeki Skaler Parçacıkların Hareketinin İncelenmesi

In the present article, we study the energy eigenvalues and wavefuntions of the spinless particles in the existance of space-dependent electric and magnetic fields. The investigation is performed for two different orientations of the external fields by solving the Schrödinger and Klein-Gordon equations via Asymptotic Iteration Method (AIM). The obtained results are discussed numerically for first few quantum levels to understand the relativistic contributions by comparing the energy eigenvalues of Schrödinger and Klein-Gordon equations. By considering the quantum conditions for the resulting equations in the case of parallel and orthogonal orientations of the fields, we obtained the plots for the energy levels.

Schwinger Pair Production via Polons and the Origin of Stokes Phenomena

Schwinger pair production of electrons and positrons in a strong electric field is a prediction of nonperturbative quantum field theory, in which the out-vacuum is superposed of multi-particle states of the in-vacuum. Solving the Dirac or Klein-Gordon equation in the background field, though a linear wave equation, and finding the pair-production rate is a difficult or nontrivial job. The phase-integral method has recently been introduced to compute the pair production in space-dependent electric fields, and a complex analysis method has been employed to calculate the pair production in time-dependent electric fields. In this paper, we apply the complex analysis method to a Sauter-type electric field and other hyperbolic-type electric fields that vanish in the past and future and show that the Stokes phenomena in pair production occur when the time-dependent frequency for a given momentum has finite simple poles (polons) with pure imaginary residues.

Magnonic Floquet Hofstadter butterfly

We introduce the magnonic Floquet Hofstadter butterfly in the two-dimensional (2D) insulating honeycomb ferromagnet using a combination of spin wave theory and quantum field theory. We show that hopping magnetic dipole moment (i.e. magnon quasiparticles) in the 2D insulating honeycomb ferromagnet accumulates the Aharonov–Casher phase when irradiated by an oscillating space- and time-dependent electric field. We study two different cases. In the first case, we consider the effects of only space-dependent electric field. In this case, we realize the magnonic Hofstadter spectrum with similar fractal structure as graphene subject to a perpendicular magnetic field, but with no spin degeneracy due to broken time-reversal symmetry by the ferromagnetic order. In addition, the magnonic Dirac points and Landau levels occur at finite energy as expected in a bosonic system. Remarkably, this discrepancy does not affect the topological invariant of the system. Consequently, the magnonic Chern number assumes odd values and the magnon Hall conductance gets quantized by odd integers. In the second case, we study the effects of both space- and time-dependent electric field. In this case, the theoretical framework is studied by the Floquet formalism. We show that the magnonic Floquet Hofstadter spectrum emerges entirely from the oscillating space- and time-dependent electric field, which is in stark contrast to electronic Floquet Hofstadter spectrum, where irradiation by circularly polarized light and a perpendicular magnetic field are applied independently. We study the deformation of the fractal structure at different laser frequencies and amplitudes, and analyze the topological phase transitions associated with gap openings in the magnonic Floquet Hofstadter butterfly.

Non Linear Response of a Harmonic Diatomic Molecule: an Exact Algebraic Approach11Most of the information presented in this contribution has already been published in Ref. [1

Even harmonic molecules display a nonlinear behavior when driven by an inhomogeneous field. Here we present the response of a single harmonic molecule to a monochromatic time and space dependent electric field E(r, t) of frequency ω employing exact algebraic methods. We evaluate the responses at the fundamental frequency ω and at succesive harmonics 2ω, 3ω, etc., as a function of the intensity of the field and compare the results with those of the first and second order perturbation theory.

Anomalous dielectric relaxation with linear reaction dynamics in space-dependent force fields.

The anomalous dielectric relaxation of disordered reaction with linear reaction dynamics is studied via the continuous time random walk model in the presence of space-dependent electric field. Two kinds of modified reaction-subdiffusion equations are derived for different linear reaction processes by the master equation, including the instantaneous annihilation reaction and the noninstantaneous annihilation reaction. If a constant proportion of walkers is added or removed instantaneously at the end of each step, there will be a modified reaction-subdiffusion equation with a fractional order temporal derivative operating on both the standard diffusion term and a linear reaction kinetics term. If the walkers are added or removed at a constant per capita rate during the waiting time between steps, there will be a standard linear reaction kinetics term but a fractional order temporal derivative operating on an anomalous diffusion term. The dielectric polarization is analyzed based on the Legendre polynomials and the dielectric properties of both reactions can be expressed by the effective rotational diffusion function and component concentration function, which is similar to the standard reaction-diffusion process. The results show that the effective permittivity can be used to describe the dielectric properties in these reactions if the chemical reaction time is much longer than the relaxation time.

