Dipole Potential Research Articles

Molecular theory of the static dielectric constant of dipolar fluids.

The link between the static dielectric constant and the microscopic intermolecular interactions is the Kirkwood g1 factor, which depends on the orientational structure of the fluid. Over the years, there have been several attempts to provide an accurate description of the orientational structure of dipolar fluids using molecular theories. However, these approaches were either limited to mean-field approximations for the pair correlation function or, more recently, limited to adjusting the orientational dependence to simulation data. Here, we derive a theory for the dielectric constant of dipolar hard-sphere fluids using the augmented modified mean-field approximation. Qualitative agreement is achieved throughout all relevant thermodynamic states, as demonstrated by a comparison with simulation data from the literature. Excellent quantitative agreement can be obtained using a single empirical scaling factor, the physical origin of which is analyzed and accounted for. In order to predict the dielectric constant of the Stockmayer fluid (Lennard-Jones plus dipole potential), we use an adjusted version of the expression for the dipolar hard-sphere fluid. Comparing theoretical predictions with newly generated simulation data, we show that it is possible to obtain excellent agreement with simulation by performing the calculations at a corresponding state using the same scaling factor. Finally, we compare the theoretical orientational structure of the Stockmayer fluid with that obtained from simulations. The simulated structure is calculated following a post-processing methodology that we introduce by deriving an original expression that relates the proposed theory to the histogram of relative dipole angles.

Long-Range Influence of a Pump on a Critical Fluid.

A pump coupled to a conserved density generates long-range modulations, resulting from the non-equilibrium nature of the dynamics. We study how these modulations are modified at the critical point where the system exhibits intrinsic long-range correlations. To do so, we consider a pump in a diffusive fluid, which is known to generate a density profile in the form of an electric dipole potential and a current in the form of a dipolar field above the critical point. We demonstrate that while the current retains its form at the critical point, the density profile changes drastically. At criticality, in d<4 dimensions, the deviation of the density from the average is given by sgn[cos(θ)]|cos(θ)/r^{(d-1)}|^{1/δ} at large distance r from the pump and angle θ with respect to the pump's orientation. At short distances, there is a crossover to a cos(θ)/r^{d-3+η} profile. Here δ and η are Ising critical exponents. The effect of the local pump on the domain wall structure below the critical point is also considered.

Mixtures of Dipolar Gases in Two Dimensions: A Quantum Monte Carlo Study

We studied the miscibility of two dipolar quantum gases in the limit of zero temperature. The system under study is composed of a mixture of two Bose gases with dominant dipolar interaction in a two-dimensional harmonic confinement. The dipolar moments are all considered to be perpendicular to the plane, turning the dipolar potential in a purely repulsive and isotropic model. Our analysis is carried out by using the diffusion Monte Carlo method, which allows for an exact solution to the many-body problem within some statistical noise. Our results show that the miscibility between the two species is rather constrained as a function of the relative dipolar moments and masses of the two components. A narrow regime is predicted where both species mix and we introduce an adimensional parameter whose value quite accurately predicts the miscibility of the two dipolar gases.

Mechanical strength and band alignment of BAs/GaN heterojunction polar interfaces: A first-principles calculation study

Recently, GaN-on-BAs has been synthesized and demonstrated as a promising architecture for efficient thermal management in GaN based high-power electronic devices with remarkably reduced thermal boundary resistance compared to that of GaN-on-diamond. In this paper, we report studies on ideal strengths and band alignments for polar BAs/GaN heterojunctions and superlattices using first-principles calculations. The results show that under normal compression, all BAs/GaN interface configurations show much higher compressive stiffness compared to that in bulk GaN [0001] direction, with the GaN softening during its structural transformation under compression markedly suppressed, which improves protection of the electronic properties under external impacts. The natural band alignments of the mismatched BAs/GaN heterojunctions are calculated by a three-step approach. Most of the heterojunction and all the superlattice interfaces show type-II staggered band offsets. Large polarization built-in electric fields are predicated in superlattice with repeatedly positive- and negative-charged N-As and Ga-B interfaces, producing a saw-tooth like dipole potential, which can effectively separate electrons and holes to different interfaces, desirable for the photocatalytic processes. Our research shows that BAs/GaN heterojunction can not only provide a much-needed alternative to GaN-on-diamond heat dissipation system in future designing of high-power electronic devices, but also offers possibilities of applications in photovoltaic and photocatalytic devices as a type-II semiconductor heterojunction or superlattice.

