Strong Dc Electric Field Research Articles

Spatially dispersive dynamical response of hot carriers in doped graphene

We study theoretically wave-vector and frequency dispersion of the complex dynamic conductivity tensor (DCT), σlm(k,ω), of doped monolayer graphene under a strong dc electric field. For a general analysis, we consider the weak ac field of arbitrary configuration given by two independent vectors, the ac field polarization and the wave vector k. The high-field transport and linear response to the ac field are described on the base of the Boltzmann kinetic equation. We show that the real part of DCT, calculated in the collisionless regime, is not zero due to dissipation of the ac wave, whose energy is absorbed by the resonant Dirac quasiparticles effectively interacting with the wave. The role of the kinematic resonance at ω=vF|k| (vF is the Fermi velocity) is studied in detail taking into account deviation from the linear energy spectrum and screening by the charge carriers. The isopower-density curves and distributions of angle between the ac current density and field vectors are presented as a map which provides clear graphic representation of the DCT anisotropy. Also, the map shows certain ac field configurations corresponding to a negative power density, thereby it indicates regions of terahertz frequency for possible electrical (drift) instability in the graphene system.

Resonance parameter calculations for molecular orbitals of H2O exposed to strong DC electric fields

SynopsisMolecular orbitals for the water molecule obtained from a potential model are represented on a three- dimensional Cartesian mesh. The Schrödinger equation describing the molecule exposed to a strong DC electric field is solved to obtain the resonance parameters (position and line width, i.e., lifetime) as a function of field orientation and for field strengths for which the outer classical turning point is within the computational grid.

Birefringence Analysis of the Effect of Electric Fields on the Order–Disorder Transition Temperature of Lamellae Forming Block Copolymers

A detailed birefringence analysis of the effect of strong dc electric fields on the order–disorder transition temperature (TODT) of lamella forming block copolymers is reported. The setup presented here enabled the measurement of the TODT with high temperature resolution while the birefringence measurements were nondestructive and straightforward compared to alternative methods. A downward shift in the transition temperature was found for all samples upon application of the electric field. The data indicate that the dominating parameter that evokes the mixing of block copolymers when exposed to electric fields is the difference in dielectric permittivity Δϵ between the block copolymer constituents. The extent to which the TODT is shifted is furthermore influenced by the degree of polymerization N. Shifts in the transition temperature of up to 7 °C were found upon application of an electric field of 5 kV/mm.

Field-induced polaron formation in the Holstein-Hubbard model

We study the effect of strong DC and pulsed electric fields on a Mott insulating system with coupling to optical phonons. A DC field of the order of the gap induces a metallic state characterized by polaronic features in the gap region and a partially inverted population. In this quasi-steady state, the field-induced doublon-hole production is balanced by a phonon-enhanced doublon-hole recombination. The photo-excitation of carriers by a pulsed field leads to similar modifications of the electronic structure in the gap region, and an associated reduction of the doublon life-time. We demonstrate that the field-induced localization of electrons effectively enhances the phonon coupling, an effect which should be measureable with time-resolved photo-emission spectroscopy.

Macroscopic and Nanoscopic Polarization Relaxation Kinetics in Lead‐Free Relaxors Bi1/2Na1/2TiO3–Bi1/2K1/2TiO3–BiZn1/2Ti1/2O3

The stability of the field‐induced ferroelectric (FE) state was studied in relaxor lead‐free ceramics (1 − y)[0.81Bi1/2Na1/2TiO3–0.19Bi1/2K1/2TiO3]–yBiZn1/2Ti1/2O3 both macroscopically and microscopically. A strong dc electric field results in the formation of a stable FE state with a large piezoelectric coefficient for compositions with a small amount of Bi(Zn1/2Ti1/2)O3, which are in the non‐ergodic relaxor state at room temperature. Increasing temperature promotes ergodic relaxor behavior, which is accompanied by the rapid destabilization of the induced state, that is, small relaxation times. Based on the obtained data, it is proposed that the depolarization is a two‐step process consisting of an initial realignment of the FE domains and their subsequent breakup into polar nanoregions. The ergodic relaxor behavior is also promoted by increasing the Bi(Zn1/2Ti1/2)O3 content. The related charge disorder results in an enhancement of random electric fields and consequently a stable FE state cannot be induced even at room temperature.

