Dielectric Susceptibility Research Articles

Emergent Vibronic Excitations in the Magnetodielectric Regime of Ce_{2}O_{3}.

The strong coupling between spin, lattice, and electronic degrees of freedom in magnetic materials can produce interesting phenomena, including multiferroic and magnetodielectric (MD) behavior, and exotic coupled excitations, such as electromagnons. We present a temperature- and magnetic field-dependent inelastic light (Raman) scattering study that reveals the emergence of vibronic modes, i.e., coupled vibrational and crystal-electric-field (CEF) electronic excitations, in the unconventional rare-earth MD material Ce_{2}O_{3}. The energies and intensities of these emergent vibronic modes are indicative of enhanced vibronic coupling and increased modulation of the dielectric susceptibility in the Néel state (T_{N}≈6.2 K). The field dependences of the energies and intensities of these vibronic modes are consistent with a decrease of both the vibronic coupling and the dielectric fluctuations associated with these modes below T_{N}. These results suggest a distinctive mechanism for MD behavior in Ce_{2}O_{3} that is associated with a field-tunable coupling between CEF and phonon states.

The dynamics hysteresis loop and dielectric susceptibility characteristics of a ferroelectric heterostructure

The dynamic properties, including hysteresis loop and dielectric susceptibility, of a ferroelectric heterostructure composed by two different films have been discussed, by means of a Landau–Devonshire theory in combination with Landau–Khalatnikov dynamic equation. The surface transition layer within each component film and a ferroelectric coupling at the interface between two films are taken into account. A parameter is introduced to describe the differences of physical properties between two constituent films. The results show that if the ferroelectric interfacial coupling is large enough, the system will lead to the severe hysteresis phenomenon for hysteresis loop and four peaks of the curve for dielectric susceptibility versus the electric field.

Development of a volume fraction scaling function for demagnetization factors in effective media theories of magnetic composites

Three effective medium theories (Maxwell-Garnett, MGT; Bruggeman, BEMT; Coherent Model Approximation, CMA) are often applied to predict dielectric or magnetic susceptibility of polymer-magnetic particle composites. Measured composite susceptibility data for multi-crystalline magnetic particulate aspect ratios near unity and bulk low frequency susceptibilities ranging from 10s to 1000s were used to evaluate EMT model accuracies. Composite volume fractions varied from 5 to 100%. Each theory was evaluated against measured data with emphasis placed on CMA and BEMT. BEMT was selected for incorporation of a scaling function relating demagnetization factors with particulate fraction. The scaled model is ScEMT. Percolation theory and micromagnetic computations were applied as guides in choice of scaling function numerical parameters. Experimental support of scale model physics was taken from measurements of magnetic sphere clustering in two dimensions. Scaling function parameters were determined by a numerical fit to one set of NiZnCuFerrite particle composites. The parameters of the scaling function were held constant and then applied to predict magnetic parameters in others. Those composites incorporated: Fe, Fe-Ni, Ni.34 Zn.65 ferrites; Mn.53 Zn.41 ferrites; NiMn.02 ferrites and Fe3O4. Scaled effective model, ScEMT, and measurement were in good agreement. When combined with Snoek’s law, the combined model improved predictions of resonant frequency in composites and relaxation frequency predictions are in progress. Overall, ScEMT shows improved accuracy for calculating low frequency susceptibility and also composite resonant frequency.

Spectral behavior of light waves on scattering from a semi-soft-boundary medium with spectral dependence.

The far-zone behavior of polychromatic light waves on scattering from an isotropic semi-soft-boundary medium with spectral dependence is considered, and both spectral shifts and spectral switches of the scattered field have been discussed. It is shown that the spectral behavior of the far-zone scattered field is influenced by the structural function width of the dielectric susceptibility of the scattering medium. In particular, the structural function width of the dielectric susceptibility of the scattering medium plays an important role in the direction at which the spectral switch occurs. These results may have potential applications in areas such as the inverse scattering problem.

Linking plasma formation in grapes to microwave resonances of aqueous dimers

The sparking of cut grape hemispheres in a household microwave oven has been a poorly explained Internet parlor trick for over two decades. By expanding this phenomenon to whole spherical dimers of various grape-sized fruit and hydrogel water beads, we demonstrate that the formation of plasma is due to electromagnetic hotspots arising from the cooperative interaction of Mie resonances in the individual spheres. The large dielectric constant of water at the relevant gigahertz frequencies can be used to form systems that mimic surface plasmon resonances that are typically reserved for nanoscale metallic objects. The absorptive properties of water furthermore act to homogenize higher-mode profiles and to preferentially select evanescent field concentrations such as the axial hotspot. Thus, beyond providing an explanation for a popular-science phenomenon, we outline a method to experimentally model subwavelength field patterns using thermal imaging in macroscopic dielectric systems.