Unified ab initio description of Fröhlich electron-phonon interactions in two-dimensional and three-dimensional materials

\textit{Ab initio} calculations of electron-phonon interactions including the polar Fr\"ohlich coupling have advanced considerably in recent years. The Fr\"ohlich electron-phonon matrix element is by now well understood in the case of bulk three-dimensional (3D) materials. In the case of two-dimensional (2D) materials, the standard procedure to include Fr\"ohlich coupling is to employ Coulomb truncation, so as to eliminate artificial interactions between periodic images of the 2D layer. While these techniques are well established, the transition of the Fr\"ohlich coupling from three to two dimensions has not been investigated. Furthermore, it remains unclear what error one makes when describing 2D systems using the standard bulk formalism in a periodic supercell geometry. Here, we generalize previous work on the \textit{ab initio} Fr\"ohlich electron-phonon matrix element in bulk materials by investigating the electrostatic potential of atomic dipoles in a periodic supercell consisting of a 2D material and a continuum dielectric slab. We obtain a unified expression for the matrix element, which reduces to the existing formulas for 3D and 2D systems when the interlayer separation tends to zero or infinity, respectively. This new expression enables an accurate description of the Fr\"ohlich matrix element in 2D systems without resorting to Coulomb truncation. We validate our approach by direct \textit{ab initio} density-functional perturbation theory calculations for monolayer BN and MoS$_2$, and we provide a simple expression for the 2D Fr\"ohlich matrix element that can be used in model Hamiltonian approaches. The formalism outlined in this work may find applications in calculations of polarons, quasiparticle renormalization, transport coefficients, and superconductivity, in 2D and quasi-2D materials.

Interface transition from Ohmic to Schottky contact in Ti3X2/MoS2 (X= B, C, N): Insights from first-principles

Ti3C2 MXene has received tremendous interest as a potential electrode in flexible electronics due to its promising electronic and mechanical properties. However, its inherent properties determine the interfacial performance and the interfacial properties with MoS2 are critical to nanoelectronics device. Based on first-principles calculations, the mechanical properties of Ti3X2 (X= B, C or N) and interfacial properties of Ti3X2/MoS2 by strain are investigated. The Ti3N2 shows advantages in strength and conductivity, and all Ti3X2/MoS2 interfaces exhibit Ohmic contact without tunneling barrier. Furthermore, depending on the nearly disappearing interface hybridization by controlling vertical strain, a transition from Ohmic to Schottky contact occurs. The differences in metal work function and interface dipole potential make the Schottky contact display n-type in Ti3B2/MoS2 and Ti3C2/MoS2, while p-type in Ti3N2/MoS2 until the vertical strain reaches 3.60 Å. Moreover, the increasing vertical strain is accompanied by the rising tunneling barrier, and weak interfacial interaction makes the variation of barrier more sensitive to strain. The results demonstrate that vertical strain is an effective way of controlling metal-semiconductor interface and provide guidance for designing Schottky devices.

Investigating the role of membrane dipole potential in liquid-liquid phase separation: ether lipids influence domain size

Lateral lipid heterogeneity is thought to play an important role in the functioning of cell membranes. Parameters such as the shape, size, and composition of domains are not well-understood in biological membranes, in part because of their chemical complexity. Factors including line tension, dipole repulsions, and thermal fluctuations contribute to these parameters. To elucidate the impact of these factors, we use simple three- or four- component model membranes. Liquid-liquid phase separation in these systems have provided numerous insights into the behavior of domains in biological membranes.

Effect of dipole potential on the orientation of Voltage-gated Alamethicin peptides regulated by chaotropic anions

• A new tBLMs model was proposed to accommodate the voltage-gated Alm peptides. • Chaotropic anions induce dipole potential decrease by disrupting the water structure. • A lower dipole potential facilitates the peptide to adopt a more upright orientation. Understanding the gating mechanism of ion channel proteins is the basis for regulating cell activity and designing new drug molecules, but it still remains a great challenge. Herein, a new tethered bilayer lipid membranes (tBLMs) model was proposed to accommodate the voltage-gated ion channel model-Alamethicin (Alm) peptides. Using surface-enhanced infrared absorption (SEIRA) spectroelectrochemistry combined with fluorescence spectroscopy, it was found that the orientation of the peptides inserted into the membranes was affected by membrane dipole potential regulated by chaotropic anions. A lower dipole potential could reduce the energy barrier of the insertion of peptides into the membranes, which facilitates the peptides to adopt a more upright orientation in the membranes. This research is of great significance for understanding the mechanism of ion channel-related diseases.