Exact solution for Bloch oscillations of a simple charge-density-wave insulator

Charge-density-wave systems have a static modulation of the electronic charge at low temperatures when they enter an ordered state. While they have been studied for decades in equilibrium, it is only recently that they have been examined in nonequilibrium with time-resolved studies. Here, we present the exact solution for the nonequilibrium response of electrons (in the simplest model for a charge density wave) when the system is placed under a strong DC electric field. This allows us to examine the formation of driven Bloch oscillations and how the presence of a current modifies the nonequilibrium density of states.

Photoelectron holography in strong optical and dc electric fields

The application of velocity map imaging for the detection of photoelectrons resulting from atomic or molecular ionization allows the observation of interferometric, and in some cases holographic structures that contain detailed information on the target from which the photoelecrons are extracted. In this contribution we present three recent examples of the use of photoelectron velocity map imaging in experiments where atoms are exposed to strong optical and dc electric fields. We discuss (i) observations of the nodal structure of Stark states of hydrogen measured in a dc electric field, (ii) mid-infrared strong-field ionization of metastable Xe atoms and (iii) the reconstruction of helium electronic wavepackets in an attosecond pump-probe experiment. In each case, the interference between direct and indirect electron pathways, reminiscent of the reference and signal waves in holography, is seen to play an important role.

Excitons in a quasi-one-dimensional quantum nanorod under a strong electric field

The response of an exciton in the ground and first excited states to a strong DC electric field is studied in a quasi-one-dimensional nano quantum well (i.e., nanorod) bounded by high symmetric barriers by studying the energy, the oscillator strength, the root-mean-square (RMS) average of the electron-hole (e-h) separation, and the average positions of the electron and the hole. The interplaying effect between the barrier confinement, e-h attraction, and the field-induced e-h separation for exciton binding is examined. We find that, for a long nanorod, the exciton energy, as well as, the oscillator strength drops abruptly as a function of the field near the exciton-dissociation field while the RMS average of the e-h separation rises rapidly. For shorter rods, the transition is more gradual due to the combined effect of the confinement and the long-range e-h interaction. A strong field is shown to transform the optically-inactive first excited state into an optically-active state in the field range between the dissociation field of the ground state and that of the first excited level. We also find that, in the ground state, the (lighter) electron is dragged by the (heavier) hole below the dissociation field. The dependence of the above mentioned properties on the rod length is also investigated for varying fields. The results are compared with those obtained for the rods with parabolic confinement.

Charge mobility and current-induced spin polarization in nonlinear surface transport of a topological insulator with hexagonal warping

The nonlinear charge mobility and current-induced spin polarization in the nonlinear surface transport of a topological insulator with a hexagonal deforming band are investigated in the presence of a strong dc electric field by means of a balance-equation approach for a single energy band. The nonlinear mobility exhibits a broad peak with respect to increasing electric field in warping systems. The in-plane perpendicular spin polarization is also sensitive to the electric field, warping constant, surface density, and temperature. An out-of-plane spin polarization, the magnitude of which is strongly carrier-density dependent, emerges in systems with warping bands at high electric fields, and drops quickly with increasing temperature.

Mixed-field orientation of molecules without rotational symmetry

The mixed-field orientation of an asymmetric-rotor molecule with its permanent dipole moment nonparallel to the principal axes of polarizability is investigated experimentally and theoretically. We find that for the typical case of a strong, nonresonant laser field and a weak static electric field complete 3D orientation is induced if the laser field is elliptically polarized and if its major and minor polarization axes are not parallel to the static field. For a linearly polarized laser field solely the dipole moment component along the most polarizable axis of the molecule is relevant resulting in 1D orientation even when the laser polarization and the static field are nonparallel. Simulations show that the dipole moment component perpendicular to the most-polarizable axis becomes relevant in a strong dc electric field combined with the laser field. This offers an alternative approach to 3D orientation by combining a linearly polarized laser field and a strong dc electric field arranged at an angle equal to the angle between the most polarizable axis of the molecule and its permanent dipole moment.

