Effective Dielectric Response Research Articles

Dielectric spectrum of a ferrofluid layer exposed to a gradient magnetic field.

A low-frequency dielectric response of a ferrofluid based on transformer oil and MnZn ferrite nanoparticles is investigated in a gradient magnetic field. Four ferrofluid samples of various nanoparticle concentrations were introduced into planar micro-capacitors located over a magnetized tip. The dielectric spectra were measured in the frequency range from 0.1Hz to 200kHz and in the local magnetic field up to 100 mT. The spectra exhibit a dielectric relaxation ascribed to nanoparticle interfacial polarization. The low-frequency spectrum of each ferrofluid decreases upon application of the magnetic field up to 20 mT. The decrease in dielectric permittivity is caused by a magnetic force acting on larger nanoparticles in the gradient magnetic field. It is assumed that the interfaces of the concentrated nanoparticles in the gradient field do not contribute to the effective dielectric response. This reduces the effective relaxation time and shifts the relaxation toward higher frequencies. The dielectric spectra are well described by a relaxation fit function consisting of one Havriliak-Negami and a conductivity term. The fitting confirms that the only effect of the gradient magnetic field on the dielectric spectra is the shift of the dielectric relaxation and the decrease of the amplitude in the imaginary permittivity. This behavior is evident from a master plot, where all dielectric relaxations are superimposed on a single line. The knowledge of the presented behavior of the ferrofluid may be valuable when applying a ferrofluid to sharply magnetized parts of various electrical equipment (wires, tips, screws, nails, edges) as a liquid dielectric medium.

Dielectric Response of Strontium Titanate (SrTiO3)/Polycarbonate Nanocomposite Electrodes for Energy Storage

In the present study we are reporting dielectric response of strontium titanate (SrTiO3 represented as ST used as filler in 1–5 mwt%)/polycarbonate (PC) nanocomposites of thickness 100 μm synthesized by solution cast method. Polycarbonate serving as passive matrix element and ceramic strontium titanate (SrTiO3) dispersed into polycarbonate to improve effective dielectric response. X-ray diffraction (XRD), Fourier transform infrared (FT-IR) and Scanning electron microscopy (SEM) have been used to investigate structural, bonding and morphological properties of fabricated nanocomposites respectively. Particular attention has been given on the examination of dielectric response for composites using Impedance analyzer. Dielectric constant has been found to be increased with increasing concentration of SrTiO3 inside the polycarbonate matrix. The observed dielectric constant reaches upto 6.61 for ST5%PC95% composite at 1 KHz. These composites of high dielectric constant can be used as high performance electrode materials for electrical energy storage devices.

Dielectric response of confined water films from a classical density functional theory perspective.

We re-examine the problem of the dielectric response of highly polar liquids such as water in confinement between two walls using simple two-variable density functional theory involving number and polarisation densities. In the longitudinal polarisation case where a perturbing field is applied perpendicularly to the walls, we show that the notion of the local dielectric constant, although ill-defined at a microscopic level, makes sense when coarse-graining over the typical size of a particle is introduced. The approach makes it possible to study the effective dielectric response of thin liquid films of various thicknesses in connection with the recent experiments of Fumagalli et al., [Science, 2018, 360, 1339-1342], and to discuss the notion of the interfacial dielectric constant. We argue that the observed properties as a function of slab dimensions, in particular the very low dielectric constants of the order of 2-3 measured for thin slabs of ∼1 nm thickness do not highlight any special properties of water but can be recovered for a generic polar solvent having similar particle size and the same high dielectric constant. Regarding the transverse polarisation case where the perturbing field is parallel to the walls, the associated effective dielectric constant as a function of slab dimensions reaches bulk-like values at much shorter widths than in the longitudinal case. In both cases, we find an oscillatory behaviour for slab thicknesses in the one nanometer range due to packing effects.

Nonlinear dielectric properties of random paraelectric-dielectric composites

The challenge of designing new tunable nonlinear dielectric materials with tailored properties has attracted an increasing amount of interest recently. Herein, we study the effective nonlinear dielectric response of a stochastic paraelectric-dielectric composite consisting of equilibrium distributions of circular and partially penetrable disks (or parallel, infinitely long, identical, partially penetrable, circular cylinders) of a dielectric phase randomly dispersed in a continuous matrix of a paraelectric phase. The random microstructures were generated using the Metropolis Monte Carlo algorithm. The evaluation of the effective permittivity and tunability were carried out by employing either a Landau thermodynamic model or its Johnson’s approximation to describe the field-dependent permittivity of the paraelectric phase and solving continuum-electrostatics equations using finite element calculations. We reveal that the percolation threshold in this composite governs the critical behavior of the effective permittivity and tunability. For microstructures below the percolation threshold, our simulations demonstrate a strong nonlinear behaviour of the field-dependent effective permittivity and very high tunability that increases as a function of dielectric phase concentration. Above the percolation threshold, the effective permittivity shows the tendency to linearization and the tunability dramatically drops down. The highly reduced permittivity and extraordinarily high tunability are obtained for the composites with dielectric impenetrable disks at high concentrations, in which the triggering of the percolation transition is avoided. The reported results cast light on distinct nonlinear behaviour of 2D and 3D stochastic composites and can guide the design of novel composites with the controlled morphology and tailored permittivity and tunability.

