External Field Strength Research Articles

Surface acoustic wave amplification by drifting electrons in semiconducting epitaxial graphene on silicon carbide

Abstract A theory is presented for the amplification of a surface acoustic wave (SAW) due to its interaction with conduction electrons in gate-controlled epitaxial graphene (epigraphene) on a SiC substrate. It is assumed that the SAW is launched onto the substrate in the direction of an external dc electric field applied to the graphene sample and causing the conduction electrons to drift at a speed greater than the speed of the SAW. The wavelength of the SAW is assumed to be shorter than the mean free path of the electrons, so that the quantum regime of interaction of those electrons with the SAW is realized. The Green’s function method is used to calculate the SAW gain as a function of the electron concentration in epigraphene and the external dc electric field strength. It is shown that the substrate-induced band gap in the electronic spectrum of epigraphene leads to a significant (at least an order of magnitude) increase in the SAW gain as compared to the case of gapless graphene. In addition, the opening of the band gap results in a non-monotonic dependence of the SAW gain on the electron concentration, controlled by the gate voltage applied to the graphene sample. This dependence is characterized by the presence of a distinct maximum at a certain value of electron concentration (of about 2 × 10 12 cm−2 for typical values of the other parameters involved), which distinguishes it from the monotonic concentration dependence of the SAW gain in gapless graphene.

Theoretical investigation of optoelectronic properties in PbS/CdS core/shell spherical quantum dots under the effect of the electric field intensity, hydrogenic impurity and geometric parameters

In this paper, we theoretically investigated the combined effects of the external electric field (EF) strength and the geometric parameters on the linear, nonlinear and total dielectric functions as well as the effective dielectric function (DF) coefficients of PbS/CdS spherical core/shell quantum dots (CSQD) in the presence of the hydrogenic impurity located at the center. The subband energy eigenvalues and their corresponding wave functions are obtained by solving the time-independent Schrödinger equation and using the variational method (VM) in the framework of the effective mass approximation (EMA). The linear, nonlinear and total DF as well as the effective DF were discussed and evaluated under the influence of EF intensity, geometric parameters, the change of the number of quantum dots (QDs) per unit volume and optical intensity I based on the compact density matrix (CDM) approach. The obtained results show that the height peaks of the linear, nonlinear and Total DF as well as the effective DF coefficients increase and their resonant peak moves towards lower energies as the EF and geometric factors increase in both cases with and without considering the hydrogenic impurity effect. Furthermore, our findings show that the impact of optical intensity and the number of QDs per unit volume does not change the resonance peaks of imaginary parts of DF and effective DF but decreases their magnitudes. As a result, we believe that numerical results will present important developments and provide great contributions in designing new optoelectronic devices related to CSQD hetero-nanostructure.

Fundamental difference between the mechanisms of electrostatic field screening in dense and thoroughly collisionless plasmas

When an external electric field appears in a homogeneous plasma, ions move into regions where their electrostatic energy is lower. Simultaneously, forces arise that counteract this effect, causing the plasma to reach equilibrium when the field disappears completely. In collisional plasma, the resulting charge inhomogeneities decrease both Coulomb energy and entropy. Randomly induced diffusion flows tend to hinder their growth, minimizing free energy at any point. Accordingly, in the Debye–Hückel theory, the external field strength decreases exponentially with distance within the plasma. In a collisionless plasma, an antiscreening mechanism operates differently. Each ion moves in a self-consistent field along distinct trajectories, following classical dynamics laws. An external field bends these trajectories, bringing ions into regions where their Coulomb energy is lower. The antiscreening mechanism occurs when ions accelerate into potential wells, increasing the distances between them along their trajectories and decreasing their number densities along these paths. The law of energy conservation for any single ion governs this principally nonlocal process, and the dependence of field strength on distance is not necessarily exponential. This paper demonstrates that the Debye–Hückel theory should not be used to describe the charge density distribution within an unrestricted stream of collisionless plasma, such as the solar wind. It also analyzes non-exponential solutions of the Poisson equation for plasma sheaths above flat surfaces, from which such a flow takes off and on which it falls, obtained in quadratures.