Femtosecond plasmon and photon wave packets excited by a high-energy electron on a metal or dielectric surface

Swift electrons generate coherent transition radiation (TR) when crossing a material surface, as well as surface plasmon polaritons (SPPs) when the material is metallic. We present analytical and numerical calculations that describe the time- and space-dependent electric fields of TR and SPPs induced by 30--300 keV electrons on a Drude metal surface. The generated SPPs form wave packets a few-hundred femtoseconds in duration, depending on the material permittivity. High-frequency components close to the plasmon resonance are strongly damped, causing the wave packets to shift to lower frequencies as they propagate further. TR is emitted to the far field as ultrashort wave packets consisting of just a few optical cycles, with an intensity and angle dependence that is determined by the material permittivity. The excitation reaches its peak amplitude within a few femtoseconds and then drops off strongly for longer times. From a correlation between material permittivity and the calculated emission behavior, we determine qualitative predictions of the TR evolution for any given material. The results presented here provide key insights into the mechanisms enabling swift electrons to serve as nanoscale optical excitation sources.

String pair production in non homogeneous backgrounds

We consider string pair production in non homogeneous electric backgrounds. We study several particular configurations which can be addressed with the Euclidean world-sheet instanton technique, the analogue of the world-line instanton for particles. In the first case the string is suspended between two D-branes in flat space-time, in the second case the string lives in AdS and terminates on one D-brane (this realizes the holographic Schwinger effect). In some regions of parameter space the result is well approximated by the known analytical formulas, either the particle pair production in non-homogeneous background or the string pair production in homogeneous background. In other cases we see effects which are intrinsically stringy and related to the non-homogeneity of the background. The pair production is enhanced already for particles in time dependent electric field backgrounds. The string nature enhances this even further. For spacial varying electrical background fields the string pair production is less suppressed than the rate of particle pair production. We discuss in some detail how the critical field is affected by the non-homogeneity, for both time and space dependent electric field backgrouds. We also comment on what could be an interesting new prediction for the small field limit. The third case we consider is pair production in holographic confining backgrounds with homogeneous and non-homogeneous fields.

Photon drag effect in(Bi1−xSbx)2Te3three-dimensional topological insulators

We report on the observation of a terahertz radiation induced photon drag effect in epitaxially grown $n$- and $p$-type (Bi$_{1-x}$Sb$_{x}$)$_{2}$Te$_{3}$ three dimensional topological insulators with different antimony concentrations $x$ varying from 0 to 1. We demonstrate that the excitation with polarized terahertz radiation results in a $dc$ electric photocurrent. While at normal incidence a current arises due to the photogalvanic effect in the surface states, at oblique incidence it is outweighed by the trigonal photon drag effect. The developed microscopic model and theory show that the photon drag photocurrent is due to the dynamical momentum alignment by time and space dependent radiation electric field and implies the radiation induced asymmetric scattering in the electron momentum space.

Microscopic conductivity of lattice fermions at equilibrium. I. Non-interacting particles

We consider free lattice fermions subjected to a static bounded potential and a time- and space-dependent electric field. For any bounded convex region ℛ ⊂ ℝd (d ≥ 1) of space, electric fields E within R drive currents. At leading order, uniformly with respect to the volume R of R and the particular choice of the static potential, the dependency on E of the current is linear and described by a conductivity (tempered, operator-valued) distribution. Because of the positivity of the heat production, the real part of its Fourier transform is a positive measure, named here (microscopic) conductivity measure of R, in accordance with Ohm’s law in Fourier space. This finite measure is the Fourier transform of a time-correlation function of current fluctuations, i.e., the conductivity distribution satisfies Green–Kubo relations. We additionally show that this measure can also be seen as the boundary value of the Laplace–Fourier transform of a so-called quantum current viscosity. The real and imaginary parts of conductivity distributions are related to each other via the Hilbert transform, i.e., they satisfy Kramers–Kronig relations. At leading order, uniformly with respect to parameters, the heat production is the classical work performed by electric fields on the system in presence of currents. The conductivity measure is uniformly bounded with respect to parameters of the system and it is never the trivial measure 0 dν. Therefore, electric fields generally produce heat in such systems. In fact, the conductivity measure defines a quadratic form in the space of Schwartz functions, the Legendre–Fenchel transform of which describes the resistivity of the system. This leads to Joule’s law, i.e., the heat produced by currents is proportional to the resistivity and the square of currents.

Pair production of Dirac particles in a $$d+1$$ d + 1 -dimensional noncommutative space–time

This work addresses the computation of the probability of fermionic particle pair production in \((d+1)\)-dimensional noncommutative Moyal space. Using Seiberg–Witten maps, which establish relations between noncommutative and commutative field variables, up to the first order in the noncommutative parameter \(\theta \), we derive the probability density of vacuum–vacuum pair production of Dirac particles. The cases of constant electromagnetic, alternating time-dependent, and space-dependent electric fields are considered and discussed.

