Image Charge Research Articles

Electric double layers with modulated surface charge density: exact 2D results

Electric double layers (EDL) with counterions only, say electrons with the elementary charge −e, in thermal equilibrium at the inverse temperature β are considered. In particular, we study the effect of the surface charge modulation on the particle number density profile and the pressure. The mobile particles are constrained to the surface of a 2D cylinder and immersed in vacuum (no dielectric image charges). An EDL corresponds to the end-circle of the cylinder which carries a (fixed) position-dependent line charge density. The geometries of one single EDL and two EDLs at distance d are considered; the particle density profile is studied for both geometries, the effective interaction of two EDLs is given by the particle pressure on either of the line walls. For any coupling constant Γ ≡ βe 2 = 2 × integer, there exists a mapping of the 2D one-component Coulomb system onto the 1D interacting anticommuting-field theory defined on a chain of sites. Using specific transformation symmetries of anticommuting variables, the contact value theorem is generalized to the EDL with modulated line charge density. For the free-fermion coupling Γ = 2 it is shown that, under certain conditions, the matrix of interaction strengths between anticommuting variables diagonalizes itself which permits one to obtain exact formulas for the particle density profile as well as the pressure. The obtained results confirm the previous indications of weak-coupling and Monte Carlo observations that the surface charge inhomogeneity implies an enhancement of the counterion density at the contact with the charged line and a diminution of the pressure between two parallel lines in comparison with the uniformly charged ones (with the same mean charge densities).

Observation of the Rydberg Resonance in Surface Electrons on Superfluid Helium Confined in a 4-$$\mu$$m Deep Channel

We report the first observation of the microwave-induced intersubband (Rydberg) resonance in the surface-bound electrons on superfluid helium confined in a single 4-$\mu$m deep channel. The resonance signal from a few thousand of surface electrons comprising the Wigner Solid (WS) is detected by observing WS melting due to the microwave absorption. The observed transition frequency for the two lowest Rydberg states, in the range of 0.4-0.5 THz, is determined by the image charges induced by the surface electrons in conducting electrodes of the microchannel and the potentials due to applied voltages, and is in a good agreement with our calculations. The observed large broadening of the resonance on the order of 10 GHz, which is due to inhomegeneous distribution of the pressing electric field in the microchannel, is also in a reasonable agreement with our finite-element modeling calculations. This study of the confined electrons is motivated by their potential use for charge and spin qubits.

Image charge detection of ion bunches using a segmented, cryogenic detector

The response of a dedicated image charge detector to a single passage of small ion bunches was studied. This detector was designed and built in our labs aiming for a maximized signal-to-noise ratio (SNR) with the motivation to enable single ion detection for deterministic ion implantation, a key technique for solid state based quantum technologies, in the future. It is shown how segmentation of the detector with the appropriate combination of the individual segment signal channels significantly increases the SNR. Additionally, the detector is cryogenically cooled to temperatures down to 163 K, further enhancing the SNR. The detection sensitivity of this detector prototype was measured to be 80 elementary charges for SNR=2, detecting 4 keV Xe+ ion bunches. At this SNR, the false-positive error rate is expected to be 0.1%. Comparing the measured sensitivity with a theoretical estimation yielding 22 elementary charges for SNR=2, the presented results lead the way to further optimizations of the detector components and the signal analysis techniques, necessary to realize single ion detection.

Energy barrier for ion field emission from a dielectric liquid sphere.

Ion field emission from the surface of a dielectric liquid is commonly modeled as a kinetic process with an energy barrier lowered by the strength of the electric field. Expressions for the energy barrier exist for simplified cases such as a planar surface and a conducting sphere. This article derives an analytical expression for the more general case of a dielectric sphere, which is the continuum model and geometry for most cases of interest. The energy barrier is computed using the method of image charges, and compared to existing expressions for simpler models. The energy barrier increases at decreasing dielectric constant and decreasing radius of curvature. Ion emission from typical systems (the ideal Taylor cone and charged nanodroplets) differs substantially when calculated with either our model or with existing simpler models.