Enhancement of controllable circular actuator using graphene oxide particles

ABSTRACT This work aims to study the responsive expansion of graphene oxide- acrylic elastomer (GO-Acry ER elastomer) under applied electric field. The GO was synthesized from graphite (GP) by using Hummer’s method based on various the ratios of GP to oxidizing agent namely potassium permanganate. GO-Acry ER elastomer was prepared by painted GO as the polarized particle on circular Acry ER as elastomer matrix. The optimum condition of synthesized GO was 1:2 ratios of GP: oxidizing agent which provided a more crystallinity, smaller particle size, and higher pore volume. The circular actuation response of GO-Acry ER elastomer was stimulated by applying external stimuli under various electric field strengths and GO characteristics. The expansion of the GO-Acry ER elastomer increased with an increasing external electric field strength. The GO-Acry ER elastomer under adding GO of 1:2 ratios was the largest expansion with 50% strain under applied electric field of 300 kV/mm. When the electric field strength was > 300 kV/mm, the ER elastomer shrank owing to electrical breakdown. Thus, GO is a suitable choice due to its ease of preparation and fast electro-responsive properties.

Effects of promoter and dedoping process on electrorheological response of polyaniline-graft-chitosan copolymer

In this study, electrorheological (ER) properties of biodegradable and conducting polyaniline-graft-chitosan (PAni-g-CS) copolymer particles were investigated. For this purpose, PAni and PAni-g-CS particles were synthesized by using in situ oxidative radicalic polymerization method. At first, PAni and PAni-g-CS/silicone oil (SO) ER suspensions (15% V/V) were subjected to external electric field and they exhibited low ER activity. When the external electric field strengths ([Formula: see text] were increased, both the suspensions showed electrical breakdown. Therefore, virgin PAni and PAni-g-CS were first subjected to dedoping process by treating with 1.0 M NaOH(aq) and non-ionic surfactant Triton X-100 (T-X) surfactant to enhance the expected ER activity and prevent the electrical breakdown. But we observed that the addition of T-X as promoter had no significant effect on the ER activity. On the other hand, electric filed-induced viscosities of both the suspensions were observed to enhance after the dedoping (DD) process and electrical breakdown prohibited. After the DD process, DD PAni-g-CS/SO ER system exhibited the highest electric field-induced viscosity by reaching 400[Formula: see text]Pa[Formula: see text]⋅[Formula: see text]s at [Formula: see text][Formula: see text]kV/mm. The highest ER efficiency was also obtained for DD PAni-g-CS/SO system at 15% (V/V) as 79. Additionally, typical shear thinning non-Newtonian viscoelastic behavior was observed under externally applied E. The conduction model of DD PAni-g-CS/SO system was determined to well fit the conduction model by showing a slope of [Formula: see text] calculated from the E vb. yield stress graph. In conclusion, conducting and biodegradable-dedoped PAni-g-CS particles would be a good candidate for potential ER applications as dry-based ER materials having high colloidal stability of 76%.

On the selective formation of cubic tetrastack crystals from tetravalent patchy particles.

Achieving the formation of target open crystalline lattices from colloidal particles is of paramount importance for their potential application in photonics. Examples of such desired structures are the diamond, tetrastack, and pyrochlore lattices. Here, we demonstrate that the self-assembly of tetravalent patchy particles results in the selective formation of cubic tetrastack crystals, both in the bulk and in the systems subjected to external fields exerted by the solid substrate. It is demonstrated that the presence of an external field allows for the formation of well-defined single crystals with a low density of defects. Moreover, depending on the strength of the applied external field, the mechanism of epitaxial growth changes. For weakly attractive external fields, the crystallization occurs in a similar manner as in the bulk, since the fluid does not wet the substrate. Nonetheless, the formed crystal is considerably better ordered than the crystals formed in bulk, since the surface induces the ordering in the first layer. On the other hand, it is demonstrated that the formation of well-ordered cubic tetrastack crystals is considerably enhanced by the increase in external field strength, and the formation of the thick crystalline film occurs via a series of layering transitions.