Multifunctional composites for elastic and electromagnetic wave propagation

Composites are ideally suited to achieve desirable multifunctional effective properties since the best properties of different materials can be judiciously combined with designed microstructures. Here, we establish cross-property relations for two-phase composite media that link effective elastic and electromagnetic wave characteristics to one another, including the respective effective wave speeds and attenuation coefficients, which facilitate multifunctional material design. This is achieved by deriving accurate formulas for the effective electromagnetic and elastodynamic properties that depend on the wavelengths of the incident waves and the microstructure via the spectral density. Our formulas enable us to explore the wave characteristics of a broad class of disordered microstructures because they apply, unlike conventional formulas, to a wide range of incident wavelengths (i.e., well beyond the long-wavelength regime). This capability enables us to study the dynamic properties of exotic disordered "hyperuniform" composites that can have advantages over crystalline ones, such as nearly optimal, direction-independent properties and robustness against defects. We specifically show that disordered "stealthy" hyperuniform microstructures exhibit novel wave characteristics (e.g., low-pass filters that transmit waves "isotropically" up to a finite wavenumber). Our cross-property relations for the effective wave characteristics can be applied to design multifunctional composites via inverse techniques. Design examples include structural components that require high stiffness and electromagnetic absorption; heat sinks for central processing units and sound-absorbing housings for motors that have to efficiently emit thermal radiation and suppress mechanical vibrations; and nondestructive evaluation of the elastic moduli of materials from the effective dielectric response.

Проблеми ідентифікації пародонтопатогенної мікрофлори та можливість їх вирішення за допомогою математичного моделювання ефективного діелектричного відгуку на прикладі S. aureus (референсного штаму)

The aim of the study was to create a multiparametric mathematical model describing the dielectric response of Staphylococcus aureus suspensions for its further use to interpret dielectric spectra and identify the parameters of periodontopathogenic bacterial flora, that would optimize diagnostic processes in periodontology. Materials and methods. The study was based on the use of the Bergman-Milton analytical spectral representation for the effective dielectric response, generalized to the case of inclusions (bacteria), which can be modeled as core-shell- spheroids. The developed software using the Matlab numerical computing environment allowed us to determine the effective dielectric conductivity and dielectric losses as a function of material and geometrical parameters of the bacterial suspension. The material parameters and the inclusion sizes (Staphylococcus aures – S. Aureus bacteria) were taken from literature sources; geometrical parameters characterizing the bacteria shape were considered as model variables. To illustrate the application of the method, the spectral density function of the simplest kind, sush as a homogeneous distribution of spheroidal shapes with a median corresponding to the spherical shape, was used. Results. The spectra of the real and imaginary part of the effective dielectric function εeff of S. Aureus aqueous suspensions in the frequency range of 1–104 KHz at different values of a nonsphericity parameter, characterizing the fluctuations of the bacterial shape around the spherical one, were calculated. It is shown that shape fluctuations can significantly affect Re εeff only at low frequencies, where they increase Re εeff. At high frequencies, the shape fluctuations only slightly reduce Re εeff. The imaginary part of the effective dielectric function (dielectric losses) shows a weak dependence on the nonsphericity parameter with in the entire frequency range. Other calculations indicate that the increase in the electrical conductivity of the plasma membrane, which characterizes S. Aureus dead bacteria, is accompanied by a marked decrease in Re εeff at frequencies below 1 MHz. Conclusion. By the example of S. Aureus, we demonstrated the use of a generalized spectral model of the effective dielectric response for the interpretation of the dielectric spectra of pathogenic microorganisms. The simplicity and analyticity of the model make it a convenient and promising tool for biophysical and medical studies. The generalized spectral model can be used to solve thel direct problem, namely to determine the influence of material and geometrical parameters of bacteria on their dielectric spectra, as well as to solve the inverse problem, which consists in finding the model parameters by processing the experimental dielectric spectra.