Lamb–Dicke localization of cold atoms in Ferris wheel optical dipole potential

We investigate the possibility of strong localization, of the Lamb–Dicke type, for cold atoms trapped by a far off-resonant Ferris wheel optical dipole potential. This optical dipole potential light field is created when a light field generated by the superposition of two similar co-propagating Laguerre–Gaussian beams, with opposite winding numbers, interacts with a two-level atom. We show that strong confinement of atoms in such a light field is possible when the light field is tightly focused, for low values of the winding number and relatively high values of power. We show that a combination of a Ferris wheel with an ordinary axial Gaussian optical lattice provides 3D cylindrically symmetric optical lattices in which the Lamb–Dicke limit can be reached for typically used experimental parameter values.

Phosphodiesterase Type 5 Inhibitors Greatly Affect Physicochemical Properties of Model Lipid Membranes.

Although phosphodiesterase type 5 inhibitors are widely used and well-studied drugs, the potential benefits of their application in the treatment of various diseases and new drug delivery systems, including liposome forms, are still being discussed. In this regard, the role of the lipid matrix of cell membranes in the pharmacological action of the inhibitors is of special interest. It was shown that sildenafil, vardenafil, and tadalafil caused a significant decrease in the boundary potential of model membranes composed of palmitoyloleoylphosphatidylcholine or its mixture with cholesterol, by 70–80 mV. The reduction in the membrane dipole potential induced by inhibitors led to a 20–25% increase in the conductance of cation-selective pores formed by the antimicrobial peptide gramicidin A. The addition of sildenafil or vardenafil also led to a significant decrease in the temperature of the main phase transition of dipalmytoylphosphatidylcholine, by about 1.5 °C, while tadalafil did not change the melting temperature. Sildenafil, vardenafil, and tadalafil enhanced the pore-forming activity of the antifungal polyene antibiotic nystatin by 11, 13, and 2 times, respectively. This fact might indicate the induction of membrane curvature stress by the inhibitors. The data obtained might be of special interest for the development of lipid-mediated forms of drugs.

Solution landscape of the Onsager model identifies non-axisymmetric critical points

We investigate critical points of the Onsager free-energy model on a sphere with different potential kernels, including the dipolar potential, the Maier–Saupe potential, the coupled dipolar/Maier–Saupe potential, and the Onsager potential. A uniform sampling method is implemented for the discretization of the Onsager model, and solution landscapes of the Onsager model are constructed using saddle dynamics coupled with downward/upward search algorithms. We first construct the solution landscapes with the dipolar and Maier–Saupe potentials, for which all critical points are axisymmetric. For the coupled dipolar/Maier–Saupe potential, the solution landscape shows a novel non-axisymmetric critical point, named tennis, which exists for a wide range of parameters. We further demonstrate various non-axisymmetric critical points in the Onsager model with the Onsager potential, including square, hexagon, octahedral, cubic, quarter, icosahedral, and dodecahedral states. The bifurcation diagram is presented to show the primary and secondary bifurcations of the isotropic state and reveal the emergence of the critical points. The solution landscape provides an efficient approach to show the global structure of the model system as well as the bifurcations of critical points, which can not only support the previous theoretical conjectures but also propose new conjectures based on the numerical findings.

Dielectric Properties of Phosphatidylcholine Membranes and the Effect of Sugars.