Generation of localized, half-frequency spin waves in micron sized ferromagnetic stripes: Experiments and simulations

Generation of localized, half-frequency spin waves in micron sized ferromagnetic stripes: Experiments and simulations

Infrared Laser Stark Spectroscopy of Methyl Fluoride in 4He Nanodroplets.

We measured the rotationally resolved infrared spectra of helium solvated methyl fluoride at 3 μm and 10 μm, wherein lies C-H and C-F stretching bands, respectively. The linewidths (FWHM) were found to increase with increasing vibrational energy and range from 0.002 cm-1 in the v3 band (C-F stretch) at ~1047 cm-1, to 0.65 cm-1 in the v4 band (asymmetric C-H stretch) at ~2997 cm-1. In between these two bands we observed the lower and upper components of the Fermi triad bands (ν1/2ν2/2ν5) at ~2859 and ~2961 cm-1. We carried out Stark spectroscopy on the lower band on account of its narrower linewidths (0.04 vs. 0.14 cm-1, respectively). The objective of performing Stark spectroscopy was to see if there is any evidence for a rotational linewidth dependence on the external field strength, due to a reduced difference in between methyl fluorides rotational energy gap and the roton-gap of superfluid helium. We did not find any evidence for such an effect, which we largely attribute to the rotational energy gap not increasing significantly enough by the external field. We point to another molecule (formaldehyde) whose energy levels are predicted to show a more promising response to application of an external field.

Magnetic field transport in geometrically thick discs: multidimensional effects on the field strength and inclination angle

ABSTRACT We theoretically investigate the magnetic flux transport in geometrically thick accretion discs which may form around black holes. We utilize a two-dimensional (2D) kinematic mean-field model for poloidal field transport which is governed by both inward advection and outward diffusion of the field. Assuming a steady state, we analytically show that the multidimensional effects prevent the field accumulation towards the centre and reduce the field inclination angle. We also numerically investigate the radial profile of the field strength and the inclination angle for two geometrically thick discs for which (quasi-)analytical solutions exist: radiatively inefficient accretion flows (RIAFs) and super-Eddington accretion flows. We develop a 2D kinematic mean-field code and perform simulations of flux transport to study the multidimensional effects. The numerical simulations are consistent with our analytical prediction. We also discuss a condition for the external field strength that RIAF can be a magnetically arrested disc. This study could be important for understanding the origin of a large-scale magnetic field that drives jets and disc winds around black holes.

Proton Transport, Electroosmotic Drag and Oxygen Permeation in Polytetrafluoroethylene Reinforced Ionomer Membranes and Their Effects on Fuel Cell Performance

Abstract With the increasing need for high power density of proton exchange membrane fuel cells, new composite membranes have been explored for superior proton transport and gas impermeability. These membranes’ physicochemical properties usually deviate from existing empirical formulas, which are poorly understood, especially when mechanical deformation occurs. This poor understanding hinders development of novelty membranes and their fuel cell applications. Here, using polytetrafluoroethylene reinforced ionomer membrane as an example, we conducted extensive water absorption experiments to determine hydration levels at different water activities. Molecular dynamics simulations and electrochemical impedance spectroscopy were used to investigate the impacts of hydration level, external electric field strength, and tensile deformation on proton transport and electroosmotic drag coefficient, as well as the impact of hydration level and free volume ratio on oxygen permeability. We proposed mathematical correlations for these impacts and incorporated them into a single-cell voltage model to analyze their effects on fuel cell performance. Results show that an increase in the electric field strength alters the proton transport pattern, but has minimal impact on the electro-osmosis coefficient. The oxygen permeability coefficient of a deformed membrane with a free volume ratio of 28.57% is more than two orders of magnitude higher than that of a non-deformed membrane. The electro-osmatic drag coefficient imposes a larger influence on fuel cell performance than oxygen permeability

Adsorption of CO, CO2 and NO2 molecules on Li/Pt decorated C-57 nanostructures and effect of applied external electric field: A DFT study