Dielectric relaxation and current conduction mechanism of Tb and Mn codoped bismuth ferrite grafted poly (vinyl alcohol) nanocomposite film

Abstract Investigation on current conduction mechanism through Tb and Mn codoped Bismuth Ferrite grafted polyvinyl alcohol (BTFMO-PVA) nanocomposite film above room temperature (300 K – 415 K) is reported here in detail. A detailed study on dielectric properties of the sample is done over a wide temperature range in a frequency range of 20 Hz - 2MHz. The conduction is attributed to correlated barrier hopping model. Bipolaron hopping dominates over single-polaron hopping in this system. Complex electric modulus spectra and complex modulus spectra are well explained by suitable models to understand the effective dielectric response. The sample responds to the externally applied magnetic field exhibiting negative magnetocapacitance at room temperature.

Impact of a charged neighboring particle on Förster resonance energy transfer (FRET)

Förster resonance energy transfer (FRET) is an important physical phenomenon which demands precise control over the FRET rate for its wide range of applications. Hence, enhancing the FRET rate using different techniques has been extensively studied in the literature. Research indicates that introducing additional particles to a system consisting of a donor-acceptor pair can change the behaviour of FRET in the system. One such technique is to utilize the collective oscillations of the surface electrons of a neighboring electrically-neutral metal nanoparticle (MNP). However, the perceived changes on the FRET rate between the donor and the acceptor, when the MNP carries excess electrical charges are yet unknown. In this paper, we study these changes by introducing a charged MNP, in the proximity of an excited donor and a ground state acceptor. We deploy the classical Green’s tensor to express the FRET rate in the system. We consider an effective dielectric response for the MNP, which accounts for the extraneous surface charge effects. We analyze the electrical potential at the acceptor position due to the changed dipole moment of the donor molecule as a result of the electric field induced at the donor position, and obtain the FRET rate of the system. This model considers arbitrary locations and orientations of the two molecular dipole moments with regard to the position of the spherical MNP. We present the enhancement of the FRET rate, predominantly caused by both the surface plasmon excitations and the extraneous surface electrical charges carried by the neighboring MNP. We obtain the results by varying the separation distance between the molecules and the MNP, the transition frequency of the donor-acceptor pair and the size of the metallic sphere. Specifically, we demonstrate that a donor-acceptor pair placed in the vicinity of an electrically-charged Silver MNP exhibits a remarkable improvement in the FRET rate. Furthermore, the aggregate FRET enhancement is determined by other characteristics such as the location of the donor, transition frequency, separation distances and the radius of the MNP. In essence, these findings reveal an approach to realize the enhanced FRET rate in a larger span in a more controlled manner that is desirable in many FRET-based applications including spectroscopic measurements.

Nonlocal electrodynamics of homogenized metal-dielectric photonic crystals

The nonlocal effective permittivity tensor for photonic crystals (PCs), having dielectric and metallic inclusions in the unit cell, is calculated and analyzed within the homogenization theory based on the Fourier formalism and the form-factor division approach. A method allowing us to extract the effective bianisotropic metamaterial parameters (permeability and chirality) from the wave vector dependence of the nonlocal effective dielectric response is proposed. Both the original nonlocal dielectric response parameters and the new bianisotropic metamaterial ones reproduce the photonic band structure of the artificial crystal far beyond the long wavelength limit and for a wide class of metal-dielectric structures. To calculate the optical spectra (reflection and transmission) of finite-size PC, the nonlocal homogenization approach is extended with the method of expansion into photonic bulk-modes (Bloch waves). The application of the developed theory is illustrated with well-known forms of metallic inclusions (slabs, thin wires, split-ring resonators) and experimentally confirmed with novel designs based on metallic crosses.

Enhancement of Coulomb blockade in epsilon near zero and hyperbolic metamaterials

Coulomb interaction of charges in a metamaterial may be expressed via its effective dielectric response function. Coulomb interaction appears considerably enhanced in artificially engineered epsilon near zero (ENZ) and hyperbolic metamaterials. Indeed, very recently it was observed that negative ENZ conditions enhance superconducting properties of a composite metamaterial via enhancement of Cooper pairing. Here we demonstrate that positive ENZ conditions enhance Coulomb repulsion of charges, leading to considerable enhancement of the Coulomb blockade effect in such metamaterials. Similar to high Tc cuprates, in hyperbolic metamaterials superconductivity and the Coulomb blockade-induced charge ordering is predicted to compete with each other.