CMIstark: Python package for the Stark-effect calculation and symmetry classification of linear, symmetric and asymmetric top wavefunctions in dc electric fields

The Controlled Molecule Imaging group (CMI) at the Center for Free Electron Laser Science (CFEL) has developed the CMIstark software to calculate, view, and analyze the energy levels of adiabatic Stark energy curves of linear, symmetric top and asymmetric top molecules. The program exploits the symmetry of the Hamiltonian to generate fully labeled adiabatic Stark energy curves.CMIstark is written in Python and easily extendable, while the core numerical calculations make use of machine optimized BLAS and LAPACK routines. Calculated energies are stored in HDF5 files for convenient access and programs to extract ASCII data or to generate graphical plots are provided. Program summaryProgram title: CMIstarkCatalogue identifier: AEQS_v1_0Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AEQS_v1_0.htmlProgram obtainable from: CPC Program Library, Queen’s University, Belfast, N. IrelandLicensing provisions: GNU General Public License version 3 or later with amendments. See code for details.No. of lines in distributed program, including test data, etc.: 4091No. of bytes in distributed program, including test data, etc.: 62805Distribution format: tar.gzProgramming language: Python (version 2.6.x, 2.7.x).Computer: Any Macintosh, PC, or Linux/UNIX workstations with a modern Python distribution.Operating system: Tested on Mac OS X and a variety of Linux distributions.RAM: 2 GB for typical calculationsClassification: 16.1.External routines:Python packages numpy and scipy; utilizes (optimized) LAPACK and BLAS through scipy. All packages available under open-source licenses.Nature of problem:Calculation of the Stark effect of asymmetric top molecules in arbitrarily strong dc electric fields in a correct symmetry classification and using correct labeling of the adiabatic Stark curves.Solution method:We set up the full M matrices of the quantum-mechanical Hamiltonian in the basis set of symmetric top wavefunctions and, subsequently, Wang transform the Hamiltonian matrix. We separate, as far as possible, the sub-matrices according to the remaining symmetry, and then diagonalize the individual blocks. This application of the symmetry consideration to the Hamiltonian allows an adiabatic correlation of the asymmetric top eigenstates in the dc electric field to the field-free eigenstates. This directly yields correct adiabatic state labels and, correspondingly, adiabatic Stark energy curves.Restrictions:The maximum value of J is limited by the available main memory. A modern desktop computer with 16 GB of main memory allows for calculations including all Js up to a values larger than 100 even for the most complex cases of asymmetric tops.Running time:Typically 1 s–1 week on a single CPU or equivalent on multi-CPU systems (depending greatly on system size and RAM); parallelization through BLAS/LAPACK. For instance, calculating all energies up to J=25 of indole (vide infra) for one field strength takes 1 CPU-s on a current iMac.

The role of inelastic processes in the temperature dependence of hall induced resistance oscillations

We develop a model of magnetoresistance oscillations induced by the Hall field in order to study the temperature dependence observed in recent experiments in two dimensional electron systems. The model is based on the solution of the von Neumann equation incorporating the exact dynamics of two-dimensional damped electrons in the presence of arbitrarily strong magnetic and dc electric fields, while the effects of randomly distributed neutral and charged impurities are perturbatively added. Both the effects of elastic impurity scattering as well as those related to inelastic processes play an important role. The theoretical predictions correctly reproduce the experimentally observed oscillations amplitude, provided that the quantum inelastic scattering rate obeys a T2 temperature dependence, consistent with electron–electron interaction effects.