The adsorption behavior of CO2, NO2, and CO gasses on C-57 carbon nanotubes is presently being investigated in this work. Utilizing density functional theory (DFT) computations at the vdW-DF3-OPT2 and PBEsol theoretical levels, the adsorption characteristics of these gases on Li-decorated nanostructures are being investigated. This article examines the NEB (Nudged Elastic Band) technique for gas adsorption on carbon nanotubes. The results of the study can help with the development and improvement of gas adsorption systems based on carbon nanotubes. Furthermore, at various strengths of external electrical fields, the adsorption behavior of CO2 and CO gasses on Pt-decorated C-57 carbon nanosheets is being investigated. The findings provide insights into how the electric field affects the adsorption energy. The objective of this study is to further our knowledge of gas adsorption on C-57 carbon nanostructures and to demonstrate how external electric fields can be used to modify the adsorption properties for a range of energy and environmental applications. We are currently studying the geometries of 1 up to 2 adsorbed gas molecules on the exterior of the nanotube and identifying preferred Li-decorated sites in terms of gas adsorption energy. The charge density difference and electronic band structures are currently being used to assess the adsorption system's characteristics. The charge accumulation between the interacting atoms is being revealed by charge density difference calculations, suggesting the possibility of covalent bond formation. This nanostructure may make a good material for gas adsorption, according to the adsorption energy data (-0.4 to 1.6 (eV)). The efficiency of gas capture and storage systems can be improved by varying the adsorption behavior through the application of an external electrical field.

External electric field driven conformation transition between lithium salts and electride-like molecules: intriguing NLO switches in Li@corannulene.

The external electric field has emerged as a powerful tool for building molecular switches with excellent properties. In this work, we investigate the impact of an external electric field on the transition between lithium salt and electride-like molecule conformations in Li@corannulene. Remarkably, the distance between the Li atom and the corannulene bottom displays a sharp increase under the influence of an external electric field strength of F-z = 110 × 10-4 a.u. As the external electric field strength increases, the Li atom brings about different directions of charge transfer (CT). The natural population analysis (NPA) charge and the molecular electrostatic potential (ESP) results show that the intermolecular CT occurs from the Li atom to the corannulene with the F-z ranging from 0 to 100 × 10-4 a.u. Interestingly, when the external electric field reaches F-z = 110 × 10-4 a.u., the CT is oriented from the corannulene to the Li atom. Moreover, electron localization function (ELF) basins are presented under an F-z of 110 × 10-4 a.u., which indicates that Li@corannulene exhibits electride-like (e-⋯[Li@corannulene]+) molecules and lithiation salt (Li+[corannulene]-) under an F-z of 0 to 100 × 10-4 a.u. Significantly, the differences in charge transfer also contribute to a significant improvement in hyperpolarizabilities (βtot) during the conformation transition from lithiation salt (Li+[corannulene]-) to electride-like (e-⋯[Li@corannulene]+) molecules. This study explores the potential of Li@corannulene as a promising second-order NLO material, and the external electric field provides an efficient strategy for designing and developing NLO switching devices.

Layer-dependent spin texture and origins of Rashba splitting quenching in the 2D CsPbI3 perovskite.

The spin-orbit coupling in inorganic perovskite materials containing heavy elements causes interesting electronic characteristics such as Rashba and Dresselhaus effects. Several studies have reported significant band splitting in the presence of asymmetry, while the impacts of the external field strength, surface termination on the electronic structure still need to be resolved. In the current study, a systematic relation between the external parameters and the band splitting in CsPbI3 slabs is clarified through first-principles calculations. Here, we examine the band splitting and spin patterns of CsPbI3 slabs exposed to an external electric field ranging from zero to hundreds of kV cm-1. Our results indicate apparent non-linear behavior of the Rashba coefficient along with a turning point for the band splitting for a definite external field. Here, the origin of this quench in the band splitting is explained in terms of spatial localization of the wave functions and clear change in their center of charge going from low to high electric fields. The findings not only explain the origin of quenching band splitting but also reveal the external field's significance in the spin texture and recombination rate of 2D perovskites. The current research outcome paves the way for atomic scale engineering of perovskite materials for a wide range of applications.