Far-infrared reflectivity spectra of nanotwinned GaV4Se8

ABSTRACT GaV Se is one of the lacunar spinel materials able to host magnetic skyrmionic textures. Earlier investigations at temperatures below its Jahn–Teller ferroelectric phase transition disclosed a nanoscale ferroelectric domain separated by 109 domain walls. Here, we have investigated the impact of this nanoscale twinning on polar phonon modes. We report dielectric and reflectivity spectra of GaV Se crystal in the phonon frequency region, derived from the ab initio predicted phonon frequencies, eigenvectors and Born-effective charges. Reflectivity spectra of the nanotwinned crystal, calculated using exact expressions for the effective macroscopic dielectric response of a perfect regular twin, are compared with the predicted reflectivity spectra of single-domain crystal. A characteristic spectral feature caused by the nanoscale twinning has been identified. While this feature is not a well-defined geometric resonance, it may still be accessible in experiments.

Fractal-Like Plasmonic Metamaterial with a Tailorable Plasma Frequency in the near-Infrared

In this work, we show that modulating the fractal dimension of nanoporous gold allows its effective dielectric response to be tailored over a wide spectral range of infrared wavelengths. In particular, the plasma edge and effective plasma frequency depend linearly on the fractal dimension, which can be controlled by varying the pore and ligament sizes. Importantly, the fractal porous metal exhibits superior plasmonic properties compared to its bulk counterpart. These properties, combined with a longer skin depth on the order of 100–200 nm, enables the penetration of optical energy deep into the nanopores where molecules can be loaded, thus, achieving more effective light–matter coupling. These findings may open new pathways to engineering the optical response of fractal-like or self-similar metamaterials without the need for sophisticated lithographic patterning.

Optical and surface enhanced Raman scattering responses of densely packed Ag-Au alloy nanoparticle films of varied composition and thickness

Sub-monolayer deposition control of pulsed laser deposition technique has been utilized to grow Ag-Au alloy nanoparticle films of different compositions and mass thicknesses using sequential ablation of pure Ag and Au targets. The films produced at high substrate temperature in single step growth have shown strong localized surface plasmon resonance (LSPR) response, which is systematically red-shifted with Au percentage and film mass thickness. The morphology of the films studied from atomic force microscopy revealed densely packed nanoparticles at all compositions with typical particle number density of ∼1010 - 1011/cm2. The optical response of the films was measured using spectroscopic ellipsometry at different angles of incidence and their effective dielectric response was determined from multiple oscillator modeling. The effective dielectric response was found to well predict the optical transmission spectra and also showed systematic red-shift of interband absorption edge with Au percentage in the films conforming alloy formation, in consistent with the results of X-ray photoelectron spectroscopy. Surface enhanced Raman scattering (SERS) measurements carried out at 488 and 633 nm excitation laser wavelengths revealed that Ag and Ag rich films exhibited relatively higher enhancement of SERS intensity as compared to other composition films. These nanoparticle films were found to be efficient SERS substrates for easy detection of trace dye molecules in nano-molar concentration range. The SERS enhancement was found to depend on film mass thickness and alloy composition dependent LSPR response having close overlap with the laser excitation wavelength.

Effective dielectric response of dispersions of graded particles.

Based upon our compact group approach and the Hashin-Shtrikman variational theorem, we propose a solution, which effectively incorporates many-particle effects in concentrated systems, to the problem of the effective quasistatic permittivity of dispersions of graded dielectric particles. After the theory is shown to recover existing analytical results and simulation data for dispersions of hard dielectric spheres with power-law permittivity profiles, we use it to describe the effective dielectric response of nonconducting polymer-ceramic composites modeled as dispersions of dielectric core-shell particles. Possible generalizations of the results are specified.

Direct measurement of the effective infrared dielectric response of a highly doped semiconductor metamaterial

We have investigated the effective dielectric response of a subwavelength grating made of highly doped semiconductors (HDS) excited in reflection, using numerical simulations and spectroscopic measurement. The studied system can exhibit strong localized surface resonances and has, therefore, a great potential for surface-enhanced infrared absorption (SEIRA) spectroscopy application. It consists of a highly doped InAsSb grating deposited on lattice-matched GaSb. The numerical analysis demonstrated that the resonance frequencies can be inferred from the dielectric function of an equivalent homogeneous slab by accounting for the complex reflectivity of the composite layer. Fourier transform infrared reflectivity (FTIR) measurements, analyzed with the Kramers–Kronig conversion technique, were used to deduce the effective response in reflection of the investigated system. From the knowledge of this phenomenological dielectric function, transversal and longitudinal energy-loss functions were extracted and attributed to transverse and longitudinal resonance modes frequencies.