Effective theory for matter in non-perturbative cavity QED

Starting from a general material system of NN particles coupled to a cavity, we use a coherent-state path integral formulation to produce a effective theory for the material degrees of freedom. We tackle the effects of image charges, the A^2A2 term and a multimode arbitrary-geometry cavity. The resulting (non-local) action has the photonic degrees of freedom replaced by an effective position-dependent interaction between the particles. In the large-NN limit, we discuss how the theory can be cast into an effective Hamiltonian where the cavity induced interactions are made explicit. The theory is applicable, beyond cavity QED, to any system where bulk material is linearly coupled to a diagonalizable bosonic bath. We highlight the differences of the theory with other well-known methods and numerically study its finite-size scaling on the Dicke model. Finally, we showcase its descriptive power with three examples: photon condensation, the 2D free electron gas in a cavity and the modification of magnetic interactions between molecular spins; recovering, condensing and extending some recent results in the literature.

Excitons in nonpolar ZnO/BeZnO quantum wells: Their binding energy and its dependence on the dimensions of the structures

Binding energy of the 1s exciton in the nonpolar ZnO/BexZn1−xO quantum well for x up to 0.3 has been determined by means of fractional dimension approach. Electron and hole wave functions have been computed by solving Schrödinger equation for each particle. The results have been represented as a function of the dimension of the structure. The effects of the optical phonon interactions and the image charges on the binding energy have been considered also. Their contribution leads to a fractional change as much as 0.53 and the binding energy increases up to nearly 140 meV within the considered ranges of the parameters. The effect of the changes in the barrier width has been found negligible, except for the case of very thin quantum wells.

Electrical image potential and solvation energies for an ion in a pore in a metallic electrode or in a nanotube.

Electrical image potentials can be important in small spaces, such as nanoscale pores in porous electrodes, which are used in capacitive desalination and in supercapacitors, as argued by Bazant's group at Massachusetts Institute of Technology. It will be shown here that inside pores in porous metallic materials the image potentials can be considerably larger than near flat walls, as a result of the fact that the dielectric constant for an electric field perpendicular to a wall is much smaller than the bulk dielectric constant of water. Calculations will be presented for the image potential in spherical and cylindrically shaped pores. The calculations for cylindrical pores can also be applied to nanotubes. It was believed for a long time, on the basis of molecular dynamics simulations, that in order to push a salt solution through a small radius nanotube, work must be done against the solvation energy of the ions, which is larger inside a narrow nanotube than it is in the bulk. The relatively large image charge potential energy in narrow nanotubes, however, tends to oppose this increase in the solvation energy. The degree to which the image potential facilitates the flow of the salt ions into nanotubes will be discussed.

Numerical Study of Single Molecular Charge Sensing by FET-Integrated Nanopore Biosensor

This report studies the charge-based sensing modality of FET-embedded nanopore biosensors through FEM simulation. PNP equation is solved to analyze the mirror charge introduced by charged biomolecule while threading through the nanopore-FET sensor. Negative and positive charged molecules are analyzed respectively. Obvious local potential change induced by the presenting of charged molecules nearby is observed. In addition, the transport-induced descreening effect is observed under intensive bias, which might explain the capability of charge sensing even under high concentrations such as 1 M for FET-nanopore biosensors.

Numerical study of effects of electrode parameters and image charge on the electric field configuration of RPCs

The working of a resistive plate chamber depends on the electric field applied inside the gas gap. The strength of this electric field inside the gas gap depends on parameters like electrode thickness, permittivity, and gas gap, among others. The applied electric field can get significantly modified by the space charge effect. This can be a major concern while working in high rate particle physics experiments. Accumulation of space charge generated due to consecutive avalanches can distort the applied field. During this investigation, we have observed that the strength of this perturbation in the electric field is a function of the amount of charge, electrode thickness, permittivity, and gas gap. In this paper, we have studied the dependence of the applied field on parameters linked to RPC fabrication mentioned above. In addition, we have used the method of image to calculate the additional polarisation field due to space charge for a basic five layers of the geometry of an RPC. The variation of the perturbation in the electric field with the same three parameters electrode thickness, permittivity, gas gap has also been discussed.

