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Statistical characterization of random electrostatic potentials

In this work we study statistical properties of random electrostatic potentials generated by one dimensional lattices with random charges. We show that the resulting random potentials are correlated Gaussian processes, satisfying the Lindeberg version of the central limit theorem, if certain restrictions are imposed on the individual potentials generated by the particles on the lattice. Since most of the point-particle electrostatic potentials occurring in nature satisfy the Lindeberg condition, the correlation properties of the random potentials are not arbitrary and must comply with the central limit theorem. Based on this theorem we can obtain explicit expressions for these correlations. We thus are able to give a characterization of a broad class of potentials yielding feasible physical scenarios. We illustrate some consequences of our findings by considering dynamical properties of a test particle interacting with the lattice. We show how the long range correlations generate statistical features in these properties, which are best exhibited when considering different length scales.

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Observation of an electrostatic force between charged surfaces in liquid crystals.

We report on the atomic force microscope observation of an electrostatic force between glass surfaces immersed in cyanobiphenil liquid crystals. The measured force is repulsive and decays exponentially with increasing surface separation. A mean field description of the electrostatic interaction in liquids has been used to determine the Debye screening length, the concentration of dissolved ions, and the surface electric potential. The effect of the observed interfacial electric field on the liquid crystal orientation at the surface has been discussed. It has been found that the coupling between the liquid crystal order and the surface electric field does not contribute considerably to the surface orienting action.

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Oblique electrostatic modes in self-gravitating dusty plasmas.

The propagation of oblique and perpendicular electrostatic modes in dusty self-gravitational magnetized plasmas is treated with due care for the small gravitational effects between charged dust particles and for the correct balance between electrostatic and gravitational forces. At purely oblique propagation, generalizations of the dust-cyclotron and dust-acoustic modes are found, where the latter can be subject to a Jeans instability. For strictly perpendicular propagation, only a mixed dust-acoustic and dust lower-hybrid mode occur at low frequencies. A Jeans collapse can be avoided by strong enough magnetic fields, the criterion for which is given.

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Nonlinear electrostatic emittance compensation in kA, fs electron bunches.

Nonlinear space-charge effects play an important role in emittance growth in the production of kA electron bunches with a bunch length much smaller than the bunch diameter. We propose a scheme employing the radial third-order component of an electrostatic acceleration field, to fully compensate the nonlinear space-charge effects. This results in minimal transverse root-mean-square emittance. The principle is demonstrated using our design simulations of a device for the production of high-quality, high-current, subpicosecond electron bunches using electrostatic acceleration in a 1 GV/m field. Simulations using the GPT code produce a bunch of 100 pC and 73 fs full width at half maximum pulse width, resulting in a peak current of about 1.2 kA at an energy of 2 MeV. The compensation scheme reduces the root-mean-square emittance by 34% to 0.4pi mm mrad.

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Electrostatic instabilities in current-carrying magnetoplasmas with equilibrium density and ion velocity gradients.

Detailed studies of electrostatic wave instabilities in a current-carrying magnetoplasma with equilibrium density and ion velocity gradients are presented. For this purpose, a general dispersion relation is derived by using a non-Boltzmann electron response as well as an ion density perturbation, which includes the ion-neutral drag. Our dispersion relation contains previously known results as limiting cases, but it also includes some additional instabilities associated with the electron-wave resonant interaction. The present results can help to understand the origin of the nonthermal electrostatic waves in laboratory and space plasmas where there are free energy sources due to ion velocity gradients and streaming particle motions.

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Monte Carlo simulations of polyampholyte-polyelectrolyte complexes: effect of charge sequence and strength of electrostatic interactions.

We present the results of Monte Carlo simulations of complexation between polyampholyte and polyelectrolyte chains. Polymers are modeled as bead-spring chains of charged Lennard-Jones particles each consisting of 32 monomers. Formation of a polyampholyte-polyelectrolyte complex is driven by polarization-induced attractive interactions. The complex is usually formed at the end of the polyelectrolyte with the polyampholyte chain elongated and aligned along the polyelectrolyte backbone. This complex structure between the polarized polyampholyte chain and the polyelectrolyte leads to maximization of the attractive and minimization of the repulsive electrostatic interactions. The size of a polyampholyte in a complex is usually larger than that of an isolated polyampholyte chain. We also observed that initially collapsed polyampholytes undergo a coil-globule transition by forming a complex. The structure of a polyampholyte-polyelectrolyte complex was analyzed by tail and loop distribution functions. We have found that the number of loops increases while their sizes decrease with the strength of the electrostatic interactions. Polyampholytes with random charge sequence form stronger complexes with polyelectrolytes than those with alternating charge sequence. Polyampholytes with long blocky sequences form a double helix with a polyelectrolyte at sufficiently large values of the Bjerrum length.

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Cluster discrimination in electrostatic heteroaggregation processes.