Monte Carlo simulation of dieletric properties of the antiferroelectric/ferroelectric BiFeO3/YMnO3 bilayer in the external electric field

Monte Carlo simulation has been used to study the dielectric properties of an antiferroelectric/ferroelectric Ising bilayer structure which consists of the top layer with spin-5/2 and the bottom layer with spin-2. The effects of the crystal-fields, the exchange couplings and the external electric field on the polarization, the dielectric susceptibility, the internal energy, the specific heat and the blocking temperature of the mixed-spin bilayer system have been examined detailedly. In particular, the phase diagrams of the blocking temperature under the effects of various physical parameters are obtained for the present system. Through a comparison, there is in agreement between our results obtained and other theoretical studies as well as experimental results.

Monte Carlo studies of the ferroelectric phase transition in KDP

The order-disorder type ferroelectric phase transition in KDP is described by the pseudospin model. Four-particle cluster approximation provides its solution which fits well with the experimental data. Here we check for the first time how this approximation is exact in the absence of proton tunneling and long-range dipole interactions by means of Monte Carlo simulations. Polarization, dielectric susceptibility and heat capacity analytical curves are reproduced well by our simulations and the difference in critical temperature turns out to be much less than a percent.

Determination of Fourier Components of Spatial Correlation Function of Dielectric Susceptibility of Random Medium

In this work, we present a new method of directly determining Fourier components of the spatial correlation function of the dielectric susceptibility of random medium. The method is based on the analysis of the ratio of the spectrum of the light scattered by the spatial correlation components of the dielectric susceptibility of tissue to the spectrum of light scattered by the randomly distributed scatterers which are independent on the value of the spectrum of the incident light and the direction of the observation. The results may find wide applications in areas such as in biomedical diagnosis.

Генерация второй гармоники от тонкого цилиндрического слоя. I. Аналитическое решение

In the Rayleigh–Gans–Debye approximation an analytical solution is obtained for the problem of second harmonic generation by a plane electromagnetic wave with elliptical polarization from a thin optically nonlinear layer on the surface of a cylindrical dielectric particle of finite size placed in a dielectric. The result is presented in tensor and vector forms in general case, in which the nonlinear dielectric susceptibility tensor has four independent components (one chiral and three non-chiral). For the first time it is shown that at generation from the end plane surfaces of a cylindrical particle the contribution of chiral components differs in phase from that of non-chiral components. It is also found that for small linear dimensions of the cylindrical particle (height and radius of the base), the radiation due to the chiral component of the second-order nonlinear dielectric susceptibility tensor makes a dominant contribution to the second harmonic generation from a non-linear cylindrical layer (end and side surfaces).

Elucidation of dipolar dynamics and the nature of structural phases in the [(CH3)2NH2][Zn(HCOO)3] hybrid perovskite framework

We demonstrate improper ferroelectric and non-relaxor behaviour of the [(CH3)2NH2][Zn(HCOO)3] hybrid perovskite framework.

Time-dependent density functional theory for interaction of ultrashort light pulse with thin materials

We present a comprehensive theoretical description for an irradiation of an ultrashort light pulse normally on thin materials based on first-principles time-dependent density functional theory. As the most elaborate scheme, we develop a microscopic description solving Maxwell equations for light electromagnetic fields and the time-dependent Kohn-Sham equation for electron dynamics simultaneously in the time domain using a common spatial grid. We call it the microscopic Maxwell-TDDFT scheme. We test this scheme for silicon thin films of various thickness, from a few atomic layers to a few tens of nm. We show that the microscopic Maxwell-TDDFT scheme provides a satisfactory description incorporating the electronic structure of thin films in the first-principles level, multiple reflections of the electromagnetic fields at the surfaces, and nonlinear light-matter interaction when the incident light pulse is strong. However, the calculation becomes expensive as the thickness increases. We then consider two limiting cases of extremely thin and sufficiently thick films and develop approximate schemes. For the extremely thin case including two-dimensional atomic-layered materials, a two-dimensional macroscopic electromagnetism is developed: a two-dimensional susceptibility is introduced for a weak field, while time evolution equation is derived for an intense field. For sufficiently thick films, the microscopic Maxwell-TDDFT scheme is expected to coincide with a description utilizing ordinary macroscopic electromagnetism. We numerically confirm it comparing the calculated results: For a weak field, a comparison is made with a description using the bulk dielectric susceptibility. For a strong field, a comparison is made with a multiscale Maxwell-TDDFT scheme which the authors' group developed previously.