Study on thermo-optic Mech-Zehnder switches with buried heater by image charge method

In this paper, the analytical solution for temperature change and switching power of the thermo-optic switches with buried heater is achieved by Image Charge Method, and the influence of different structure parameters on the switching power, such as the offset distance away from the heater to the cladding oxide layer surface, the heater width and length, the thickness of cladding and buried oxide layer is studied in detail. Results show that the analytical solution of Image Charge Method is a convenient way of less computational time-consuming to solve temperature field of thermo-optic switches with buried heater, meanwhile, higher heat efficiency and lower switching power can be obtained with narrower, buried heater of the thermo-optic switches, which may provide guides in thermo-optic switches’ designs and their applications.

Development of a 1.4-cell RF photocathode gun for single-shot MeV ultrafast electron diffraction devices with femtosecond resolution

The MeV ultrafast electron diffraction (UED) has proven to be a powerful tool for exploring the ultrafast process at the atomic level with femtosecond resolution. However, MeV UED facilities are struggling to achieve sub-100 fs temporal resolution at pC level charges due to the rapid and severe bunch lengthening near the cathode region. Both analytical work and simulation show that the bunch lengthening near the cathode originates from both the strong space charge effect (SCE) within the sub-relativistic electron beam and the bunch tail trailing due to the mirror charge effect (MCE). In this paper, we report the development of a new 1.4-cell RF gun that significantly increases the extraction field at the cathode to suppress both the SCE and the MCE. Compared to the conventional 1.6-cell RF gun, the new 1.4-cell RF gun is more suitable to produce an electron beam with a shorter bunch length and less distorted longitudinal phase space. The shorter bunch length leads directly to a temporal resolution improvement in UEDs. Meanwhile, the beam longitudinal phase space with better linearity ensures a smaller compressed bunch length when the bunch compression schemes are applied.

Image Charge effect on Metallic Particle Movement in a single phase Gas Insulated Bus Duct with Dielectric Coated Enclosure

Image Charge effect on Metallic Particle Movement in a single phase Gas Insulated Bus Duct with Dielectric Coated Enclosure

HSMA: An O(N) electrostatics package implemented in LAMMPS

We implement two recently developed fast Coulomb solvers, HSMA3D (Zhao et al. (2018) [49]) and HSMA2D (Liang et al. (2020) [50]), into a new user package HSMA for the molecular dynamics simulation engine LAMMPS. The HSMA package is designed for efficient and accurate modeling of electrostatic interactions in 3D and 2D periodic systems with dielectric effects at O(N) cost. The implementation is hybrid MPI and OpenMP parallelized and compatible with existing LAMMPS functionalities. The vectorization technique following AVX512 instructions is adopted for acceleration. To establish the validity of our implementation, we have presented extensive comparisons to the widely used particle-particle particle-mesh (PPPM) algorithm in LAMMPS and other dielectric solvers. With the proper choice of algorithm parameters and parallelization setup, the package enables calculations of electrostatic interactions that is efficient especially for systems with dielectric mismatch. Program summaryProgram Title: HSMACPC Library link to program files:https://doi.org/10.17632/z39zyh573s.1Developer's repository link:https://github.com/LiangJiuyang/HSMA-Harmonic-surface-mapping-algorithm-in-LAMMPSLicensing provisions: GPLv3Programming language: C++Nature of problem: Evaluation of long-range electrostatic interactions for charged system with fully periodic condition or confined by planar dielectric interfaces.Solution method: We implement the Harmonic Surface Mapping algorithm (HSMA), which combines the image-charge method with the harmonic surface mapping and converts the contribution of infinite images into a finite number of surface charges on an auxiliary sphere, into simulation package LAMMPS. The HSMA package works for both fully periodic systems and partially periodic systems with planar dielectric interfaces, achieving truly linear O(N) complexity by employing fast multipole method. Our package can be applied to general all-atom simulations and a broad range of charged complex fluids under dielectric confinement.Additional comments including restrictions and unusual features: Hybrid MPI plus OpenMP parallelization is used in HSMA package for high-performance computing. We provide vector optimization by using the AVX512 instructions and Intel MKL library for further acceleration. The Intel Parallel Studio is required for these techniques.