Electrostatic heteroaggregation processes arising in 1:1 mixtures of oppositely charged microspheres at low and very low electrolyte concentrations were investigated by means of single-cluster light scattering. Cluster discrimination, i.e., the fact that clusters differing by only one constituent particle behave quite differently, was found for monomers and dimers. This effect was recently predicted by Brownian dynamic simulations but, to the best of our knowledge, not yet confirmed by experiments. The experimental data were confronted with the simulations and a good qualitative and quantitative agreement was obtained. The origin of the cluster discrimination phenomenon could be related to the range of the attractive electrostatic interactions.

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Nonlinear screening and effective electrostatic interactions in charge-stabilized colloidal suspensions.

A nonlinear response theory is developed and applied to electrostatic interactions between spherical macroions, screened by surrounding microions, in charge-stabilized colloidal suspensions. The theory describes leading-order nonlinear response of the microions (counterions, salt ions) to the electrostatic potential of the macroions and predicts microion-induced effective many-body interactions between macroions. A linear response approximation [A.R. Denton, Phys. Rev. E 62, 3855 (2000)] yields an effective pair potential of screened-Coulomb (Yukawa) form, as well as a one-body volume energy, which contributes to the free energy. Nonlinear response generates effective many-body interactions and essential corrections to both the effective pair potential and the volume energy. By adopting a random-phase approximation (RPA) for the response functions, and thus neglecting microion correlations, practical expressions are derived for the effective pair and triplet potentials and for the volume energy. Nonlinear screening is found to weaken repulsive pair interactions, induce attractive triplet interactions, and modify the volume energy. Numerical results for monovalent microions are in good agreement with available ab initio simulation data and demonstrate that nonlinear effects grow with increasing macroion charge and concentration and with decreasing salt concentration. In the dilute limit of zero macroion concentration, leading-order nonlinear corrections vanish. Finally, it is shown that nonlinear response theory, when combined with the RPA, is formally equivalent to the mean-field Poisson-Boltzmann theory and that the linear response approximation corresponds, within integral-equation theory, to a linearized hypernetted-chain closure.

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Electrostatics of charged dielectric spheres with application to biological systems.

Because electrostatic forces are crucial in biological systems, molecular dynamics simulations of biological systems require a method of computing electrostatic forces that is accurate and rapid. We propose a surface charge method, apply it to a system of arbitrary number of charged dielectric spheres, and obtain an exact solution for an arbitrary configuration of the spheres. The precision depends only on the number of terms kept in a series expansion and can therefore be controlled at will. It appears that the first few terms are usually adequate. The exact result exhibits a phenomenon that we call asymmetric screening. Namely, the magnitude of attractive interactions is decreased (relative to point charges in an infinite solvent) while the magnitude of repulsive interactions is increased (again, relative to point charges in an infinite solvent). This effect might aid in the adoption of correct conformations and in intermolecular recognition. Evaluation of the energy involves only matrix inversion. The surface charge method can be transformed easily to a numerical method for use with arbitrary surfaces. With modest additions, the model also describes an electrorheological fluid. Such a system provides the cleanest opportunity to apply the model.

Biology↗

Time-dependent alignment and orientation of molecules in combined electrostatic and pulsed nonresonant laser fields.

We examine the time evolution of states created by the nonadiabatic interaction of a polar molecule with combined electrostatic and pulsed nonresonant laser fields and show that the orientation due to the electrostatic field alone can be greatly enhanced by restricting the angular amplitude of the molecule by the pulsed laser field. An analytic model indicates that in the short-pulse limit the interaction is governed by an impulsive transfer of action from the radiative field to the molecule.

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Mapping of electrostatic potential in deep submicron CMOS devices by electron holography.

Quantitative two-dimensional maps of electrostatic potential in device structures are obtained using off-axis electron holography with a spatial resolution of 6 nm and a sensitivity of 0.17 V. Estimates of junction depth and variation in electrostatic potential obtained by electron holography, process simulation, and secondary ion mass spectroscopy show close agreement. Measurement artifacts due to sample charging and surface "dead layers" do not need to be considered provided that proper care is taken with sample preparation. The results demonstrate that electron holography could become an effective method for quantitative 2D analysis of dopant diffusion in deep-submicron devices.

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Controlling the orientation of polar molecules with combined electrostatic and pulsed, nonresonant laser fields.

We demonstrate that molecules with a moderate permanent dipole moment can be oriented with combined electrostatic and pulsed, nonresonant laser fields. We use OCS molecules as a sample. The degree of orientation can be increased by increasing the magnitude of electrostatic field and the peak intensity of the laser field or by decreasing the rotational temperature of the molecules.

Carbon Monoxide↗

Molecular dynamics simulations of electrostatic layer-by-layer self-assembly.