Renovation of Interest in the Magnetoelectric Effect in Nanoferroics

Recent theoretical studies of the influence of the magnetoelectric effect on the physical properties of nanosized ferroics and multiferroics have been reviewed. Special attention is focused on the description of piezomagnetic, piezoelectric, and linear magnetoelectric effects near the ferroid surface in the framework of the Landau–Ginzburg–Devonshire phenomenological theory, where they are considered to be a result of the spontaneous surface-induced symmetry reduction. Therefore, nanosized particles and thin films can manifest pronounced piezomagnetic, piezoelectric, and magnetoelectric properties, which are absent for the corresponding bulk materials. In particular, the giant magnetoelectric effect induced in nanowires by the surface tension is possible. A considerable influence of size effects and external fields on the magnetoelectric coupling coefficients and the dielectric, magnetic, and magnetoelectric susceptibilities in nanoferroics is analyzed. Particular attention is paid to the influence of a misfit deformation on the magnetoelectric coupling in thin ferroic films and their phase diagrams, including the appearance of new phases absent in the bulk material. In the framework of the Landau–Ginzburg–Devonshire theory, the linear magnetoelectric and flexomagnetoelectric effects induced in nanoferroics by the flexomagnetic coupling are considered, and a significant influence of the flexomagnetic effect on the nanoferroic susceptibility is marked. The manifestations of size effects in the polarization and magnetoelectric properties of semiellipsoidal bismuth ferrite nanoparticles are discussed.

The influence of external stress/strain on the phase transitions and dielectric properties of potassium niobate crystal

We investigated the effects of one-dimensional shock wave compression on the structural, dielectric, and ferroelectric properties of a potassium niobate ferroelectric crystal using Landau–Ginsburg–Devonshire phenomenological theory. Our results show that the cubic to tetragonal and tetragonal to orthorhombic phase transition temperatures increase with increasing shock compression and the derivative of the phase transition temperatures with respect to stain induced by shock compression was estimated to be 21.9 °C/10−3. Additionally, the orthorhombic to rhombohedral phase transition temperature decreased with increasing shock compression and the derivative of the phase transition temperatures with respect to stain induced by shock compression was estimated to be −144.3 °C/10−3. This result represents a 15.3% increase in the remnant polarization of the tetragonal KNbO3 crystal when the stain induced by shock compression increased from 0 to 4 × 10−3 at 400 °C. The dielectric susceptibilities ε33 and ε22 were calculated to decrease by approximately 20% and 40%, respectively, when the stain induced by shock compression increased from 0 to 1.6 × 10−3 at 350 °C. The increase in the remnant polarization and the decrease in the dielectric properties induced by shock compression provide an effective way to improve the power output capacity of a ferroelectric-pulsed power supply.

Coulomb effects on the photoexcited quantum dynamics of electrons in a plasmonic nanosphere

With recent experiments investigating the optical properties of progressively smaller plasmonic particles, quantum effects become increasingly more relevant, requiring a microscopic description. Using the density matrix formalism we analyze the photo-excited few-electron dynamics of a small nanosphere. Following the standard derivation of the bulk plasmon we particularly aim on elucidating the role of the Coulomb interaction. Calculating the dielectric susceptibility spectrum in the linear optical response we find discrete resonances resulting from a collective response mediated by the Coulomb interaction between the electrons. In the nonlinear regime, the occupations of the system exhibit oscillations between the interacting eigenstates. Our approach provides an ideal platform to study and explain nonlinear and quantum plasmonics, revealing that the photo-excited dynamics of plasmonic nano spheres has similarities with and combines characteristics of both, the well-known two-level Rabi dynamics, and the collective many-electron behavior typical of plasmons.

Terahertz Switching Focuser Based on Thin Film Vanadium Dioxide Zone Plate

In this paper, we propose a switchable focuser device based on a Fresnel zone plate (FZP) structure for terahertz (THz) applications. Each FZP contains seven rings, etched in thin VO2 film with the designed focal lengths of 50 and 100 mm for 3.7-THz frequency. Temperature-induced VO2 phase transition leads to the change in dielectric susceptibility of the material, which allows one to switch on and off the focusing properties of the device. The devices were tested with radiation of 3.1 and 3.7 THz emitted by quantum cascade lasers. Experimental results were compared with numerical simulations. In this article, we compare the FZP based on VO2 films with different properties and show that a thicker VO2 film reveals higher focusing efficiency, while a thinner one reveals a higher modulation ratio for the peak intensity at the focal point of FZP. We demonstrate experimentally the near-diffraction-limited size of the beam in the focal point of the device. Switching between two phase states of the VO2 films results in up to the 38-fold change of intensity in the focal point.