Waveform conversion and validation of transient magnetic field due to ESD using equivalent circuit of magnetic near‐field probe

AbstractWith a 6 GHz band high resolution magnetic near‐field probe (XF‐R 3‐1) produced by Langer, the transient magnetic fields due to collision ESDs (electrostatic discharges) between metal balls at a charging voltage of 600 V were measured near the spark point to investigate a dipole radiation mechanism. In this study, as an object of considering the measured magnetic field waveforms, two different equivalent circuits of the magnetic near‐field probe are derived based on the probe response waveform observed by a TDR (time domain reflectometer) with a 10 ps rise‐time pulse and the probe reflection coefficient measured with a 26 GHz network analyzer. Waveform conversion formulae from the probe output voltage to the magnetic field are given. The validity of the conversion methods is verified by comparing the measured conversion waveforms and their frequency spectra in addition to the converted spectra by the Langer field correction curve with the calculated waveforms of the transient magnetic far‐fields from a dipole model consisting of image charge pairs and the Rompe‐Weizel spark resistance formula. The probe conversion formulae presented here are valid within the frequency range of the 6 GHz probe band, however, beyond the band the resonance peaks at multiple frequencies over 8 GHz appear on the spectra, which causes damping oscillations peculiar to the probe with multiple frequencies to the time domain waveform.

Remote sensing array (RSA) for linac beam monitoring

The purpose of the present work is to evaluate the feasibility of a novel real-time beam monitoring device for medical linacs which remotely senses charge carriers produced in air by the beam without intersecting and attenuating the beamline. The primary goal is to elaborate a theoretical concept of a possible detector geometry and underlying physical model that allows for determination of clinically relevant beam data in real time, namely MLC leaf positions and dose rate. The detector consists of two opposing electrode arrays arranged in two possible orientations around the beamline. Detection of charge carriers is governed by electromagnetic principles described by Shockley–Ramo theorem. Ions produced by ionization of the air column upstream of patient move laterally in an external electric field. According to the method of images, mirror charges and mirror currents are formed in the strip electrodes. Determination of MU rate and MLC positions using the measured signal requires solution of an inverse problem. In the present work we adopted a Least-Square approach and characterized detector response and sensitivity to detection of beam properties for different electrode geometries and MLC shapes. Results were dependent on MLC field shape and the leaf position within the active volume. The accuracy of determination of leaf positions were in the sub-mm range (up to 0.25–1 mm). Additionally, detector sensitivity was quantified by simulating ions/pulse delivered with a radiation transport deterministic computation in 1D in CEPXS/ONEDANT. For a 6 MV linac pulse, signal amplitude per pulse was estimated to be in the lower pA to fA range. We computationally demonstrated feasibility of the remote sensing detector capable of measuring beam parameters such as MLC leaf positions and dose range for each pulse. Future work should focus on optimizing the electrode geometry to increase sensitivity and better reconstruction algorithms to provide more accurate solutions of the inverse problem.

Dark electron tunneling current in metal–insulator–metal structures: modeling, fabrication, and measurement

Metal–insulator–metal (MIM) structures have been proposed for infrared photodetectors, optical rectennas, and alternative choices to conventional solar cells. Generally, if two dissimilar electrodes are separated by a sufficiently thin insulator, electrons can travel through the insulating gap region due to the quantum tunneling effect. This tunneling through the insulating gap can be driven by an electromagnetic field or an external bias voltage. In our work, we incorporate the contributions of the Poole–Frenkel (PF) effect on the energy barrier of the MIM junction and the effect of Coulomb interaction between the electron charge and its virtual image charges. Using this modified PF-Coulombic barrier, we perform quantum calculations for electron tunneling using the transfer matrix method and Wentzel–Kramers–Brillouin approximations. This, along with the density of states, the Fermi occupation probability, and the quantum confinement effect is used to derive the dark current density for the MIM diode. To test the theory, MIM Au-TiO2-Ti diodes are designed and fabricated in-house using photolithography, along with electron beam evaporation and sputter depositions. The dark current–voltage (I − V) characteristics of the fabricated MIM photodiodes are measured and show good agreement with theoretical predictions.