Electrostatic assembly of multilayered thin films through sequential adsorption of polyions in layer-by-layer fashion utilizes the strong electrostatic attraction between oppositely charged molecules. We perform molecular dynamics simulations of multilayers of flexible polyelectrolytes around a charged spherical particle. Our simulations establish that the charge reversal after each deposition step is a crucial factor for the steady layer growth. The multilayers appear to be nonequilibrium structures.

Algorithms↗

Novel formulation of nonlocal electrostatics.

The accurate modeling of the dielectric properties of water is crucial for many applications in physics, computational chemistry, and molecular biology. This becomes possible in the framework of nonlocal electrostatics, for which we propose a novel formulation allowing for numerical solutions for the nontrivial molecular geometries arising in the applications mentioned before. Our approach is based on the introduction of a secondary field psi, which acts as the potential for the rotation free part of the dielectric displacement field D. For many relevant models, the dielectric function of the medium can be expressed as the Green's function of a local differential operator. In this case, the resulting coupled Poisson (-Boltzmann) equations for psi and the electrostatic potential phi reduce to a system of coupled partial differential equations. The approach is illustrated by its application to simple geometries.

Biopolymers↗

Dominant electrostatic nature of the reversed field pinch dynamo.

The origin of the dynamo velocity field of the reversed field pinch within the visco-resistive MHD modeling is uncovered. The main component of this field is an electrostatic drift. The corresponding electrostatic field is related to a small charge separation which is consistent with the quasineutrality approximation, and which should be present in real plasmas, too. While quite natural in the stationary single helicity state, this analysis is shown to extend also to the nonstationary multiple helicity regime. Numerical simulations provide the spatial distribution of fields and of charge separation.

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Complex oscillations and chaos in electrostatic microelectromechanical systems under superharmonic excitations.

In this Letter, the formation of complex oscillations of the type 2n M oscillations per period at the Mth superharmonic excitation is reported for electrostatic microelectromechanical systems. A dc bias (beyond "dc symmetry breaking") and an ac signal (at the Mth superharmonic frequency) with an amplitude around "ac symmetry breaking" gives rise to M oscillations per period or period M response. On increasing the ac voltage, a cascade of period doubling bifurcations take place giving rise to 2n M oscillations per period. An interesting chaotic transition (1-band and 2-band chaos) is observed during the first period doubling bifurcation. The nonlinear nature of the electrostatic force is shown to be responsible for the reported observations.

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Inductive and electrostatic acceleration in relativistic jet-plasma interactions.

We report on the observation of rapid particle acceleration in numerical simulations of relativistic jet-plasma interactions and discuss the underlying mechanisms. The dynamics of a charge-neutral, narrow, electron-positron jet propagating through an unmagnetized electron-ion plasma was investigated using a three-dimensional, electromagnetic, particle-in-cell computer code. The interaction excited magnetic filamentation as well as electrostatic plasma instabilities. In some cases, the longitudinal electric fields generated inductively and electrostatically reached the cold plasma-wave-breaking limit, and the longitudinal momentum of about half the positrons increased by 50% with a maximum gain exceeding a factor of 2 during the simulation period. Particle acceleration via these mechanisms occurred when the criteria for Weibel instability were satisfied.

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Electrostatic Changes in Lycopersicon esculentum Root Plasma Membrane Resulting from Salt Stress.

Salinity-induced alterations in tomato (Lypersicon esculentum Mill. cv Heinz 1350) root plasma membrane properties were studied and characterized using a membrane vesicle system. Equivalent rates of MgATP-dependent H(+)-transport activity were measured by quinacrine fluorescence (DeltapH) in plasma membrane vesicles isolated from control or salt-stressed (75 millimolar salt) tomato roots. However, when bis-[3-phenyl-5-oxoisoxazol-4-yl] pentamethine was used to measure MgATP-dependent membrane potential (DeltaPsi) formation, salt-stressed vesicles displayed a 50% greater initial quench rate and a 30% greater steady state quench than control vesicles. This differential probe response suggested a difference in surface properties between control and salt-stressed membranes. Fluorescence titration of vesicles with the surface potential probe, 8-anilino-1-napthalenesulphonic acid (ANS) provided dissociation constants (K(d)) of 120 and 76 micromolar for dye binding to control and salt-stressed vesicles, respectively. Membrane surface potentials (Psi(o)) of-26.0 and -13.7 millivolts were calculated for control and salt-stressed membrane vesicles from the measured K(d) values and the calculated intrinsic affinity constant, K(i). The concentration of cations and anions at the surface of control and salt-stressed membranes was estimated using Psi(o) values and the Boltzmann equation. The observed difference in membrane surface electrostatic properties was consistent with the measured differences in K(+)-stimulated kinetics of ATPase activity between control and salt-stressed vesicles and by the differential ability of Cl(-) ions to stimulate H(+)-transport activity. Salinity-induced changes in plasma membrane electrostatic properties may influence ion transport across the plasma membrane.

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