Simple carbohydrates are associated with the enhanced risk of cardiovascular disease and adverse changes in lipoproteins in the organism. Conversely, sugars are known to exert a stabilizing effect on biological membranes, and this effect is widely exploited in medicine and industry for cryopreservation of tissues and materials. In view of elucidating molecular mechanisms involved in the interaction of mono- and disaccharides with biomimetic lipid systems, we study the alteration of dielectric properties, the degree of hydration, and the rotational order parameter and dipole potential of lipid bilayers in the presence of sugars. Frequency-dependent deformation of cell-size unilamellar lipid vesicles in alternating electric fields and fast Fourier transform electrochemical impedance spectroscopy are applied to measure the specific capacitance of phosphatidylcholine lipid bilayers in sucrose, glucose and fructose aqueous solutions. Alteration of membrane specific capacitance is reported in sucrose solutions, while preservation of membrane dielectric properties is established in the presence of glucose and fructose. We address the effect of sugars on the hydration and the rotational order parameter for 1-palmitoyl-2-oleoyl-sn-glycero-3- phosphocholine (POPC) and 1-stearoyl-2-oleoyl-sn-glycero-3- phosphocholine (SOPC). An increased degree of lipid packing is reported in sucrose solutions. The obtained results provide evidence that some small carbohydrates are able to change membrane dielectric properties, structure, and order related to membrane homeostasis. The reported data are also relevant to future developments based on the response of lipid bilayers to external physical stimuli such as electric fields and temperature changes.

Surface Characterization of Lipid Biomimetic Systems.

Zeta potential and dipole potential measures are direct operational methodologies to determine the adsorption, insertion and penetration of ions, amphipathic and neutral compounds into the membranes of cells and model systems. From these results, the contribution of charged and dipole groups can be deduced. However, although each method may give apparent affinity or binding constants, care should be taken to interpret them in terms of physical meaning because they are not independent properties. On the base of a recent model in which the lipid bilayer is considered as composed by two interphase regions at each side of the hydrocarbon core, this review describes how dipole potential and zeta potential are correlated due to water reorganization. From this analysis, considering that in a cell the interphase region the membrane extends to the cell interior or overlaps with the interphase region of another supramolecular structure, the correlation of dipole and electrostatic forces can be taken as responsible of the propagation of perturbations between membrane and cytoplasm and vice versa. Thus, this picture gives the membrane a responsive character in addition to that of a selective permeability barrier when integrated to a complex system.

Finite-element dynamic-matrix approach for spin-wave dispersions in magnonic waveguides with arbitrary cross section

We present a numerical approach to efficiently calculate spin-wave dispersions and spatial mode profiles in magnetic waveguides of arbitrarily shaped cross section with any non-collinear equilibrium magnetization that is translationally invariant along the waveguide. Our method is based on the propagating-wave dynamic-matrix approach by Henry et al. (Ref. 19) and extends it to arbitrary cross sections using a finite-element method. We solve the linearized equation of motion of the magnetization only in a single waveguide cross section, which drastically reduces computational effort compared to common three-dimensional micromagnetic simulations. In order to numerically obtain the dipolar potential of individual spin-wave modes, we present a plane-wave version of the hybrid finite-element/boundary-element method by Fredkin and Koehler which we extend to a modified version of the Poisson equation. Our method is applied to several important examples of magnonic waveguides including systems with surface curvature, such as magnetic nanotubes, where the curvature leads to an asymmetric spin-wave dispersion. In all cases, the validity of our approach is confirmed by other methods. Our method is of particular interest for the study of curvature-induced or magnetochiral effects on spin-wave transport and also serves as an efficient tool to investigate standard magnonic problems.

On critical dipoles in dimensions n ⩾ 3

We reconsider generalizations of Hardy's inequality corresponding to the case of (point) dipole potentials Vγ(x)=γ(u,x)|x|−3, x∈Rn﹨{0}, γ∈[0,∞), u∈Rn, |u|=1, n∈N, n⩾3. More precisely, for n⩾3, we provide an alternative proof of the existence of a critical dipole coupling constant γc,n>0, such thatfor all γ∈[0,γc,n], and all u∈Rn, |u|=1,∫Rndnx|(∇f)(x)|2⩾±γ∫Rndnx(u,x)|x|−3|f(x)|2,f∈D1(Rn) with D1(Rn) denoting the completion of C0∞(Rn) with respect to the norm induced by the gradient. Here γc,n is sharp, that is, the largest possible such constant. Moreover, we discuss upper and lower bounds for γc,n>0 and develop a numerical scheme for approximating γc,n.This quadratic form inequality will be a consequence of the fact[−Δ+γ(u,x)|x|−3]|C0∞(Rn﹨{0})‾⩾0 if and only if 0⩽γ⩽γc,n in L2(Rn) (with T‾ the operator closure of the linear operator T).We also consider the case of multicenter dipole interactions with dipoles centered on an infinite discrete set.

Virtual Electric Dipole Field Applied to Autonomous Formation Flight Control of Unmanned Aerial Vehicles.