Vacuum decay and the transmission resonances in space-dependent electric fields

We investigate the decay of quantum electrodynamical (QED) vacuum in arbitrary space-dependent electric fields. In particular, we analyze the resonance peaks of the positron emission spectrum for the external fields with subcycle structure. For this, we study the transmission probability in the framework of scattering approach to vacuum pair production. In under-the-barrier scattering regime, we show that the width of a transmission resonance can be enhanced when the effective scattering potential contains multiple wells. Such a broadening in the resonance width corresponds to a decrease in the tunneling time. This may be relevant for observing the vacuum decay at shorter timescales before the external field is adiabatically turned off. In above-the-barrier scattering regime, we give a set of coupled differential equations for the numerical computation of the Bogoliubov coefficients.

Spinless Particles in Exponentially Varying Electric and Magnetic Fields

We investigate the motion of the charged spin-0 particles subjected to the space-dependent electric and magnetic fields. By selecting the external fields oriented parallel and orthogonal to each other, exact solutions of the motion are obtained for the nonrelativistic and the relativistic cases. The quantized energy spectrum is determined by using the solutions obtained for the motion of the particles and dependence of the energy on the strengths of the electric and magnetic fields is discussed. We compared the energy spectrum of the nonrelativistic and the relativistic particles by using the numerical results obtained for the first few quantum levels.

Effective action of QED in electric field backgrounds. II. Spatially localized fields

We find the Bogoliubov coefficient from the tunneling boundary condition on a charged particle coupled to a static electric field $E_0 sech^2 (z/L)$ and, using the regularization scheme in Phys. Rev. D 78, 105013 (2008), obtain the exact one-loop effective action in scalar and spinor QED. It is shown that the effective action satisfies the general relation between the vacuum persistence and the mean number of produced pairs. We advance an approximation method for general electric fields and show the duality between the space-dependent and time-dependent electric fields of the same form at the leading order of the effective actions.

A coupled electro-hydrodynamic numerical modeling of droplet actuation by electrowetting

A numerical study is performed to investigate droplet actuation under electrowetting forces. A volume of fluid, VOF, technique is applied to track the 3D interface. The Laplace equation with proper boundary conditions is solved to predict the time- and space-dependent electric field in the domain. The droplet actuation with respect to the powered electrode, the contact angle distribution at the three-phase line, and the droplet morphology are studied. Additionally, other issues such as droplet velocity, contact angle hysteresis, and droplet location with respect to the powered electrode (switching frequency) are investigated to characterize the droplet behavior between two parallel substrates.

Worldline instantons and pair production in inhomogenous fields

We show how to do semiclassical nonperturbative computations within the worldline approach to quantum field theory using ``worldline instantons.'' These worldline instantons are classical solutions to the Euclidean worldline loop equations of motion, and are closed spacetime loops parametrized by the proper time. Specifically, we compute the imaginary part of the one-loop effective action in scalar QED using ``worldline instantons'' for a wide class of inhomogeneous electric field backgrounds. We treat both time-dependent and space-dependent electric fields and note that temporal inhomogeneities tend to shrink the instanton loops, while spatial inhomogeneities tend to expand them. This corresponds to temporal inhomogeneities tending to enhance local pair production, with spatial inhomogeneities tending to suppress local pair production. We also show how the worldline instanton technique extends to spinor QED.

Nonlinear response of a harmonic diatomic molecule: Algebraic nonperturbative calculation

Even harmonic molecules display a nonlinear behavior when driven by an inhomogeneous field. We calculate the response of single harmonic molecules to a monochromatic time and space dependent electric field $E(r,t)$ of frequency $\ensuremath{\omega}$ employing exact algebraic methods. We evaluate the responses at the fundamental frequency $\ensuremath{\omega}$ and at successive harmonics $2\ensuremath{\omega}$, $3\ensuremath{\omega}$, etc., as a function of the intensity and of the frequency of the field and compare the results with those of first and second order perturbation theory.

Kinetic theory of radio frequency quadrupole ion traps. I. Trapping of atomic ions in a pure atomic gas.

A kinetic theory based on the Boltzmann equation is developed for the trapping of atomic ions in a radio-frequency quadrupole ion trap containing enough neutral atoms that ion-neutral collisions cannot be ignored. The collisions are treated at the same level of sophistication and detail as is used to deal with the time- and space-dependent electric fields in the trap. As a result, microscopic definitions are obtained for the damping and stochastic forces that originate from such collisions. These definitions contrast with corresponding phenomenological terms added ad hoc in previous treatments to create damped Mathieu and Langevin equations, respectively. Furthermore, the theory indicates that either collisional cooling or heating of the ions is possible, depending upon details of the ion-neutral mass ratios and interaction potential. The kinetic theory is not dependent on any special assumptions about the electric field strengths, the ion-neutral interaction potentials, or the ion-neutral mass ratio. It also provides an ab initio way to describe the ion kinetic energies, temperatures, and other properties by a series of successive approximations.