Rearrangement of the Conformations of Polyampholitic Macromolecules on the Surface of a Charged Spherical Metal Nanoparticle in an Alternating Electric Field: Molecular Dynamic Simulation

The rearrangement of the conformational structure of polyampholytic polypeptides on the surface of a charged spherical gold nanoparticle with its polarity intermittently changing over time was studied using molecular dynamics modeling. The angular distributions of the polypeptide atoms, as well as the radial distributions of the macrochain atomic density in the equatorial region of the nanoparticle with differentiation according to the types of links, were calculated. The polyampholyte shell acquired an annular shape, and the resulting macromolecular ring was located around the charged nanoparticle perpendicularly to the vector of the external electric field strength. With an increase in the charge of the nanoparticle, the ring belt was ordered according to the types of macrochain links, forming concentric annular layers. The diameter of the macromolecular ring depended on the law of distribution of charged units in the macrochain. At elevated temperatures the annular macromolecular ring was deformed at the moments of the highest polarization of the nanoparticle.

Targeted Spin-State Regulation to Boost Oxygen Reduction Reaction.

Catalytic reactions are known to be significantly affected by spin states and their variations during reaction processes, yet the mechanisms behind them remain not fully understood, thus preventing the rational optimization of catalysis. Here, we explore the relationship between the spin states of active sites and their catalytic performance, taking the oxygen reduction reaction as an example. We demonstrate that the catalytic performance is spin-state-dependent and can be improved by adjusting spin states during the catalytic process. To this end, we further investigate the possibility of altering the spin states of transition metals through the application of external fields, such as adsorbed species. By studying the influence of the strength of adsorbed ligands on spin states and its impact on catalytic performance, our results show that optimal catalytic performance is achieved when the strength of the external field is neither too strong nor too weak, forming a volcano-like relationship between the catalytic performance and the external field strength. Our findings can have far-reaching implications for the rational design of high-performance catalysis.

Tensor-conductance model for reducing the computational artifact in target tissue for low-frequency dosimetry

Objective. In protecting human from low-frequency (<100 kHz) exposure, an induced electric field strength is used as a physical quantity for assessment. However, the computational assessment suffers from a staircasing error because of the approximation of curved boundary discretized with cubic voxels. The international guidelines consider an additional reduction factor of 3 when setting the limit of external field strength computed from the permissible induced electric field. Here, a new method was proposed to reduce the staircasing error considering the tensor conductance in human modeling for low-frequency dosimetry. Approach. We proposed a tensor-based conductance model, which was developed on the basis of the filling ratio and the direction of the tissue interface to satisfy the electric field boundary condition and reduce staircasing errors in the target tissue of a voxel human model. Main results. The proposed model was validated using two-layer nonconcentric cylindrical and spherical models with different conductivity contrasts. A comparison of induced electric field strengths with solutions obtained using an analytical formula and finite element method simulation indicated that for a wide range of conductivity ratios, staircasing errors were reduced compared with a conventional scalar-potential finite-difference method. The induced electric field in a simple anatomical head model using our approach was in good agreement with finite element method for exposure to uniform magnetic field exposure and that from coil, simulating transcranial magnetic stimulation. Significance. The proposed tensor-conductance model demonstrated that the staircasing error in an inner target tissue of a voxel human body can be reduced. This finding can be used for the electromagnetic compliance assessment and dose evaluation in electric or magnetic stimulation at low frequencies.

Effects of Oxygen Flow during Fabrication by Magnetron Sputtering on Structure and Performance of Zr-Doped HfO2 Thin Films.