The following paper presents a method for the use of a virtual electric dipole potential field to control a leader-follower formation of autonomous Unmanned Aerial Vehicles (UAVs). The proposed control algorithm uses a virtual electric dipole potential field to determine the desired heading for a UAV follower. This method’s greatest advantage is the ability to rapidly change the potential field function depending on the position of the independent leader. Another advantage is that it ensures formation flight safety regardless of the positions of the initial leader or follower. Moreover, it is also possible to generate additional potential fields which guarantee obstacle and vehicle collision avoidance. The considered control system can easily be adapted to vehicles with different dynamics without the need to retune heading control channel gains and parameters. The paper closely describes and presents in detail the synthesis of the control algorithm based on vector fields obtained using scalar virtual electric dipole potential fields. The proposed control system was tested and its operation was verified through simulations. Generated potential fields as well as leader-follower flight parameters have been presented and thoroughly discussed within the paper. The obtained research results validate the effectiveness of this formation flight control method as well as prove that the described algorithm improves flight formation organization and helps ensure collision-free conditions.

Statin-boosted cellular uptake and endosomal escape of penetratin due to reduced membrane dipole potential.

Cell penetrating peptides are promising tools for delivery of cargo into cells, but factors limiting or facilitating their cellular uptake are largely unknown. We set out to study the effect of the biophysical properties of the cell membrane on the uptake of penetratin, a cell penetrating peptide. Using labelling with pH-insensitive and pH-sensitive dyes, the kinetics of cellular uptake and endo-lysosomal escape of penetratin were studied by flow cytometry. We report that escape of penetratin from acidic endo-lysosomal compartments is retarded compared with its total cellular uptake. The membrane dipole potential, known to alter transmembrane transport of charged molecules, is shown to be negatively correlated with the concentration of penetratin in the cytoplasmic compartment. Treatment of cells with therapeutically relevant concentrations of atorvastatin, an inhibitor of HMG-CoA reductase and cholesterol synthesis, significantly increased endosomal escape of penetratin in two different cell types. This effect of atorvastatin correlated with its ability to decrease the membrane dipole potential. These results highlight the importance of the dipole potential in regulating cellular uptake of cell penetrating peptides and suggest a clinically relevant way of boosting this process.

Vacuum Field Photonic Trap for Excitons

AbstractThe spectral and spatial dependence of the resonant Lamb shift of an exciton inside a photonic vacuum field landscape is considered to predict an optical dipole potential generated via virtual photons quantum fluctuations. This trapping potential can be exploited for exciton self‐positioning in electric field hotspots at the nanoscale in view of quantum electrodynamics effects. By non Hermitian decomposition of the electromagnetic Green tensor in terms of quasi normal modes, the resonant part of the Lamb shift is expressed as Fano profile strictly interlinked to the Purcell enhancement of the exciton radiative lifetime. This approach explains how to tailor the depth of the vacuum photonic potential in several possible scenarios.

Effect of the Stark shift on the low-energy interference structure in strong-field ionization

An improved quantum trajectory Monte Carlo method including the Stark shift of the initial state, Coulomb potential, and multielectron polarization-induced dipole potential is adopted to revisit the origin of the low-energy interference structure in the photoelectron momentum distribution of the xenon atom subjected to an intense laser field, and resolve the different contributions of these three effects. We found that the Stark shift plays an essential role on the low-energy interference structure, which moves the ringlike constructive interference structure to the lower momentum region. The formation of the low-energy interference structure is a result of the combined effects of Stark shift, laser, and Coulomb fields, while the multielectron polarization mainly enhance the probability of the low energy photoelectron spectrum. Our finding provides insight into the electron dynamics of atoms and molecules when driven by the intense laser fields.

Spontaneous atomic crystallization via diffractive dephasing in optical cavities

The design of an experiment on the spontaneous crystallization of a laser-cooled, but thermal atomic cloud into a hexagonally structured phase is discussed. Atomic interaction is mediated by the dipole potential of an optical lattice formed spontaneously in a multi-mode degenerate cavity from single-mode longitudinal pumping. The length scale of the structure is given by the diffractive dephasing between the spontaneous sidebands and the on-axis pump. A linear stability indicates that the transition can be observed in a cavity of moderate finesse compatible with having the cavity mirrors outside the vacuum cell. A new anti-reflection coated cell has been assembled for this purpose.