Electrohydrodynamic interaction of spherical particles under Quincke rotation

Weakly conducting dielectric particles suspended in a dielectric liquid of higher conductivity can undergo a transition to spontaneous sustained rotation when placed in a sufficiently strong dc electric field. This phenomenon of Quincke rotation has interesting implications for the rheology of these suspensions, whose effective viscosity can be controlled and reduced by application of an external field. While previous models based on the rotation of isolated particles have provided accurate estimates for this viscosity reduction in dilute suspensions, discrepancies have been reported in more concentrated systems where particle-particle interactions are likely significant. Motivated by this observation, we extend the classic description of Quincke rotation based on the Taylor-Melcher leaky dielectric model to account for pair electrohydrodynamic interactions between two identical spheres using the method of reflections. A coupled system of evolution equations for the dipole moments and angular velocities of the spheres is derived that accounts for electric dipole-dipole interactions and hydrodynamic rotlet interactions up to order O(R(-5)), where R is the separation distance between the spheres. A linear stability analysis of this system shows that interactions modify the value of the critical electric field for the onset of Quincke rotation: both electric and hydrodynamic interactions can either stabilize or destabilize the system depending on the orientation of the spheres, but the leading effect of interactions on the onset of rotation is hydrodynamic. We also analyze the dynamics in the nonlinear regime by performing numerical simulations of the governing equations. In the case of a pair of spheres that are fixed in space, we find that particle rotations always synchronize in magnitude at long times, though the directions of rotation of the spheres need not be the same. The steady-state angular velocity magnitude depends on the configuration of the spheres and electric field strength and agrees very well with an asymptotic estimate derived for corotating spheres. In the case of freely-suspended spheres, dipolar interactions are observed to lead to a number of distinct behaviors depending on the initial relative configuration of the spheres and on any infinitesimal initial perturbation introduced in the system: in some cases the spheres slowly separate in space while steadily rotating, while in other cases they pair up and either corotate or counterrotate depending on their orientation relative to the field.

Dielectric breakdown of Mott insulators – doublon production and doublon heating

Using dynamical mean-field theory and the non-crossing approximation as impurity solver, we study the response of a Mott insulator to strong dc electric fields. The breakdown of the Mott insulating state is triggered by field-induced creation of doublon-hole pairs. In a previous investigation, Ref. [1], it was found that the system approaches a long-lived quasi-steady state in which the current is time-independent although the number of carriers constantly increases. Here we investigate and clarify the nature of this state, which exists only because thermalization is slow in the Hubbard model at strong coupling. The current is time-independent because doublons and holes have an infinite temperature distribution. Evidence for this fact is obtained from spectral functions and by comparing the electric current with the field-induced doublon-hole creation rate. Implications to real experiments, in systems with energy dissipation, are discussed.

Structure and dielectric properties of niobium-rich potassium lithium tantalate niobate single crystals

Abstract The structure and lattice parameters of high quality niobium-rich potassium lithium tantalate niobate (K 0.95 Li 0.05 )(Ta 1− x Nb x )O 3 ( x =0.44–0.66) single crystals were determined by the X-ray powder diffraction technique. Dielectric properties were studied in the temperature range from 20 to 500 K. The transition temperatures were determined, and the Curie temperatures adjusted between 330 K and 480 K through the suitable selection of Ta-to-Nb ratio. The degree of relaxation ( γ ) has been estimated using the modified Curie–Weiss law. The (K 0.95 Li 0.05 )(Ta 1− x Nb x )O 3 single crystals have also shown strong dc electric field dependent dielectric behavior, and high dielectric tunability (∼43%) for x =0.44.

Electric field induced needle-pulsed arc discharge carbon nanotube production apparatus: Circuitry and mechanical design

A simple and low cost apparatus is reported to produce multiwall carbon nanotubes and carbon nano-onions by a low power short pulsed arc discharge reactor. The electric circuitry and the mechanical design details and a micro-filtering assembly are described. The pulsed-plasma is generated and applied between two graphite electrodes. The pulse width is 0.3 μs. A strong dc electric field is established along side the electrodes. The repetitive discharges occur in less than 1 mm distance between a sharp tip graphite rod as anode, and a tubular graphite as cathode. A hydrocarbon vapor, as carbon source, is introduced through the graphite nozzle in the cathode assembly. The pressure of the chamber is controlled by a vacuum pump. A magnetic field, perpendicular to the plasma path, is provided. The results show that the synergetic use of a pulsed-current and a dc power supply enables us to synthesize carbon nanoparticles with short pulsed plasma. The simplicity and inexpensiveness of this plan is noticeable. Pulsed nature of plasma provides some extra degrees of freedom that make the production more controllable. Effects of some design parameters such as electric field, pulse frequency, and cathode shape are discussed. The products are examined using scanning probe microscopy techniques.