Diaelectric material as permanent magnet: Frog levitation from magnet

We previously derived direct-current type negative dielectric susceptibility χ, a state which was termed “diaelectric” in previous work. One candidate for a strong diaelectric material was postulated to be a permanent magnet. According to conventional theories, superconductors or room temperature diamagnets can be thought of as fingers or frogs as in the case of frog levitation by magnets and magnetic levitation between fingertips as explained in previous studies. In this study, we regard magnets as anti-electric materials such that this diaelectric phenomenon may satisfy all conditions necessary to repel the external charges outside the magnet. This dielectric state stems from the concept of an image charge in which the opposing charge may be induced by an external charge that is referenced to a centered plane, and the concurrent same polar charge then coexists as a counterpart of the dipole in the image charge portion, thus repelling the external charge. The resulting finite magnetic fields in permanent magnets may stem from non-zero permanent dipole currents.

Information content of the differences in the charge radii of mirror nuclei

Differences in the charge radii of mirror nuclei have been recently suggested to contain information on the slope of the symmetry energy $L$. To test this hypothesis, we perform statistical correlation analysis using quantified energy density functionals that are consistent with our previous knowledge on global nuclear observables such as binding energies and charge radii. We conclude that the difference in charge radii between a mirror pair, $\mathrm{\ensuremath{\Delta}}{R}_{\mathrm{ch}}^{\mathrm{mir}}$, is an inferior isovector indicator compared to other observables, such at the neutron skin or electric dipole polarizability ${\ensuremath{\alpha}}_{\mathrm{D}}$. In particular, this quantity correlates poorly with both the neutron skin and $L$. We demonstrate that $\mathrm{\ensuremath{\Delta}}{R}_{\mathrm{ch}}^{\mathrm{mir}}$ is influenced by pairing correlations in the presence of low-lying proton continuum in the proton-rich mirror-partner nucleus. Considering the large theoretical uncertainties on $\mathrm{\ensuremath{\Delta}}{R}_{\mathrm{ch}}^{\mathrm{mir}}$, we conclude that the precise data on mirror charge radii cannot provide a stringent constraint on $L$.

High-Resolution Detection of Rock-Forming Minerals by Permittivity Measurements with a Near-Field Scanning Microwave Microscope.

The identification of the minerals composing rocks and their dielectric characterization is essential for the utilization of microwave energy in the rock industry. This paper describes the use of a near-field scanning microwave microscope with enhanced sensitivity for non-invasive measurements of permittivity maps of rock specimens at the micrometer scale in non-contact mode. The microwave system comprises a near-field probe, an in-house single-port vectorial reflectometer, and all circuitry and software needed to make a stand-alone, portable instrument. The relationship between the resonance parameters of the near-field probe and the dielectric properties of materials was determined by a combination of classical cavity perturbation theory and an image charge model. The accuracy of this approach was validated by a comparison study with reference materials. The device was employed to determine the permittivity maps of a couple of igneous rock specimens with low-loss and high-loss minerals. The dielectric results were correlated with the minerals comprising the samples and compared with the dielectric results reported in the literature, with excellent agreements.

Absence of induced magnetic monopoles in Maxwellian magnetoelectrics

The electromagnetic response of topological insulators is governed by axion electrodynamics, which features a topological magnetoelectric term in the Maxwell equations. As a consequence magnetic fields become the source of electric fields and vice versa, a phenomenon that is general for any material exhibiting a linear magnetoelectric effect. Axion electrodynamics has been associated with the possibility to create magnetic monopoles, in particular, by an electrical charge that is screened above the surface of a magnetoelectric material. Here we explicitly solve for the electromagnetic fields in this geometry and show that while vortexlike magnetic screening fields are generated by the electrical charge their divergence is identically zero at every point in space, which implies an absence of induced magnetic monopoles. Nevertheless magnetic image charges can be made explicit in the problem, and even if no bound state with electric charges yielding a dyon arises, a dyonlike angular momentum follows from our analysis. Because of its dependence on the dielectric constant this angular momentum is not quantized, which is consistent with a general argument that precludes magnetic monopoles to be generated in Maxwell magnetoelectrics. We also solve for topologically protected zero modes in the Dirac equation induced by the point charge. Since the induced topological defect on the topological insulator's surface carries an electric charge as a result of the axion term, these zero modes are not self-conjugated.