Dielectric relaxation and charged domain walls in (K,Na)NbO3-based ferroelectric ceramics

The influence of domain walls on the macroscopic properties of ferroelectric materials is a well known phenomenon. Commonly, such “extrinsic” contributions to dielectric permittivity are discussed in terms of domain wall displacements under external electric field. In this work, we report on a possible contribution of charged domain walls to low frequency (10–106 Hz) dielectric permittivity in K1-xNaxNbO3 ferroelectric ceramics. It is shown that the effective dielectric response increases with increasing domain wall density. The effect has been attributed to the Maxwell-Wagner-Sillars relaxation. The obtained results may open up possibilities for domain wall engineering in various ferroelectric materials.

Theoretical Approach to Electroresistance in Ferroelectric Tunnel Junctions

In this paper, a theoretical approach, comprising the non-equilibrium Green's function method for electronic transport and Landau-Khalatnikov equation for electric polarization dynamics, is presented to describe polarization-dependent tunneling electroresistance (TER) in ferroelectric tunnel junctions. Using appropriate contact, interface, and ferroelectric parameters, measured current-voltage characteristic curves in both inorganic (Co/BaTiO$_{3}$/La$_{0.67}$Sr$_{0.33}$MnO$_{3}$) and organic (Au/PVDF/W) ferroelectric tunnel junctions can be well described by the proposed approach. Furthermore, under this theoretical framework, the controversy of opposite TER signs observed experimentally by different groups in Co/BaTiO$_{3}$/La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ systems is addressed by considering the interface termination effects using the effective contact ratio, defined through the effective screening length and dielectric response at the metal/ferroelectric interfaces. Finally, our approach is extended to investigate the role of a CoO$_{x}$ buffer layer at the Co/BaTiO$_{3}$ interface in a ferroelectric tunnel memristor. It is shown that, to have a significant memristor behavior, not only the interface oxygen vacancies but also the CoO$_{x}$ layer thickness may vary with the applied bias.

Efective infrared reflectivity and dielectric function of polycrystalline alumina ceramics

Room‐temperature infrared (IR) reflectivity, terahertz transmission and evaluated effective complex dielectric response of four polycrystalline alumina ceramics (corundum) with highly anisotropic grains in the IR range were determined. The spectra were compared with modelled spectra based on the known IR response of sapphire single crystal and Bruggeman and Lichtenecker models of the effective medium approximation (EMA). The results are extremely sensitive to the surface treatment (polishing), but do not depend on the grain size in the range of ∼0.3–1 μm and on the weak doping, needed for processing of optically transparent ceramics. As all the samples show similar grain shapes and topology, no measurable differences among the fully dense samples were observed. Agreement with the modelled spectra is reasonable, but shows a higher effective mode damping. A weak geometrical resonance was revealed near 500 cm−1, better described by the Bruggeman model. The small sample porosity up to 6.2% is revealed mainly as a reduced reflectivity above ∼650 cm−1, which produces weak losses in the high‐frequency range above the TO phonon modes, better described by the Lichtenecker model.

Dielectric Response of Ba0.5Sr0.5TiO3-Mg2SiO4 Composites Synthesized by a Citrate Gel In-Situ Process

(1-x)Ba0.5Sr0.5TiO3(BST)-xMg2SiO4 (MS) composite ceramics have been synthesized in-situ via a citrate-nitrate combustion process, and their structure and effective dielectric response have been investigated. The compositions with x ≥ 0.3 formed a second phase Ba2TiSi2O8, besides BST and MS phases. The BST-MS composites showed an uniform microstructure with MS particles homogeneously dispersed in BST particles. The temperature at the permittivity maxima of the composites was nearly unchanged with increasing the volume fraction of MS (ƒMS) from 0.1 to 0.4 and then slightly decreased with further increasing ƒMS. According to the modified effective medium approx-imation couple with the spherical inclusion model, the permittivity (ϵBST) of BST in the composites was strongly dependent on ƒMS, which was defined by the relation .

In situ synthesis of Ba0.5Sr0.5TiO3–Mg3B2O6 composites and their dielectric response

Ba0.5Sr0.5TiO3–Mg3B2O6 (BST–MB) composites have been prepared in situ by a citrate gel process, and their structure and effective dielectric response have been investigated. The precursor with pH≥7 is suitable for in situ formation of the diphase structure consisting of BST and MB. Accordingly, MB particles homogeneously disperse in BST particles, accompanied by the formation of boron-rich grain boundary resulting from liquid phases sintering of B2O3. Related with the existence of boron-rich grain boundary and the incorporation of Mg2+ into BST lattice, permittivity decreases rapidly with increasing volume fraction of MB from 0.0 to 0.2 and then decreases slowly with further increasing, which coincides with theoretical prediction of the layered model.