Oxygen defects in Hafnium Oxide (HfO2)-based ferroelectric thin films not only are related to the cause of ferroelectricity but also affect the ferroelectric properties of the thin films. This paper, therefore, focuses on the fabrication of Zr:HfO2 thin films by RF (Radio Frequency) magnetron sputtering with Zr-doped HfO2 as the target and examines how oxygen flow impacts the oxygen vacancies and electrical properties thereof. Additionally, TiN thin-film electrodes were prepared by direct current (DC) magnetron reactive sputtering using nitrogen as the reaction gas, the influences of the substrate temperature on the film deposition rate and crystal phase structure were investigated, and the resultant thin-film electrodes with the lowest resistivity were obtained. Furthermore, the ferroelectric hysteresis loop and leakage current density of metal-insulator-metal (MIM) ferroelectric capacitors formed by annealing the 30 nm thick deposited Zr:HfO2 sandwiched between the top and bottom TiN electrodes were measured. The results demonstrate that varying oxygen flow has a considerable effect on oxygen vacancies and the Zr doping concentration of deposited Zr:HfO2 ferroelectric thin films. When the oxygen flow is set to 40 sccm (standard cubic centimeters per minute) and an external electric field strength of 2 mV/cm is applied, the remnant polarization reaches 18 μC/cm2, with a decrease in the leakage current density of 105-6 orders of magnitude.

Numerical Study of Free-Surface Electro-Hydrodynamic Wave Turbulence

Direct numerical simulation of threedimensional chaotic motion of a dielectric liquid with a free surface under the action of external horizontal electric field is carried out. The numerical model takes into account the effects of surface tension, viscosity, and external isotropic random forcing acting at large scales. A transition from dispersive capillary wave turbulence to quasi-isotropic nondispersive EHD surface turbulence with increase of the external electric field strength is numerically observed for the first time. At the regime of developed EHD wave turbulence, the total electrical energy is found to be much greater than the energy of capillary waves, i.e., electrohydrodynamic effects play a dominant role. At the same time, anisotropic effects are detected that lead to the generation of capillary wave packets traveling perpendicular to the external field direction. Despite the revealed anisotropy, the calculated spectrum of EHD wave turbulence is in very good agreement with the analytical spectrum obtained on the basis of dimensional analysis of weak turbulence spectra.

Electric Field‐Induced Phase Transition on HPX6 (X = C, Si, Ge, Sn) Monolayers

Herein, the electronic, thermoelectric, and optical properties of semimetallic HPX6 (X = C, Si, Ge, Sn) monolayers are systematically studied under the influence of external electric field in the framework of density functional theory. A band tuning has been achieved in these structures by the application of an external electric field of appropriate strength. It is predicted that Dirac cone splitting is nearly proportional to the external electric field strength. The modulation of electric properties induced by external field can alter the position of chemical potential in the band diagram and brings significant improvement in thermoelectric responses. The application of an external electric field significantly modulates the optical properties. The electric field‐induced HPX6 system provides better thermoelectric and optical response for nanodevice applications.

Mean Field Spin Glass Models Under Weak External Field

We study the fluctuation and limiting distribution of free energy in mean-field spin glass models with Ising spins under weak external fields. We prove that at high temperature, there are three sub-regimes concerning the strength of external field $h \approx \rho N^{-\alpha}$ with $\rho,\alpha\in (0,\infty)$. In the super-critical regime $\alpha < 1/4$, the variance of the log-partition function is $\approx N^{1-4\alpha}$. In the critical regime $\alpha = 1/4$, the fluctuation is of constant order but depends on $\rho$. Whereas, in the sub-critical regime $\alpha>1/4$, the variance is $\Theta(1)$ and does not depend on $\rho$. We explicitly express the asymptotic mean and variance in all three regimes and prove Gaussian central limit theorems. Our proofs mainly follow two approaches. One utilizes quadratic coupling and Guerra's interpolation scheme for Gaussian disorder, extending to many other spin glass models. However, this approach can prove the CLT only at very high temperatures. The other one is a cluster-based approach for general symmetric disorders, first used in the seminal work of Aizenman, Lebowitz, and Ruelle~(Comm.~Math.~Phys.~112 (1987), no.~1, 3--20) for the zero external field case. It was believed that this approach does not work if the external field is present. We show that if the external field is present but not too strong, it still works with a new cluster structure. In particular, we prove the CLT up to the critical temperature in the Sherrington-Kirkpatrick (SK) model when $\alpha \ge 1/4$. We further address the generality of this cluster-based approach. Specifically, we give similar results for the multi-species SK model and diluted SK model.