Development of a New Glass for both Visible and Near-Infrared Optical Applications

A glass system of composition 40P2O5-40ZnO-(20-x)Na2O-xCdO (where, x = 1, 2, 3, 4, 5 and 6 mol%) was prepared using the conventional melt quenching technique. The glass formability of the prepared samples was inspected using XRD technique. Archimedes’ method was used to determine the density of the prepared glass samples then the molar volume was calculated. The optical spectroscopic investigations of the prepared glass samples were carried out over the spectral range (190-2500 nm). The proposed glass showed a successive transparency in both visible and near-IR ranges of spectrum till 2500 nm with considerably high transmission of about 78%. The refractive index of the glass samples with some other useful parameters such as dielectric constant, electric susceptibility and electronic polarizability of the prepared glass were evaluated. The results suggest the practicality of utilizing such new glass in the fabrication of optical supplies such as lenses and optical windows used for Nd:YAG lasers. The metallization criteria data of the prepared glass propose a good basis for predicting new nonlinear optical materials.

Absence of a multiglass state in some transition metal doped quantum paraelectrics

We critically investigate the purported existence of a multiglass state in the quantum paraelectrics $\mathrm{SrTiO}{}_{3}$ and $\mathrm{KTaO}{}_{3}$ doped with magnetic $3d$ transition metals. We observe that the transition metals have limited solubility in these hosts, and that traces of impurity magnetic oxides persist even in the most well-processed specimens. Our dielectric measurements indicate that the polar nanoregions formed as a consequence of doping appear to lack co-operativity, and the associated relaxation process exhibits a thermally activated Arrhenius form. At lower temperatures, the dielectric susceptibility could be fit using the Barrett's formalism, indicating that the quantum-paraelectric nature of the host lattices is unaltered by the doping of magnetic transition metal oxides. All these doped quantum paraelectrics exhibit a crossover from the high-temperature Curie-Weiss regime to one dominated by quantum fluctuations, as evidenced by a $T{}^{2}$ dependence of the temperature-dependent dielectric susceptibility. The temperature dependence of the magnetic susceptibility indicates that magnetic signatures observed in some of the specimens could be solely ascribed to the presence of impurity oxides corresponding to the magnetic dopants used. Hence, the doped quantum paraelectrics appear to remain intrinsically paramagnetic down to the lowest measured temperatures, ruling out the presence of a multiglass state.

Doping induced effect on optical and band structure properties of Sr2Si5N8 based phosphors: DFT approach

Nitrido silicates are emerged as highly efficient luminescent materials (phosphors) that found considerable industrial application as white light emitting diodes (LEDs) have been studied with respect to band structure and related electronic structure parameters. They have tunable optical properties, as the band gap is of indispensable because it determines both the electrical and optical features of the material, which can be varied by the material composition (by doping technique). It is found also that the nitride (alumo) silicates “Sr2Si5N8: Eu2+” have wide industrial application as highly efficient red-emitting phosphor materials in pc-LEDs. In this report we apply density functional theory (DFT) within the GGA+U approach to study the structural, electronic and optical properties of Eu2+ and Ce3+ doped Sr2Si5N8. The total energy has been optimized as a function of the unit cell volume. Electronic structure including, the electronic density of state (DOS), the band structure and the linear optical susceptibility are calculated for the relaxed structure applying the optimized lattice constant. The calculated optical dispersion of dielectric susceptibility are closely related to the corresponding electronic structure and our results are in very good agreement with experimental data.

Dielectric susceptibility of quantum Ising model: Simulated monoclinic A2BX4-type ferroelectrics

To explain the anomalous increase in the permittivity of monoclinic Rb2ZnI4 at low temperatures, the dielectric susceptibilities of modulated structures are calculated. A quantum Ising model is reduced from a lattice Hamiltonian by the mean field approximation and two-quantum-level approximation. The susceptibilities of modulated phases remain to be finite values at zero temperature as in the uniform ferroelectric phase, if the quantum effect works. Furthermore, the susceptibilities of the incommensurate and high-order commensurate phases increase at low temperatures. Since the ferroelectric phase can be realized with accompanying strong discontinuity, such an increase is different from the phenomenon during the lock-in transition. The phase diagram and the temperature dependence of susceptibility are calculated. With the selection of a proper set of parameters, the theoretical results agree qualitatively with experimental reports of (Rb1−xKx)2ZnI4.

Nonlinear dielectric constant in Pb(Mg1/3Nb2/3)O3–PbTiO3 relaxor ferroelectrics

The nonlinear dielectric constant ε3 induced by the cube of the electric field was investigated in the paraelectric phase of the relaxor ferroelectric 70.5% Pb(Mg1/3Nb2/3)O3–29.5% PbTiO3. ε3 was confirmed to be positive near Tm at which the linear permittivity is maximum, and becoming negative at 158.5 °C. The temperature dependences of the linear and nonlinear dielectric susceptibilities are discussed on the basis of the Landau-type free energy function.