Ab initiostudy of high-lying doubly excited states of helium in static electric fields: Complex-scaling generalized pseudospectral method in hyperspherical coordinates

We present a complex-scaling (CS) generalized pseudospectral (GPS) method in hyperspherical coordinates (HSC) for $\mathit{ab}$ initio and accurate treatment of the resonance energies and autoionization widths of two-electron atomic systems in the presence of a strong dc electric field. The GPS method allows nonuniform and optimal spatial discretization of the two-electron Hamiltonian in HSC with the use of only a modest number of grid points. The procedure is applied for the first precision calculation of the energies and autoionization widths for the high-lying ${}^{1}\phantom{\rule{-0.16em}{0ex}}{S}^{e}$, ${}^{1}\phantom{\rule{-0.16em}{0ex}}{P}^{o}$, ${}^{1}\phantom{\rule{-0.16em}{0ex}}{D}^{e}$, and ${}^{1}\phantom{\rule{-0.16em}{0ex}}{F}^{o}$ ($n=10$--20) doubly excited resonance states of He atoms. In addition, we present a theoretical prediction of the energies and widths of high-lying doubly excited resonance states of ${}^{1}\phantom{\rule{-0.16em}{0ex}}{P}^{o}$ ($n=$ 8--15) in external dc electric field strengths of 3.915--10.44 kV/cm. The calculated dc-field perturbed high-lying resonance energies are in good agreement with the latest experimental data.

Guided self-assembly of nanostructured titanium oxide

A series of nanostructured titanium oxide particles were synthesized by a simple wet chemical method and characterized by means of small-angle x-ray scattering (SAXS)/wide-angle x-ray scattering (WAXS), atomic force microscope (AFM), scanning electron microscope (SEM), transmission electron microscope (TEM), thermal analysis, and rheometry. Tetrabutyl titanate (TBT) and ethylene glycol (EG) can be combined to form either TiOx nanowires or smooth nanorods, and the molar ratio of TBT:EG determines which of these is obtained. Therefore, TiOx nanorods with a highly rough surface can be obtained by hydrolysis of TBT with the addition of cetyl-trimethyl-ammonium bromide (CTAB) as surfactant in an EG solution. Furthermore, TiOx nanorods with two sharp ends can be obtained by hydrolysis of TBT with the addition of salt (LiCl) in an EG solution. The AFM results show that the TiOx nanorods with rough surfaces are formed by the self-assembly of TiOx nanospheres. The electrorheological (ER) effect was investigated using a suspension of titanium oxide nanowires or nanorods dispersed in silicone oil. Oil suspensions of titanium oxide nanowires or nanorods exhibit a dramatic reorganization when submitted to a strong DC electric field and the particles aggregate to form chain-like structures along the direction of applied electric field. Two-dimensional SAXS images from chains of anisotropically shaped particles exhibit a marked asymmetry in the SAXS patterns, reflecting the preferential self-assembly of the particles in the field. The suspension of rough TiOx nanorods shows stronger ER properties than that of the other nanostructured TiOx particles. We find that the particle surface roughness plays an important role in modification of the dielectric properties and in the enhancement of the ER effect.

Nonlinear transport in a two-dimensional electron gas with a periodically modulated potential

We study the nonlinear response of the current of a two-dimensional electron gas with a periodically modulated potential in one direction in a strong DC electric field at low temperatures. The dependence of the current on the field, the electron density, and the temperature is investigated by using a relaxation-time approximation for inelastic scattering. Elastic scattering is treated microscopically including interband scattering. The roles of elastic and inelastic scattering on the nonlinear current are examined. The result is applied to the Kronig-Penney model. It is found that, for a fixed total scattering rate, the field dependence of the current is insensitive to the ratio of the elastic and inelastic scattering rates in contrast with the recent results of the nonlinear current of similar models of a single miniband in one dimension and three dimensions. The result from the Kronig-Penney model is compared with the recent data from a modulated two-dimensional GaAs quantum well which show evidence of the negative differential conductance.

Lane formation in confined cation mixtures in strong DC electric fields

On applying an external electric potential field at the ends of a flexible nanopore, the resulting onward current flow parallel to the field has been simulated with molecular dynamic simulation. This is suitable to study different effects of self-organisation, such as lane formation. The electrolyte model was composed of three Na+/K+ (30/70, 50/50 and 70/30) concentration ratios in aqueous solutions.