Static Dielectric Susceptibility Research Articles

Sound Velocity, Electronic, Optical, and Mechanical Properties for Nano Semiconductor Materials (CdTe, ZnTe) under the Influence of Pressure

The energy band structure, energy band gaps, refractive index, high-frequency dielectric constant, static dielectric constant, reflectivity, and susceptibility for CdTe and ZnTe have been determined. The elastic parameters, Young’s, bulk, shear moduli, Poisson ratio, anisotropic factor, and the internal strain parameter have been calculated for the CdTe and ZnTe. The acoustic velocity in the directions [001], [110], and [111] has been determined for the studied compounds. The pressure dependence of the investigated properties has been studied. The empirical pseudopotential method (EPM) was used to calculate our results. Our results at high values of pressure could be taken as a reference for future experimental and theoretical works. Generally, our results are found to be in good accord with experimental and theoretical data published in the literature. The data gained in this study could be useful in the development of optoelectronic devices under high values of pressure.

Using Gaussian process regression to simulate the vibrational Raman spectra of molecular crystals

Vibrational properties of molecular crystals are constantly used as structural fingerprints, in order to identify both the chemical nature and the structural arrangement of molecules. The simulation of these properties is typically very costly, especially when dealing with response properties of materials to e.g. electric fields, which require a good description of the perturbed electronic density. In this work, we use Gaussian process regression (GPR) to predict the static polarizability and dielectric susceptibility of molecules and molecular crystals. We combine this framework with ab initio molecular dynamics to predict their anharmonic vibrational Raman spectra. We stress the importance of data representation, symmetry, and locality, by comparing the performance of different flavors of GPR. In particular, we show the advantages of using a recently developed symmetry-adapted version of GPR. As an examplary application, we choose Paracetamol as an isolated molecule and in different crystal forms. We obtain accurate vibrational Raman spectra in all cases with fewer than 1000 training points, and obtain improvements when using a GPR trained on the molecular monomer as a baseline for the crystal GPR models. Finally, we show that our methodology is transferable across polymorphic forms: we can train the model on data for one crystal structure, and still be able to accurately predict the spectrum for a second polymorph. This procedure provides an independent route to access electronic structure properties when performing force-evaluations on empirical force-fields or machine-learned potential energy surfaces.

Real space electrostatics for multipoles. III. Dielectric properties.

In Papers I and II, we developed new shifted potential, gradient shifted force, and Taylor shifted force real-space methods for multipole interactions in condensed phase simulations. Here, we discuss the dielectric properties of fluids that emerge from simulations using these methods. Most electrostatic methods (including the Ewald sum) require correction to the conducting boundary fluctuation formula for the static dielectric constants, and we discuss the derivation of these corrections for the new real space methods. For quadrupolar fluids, the analogous material property is the quadrupolar susceptibility. As in the dipolar case, the fluctuation formula for the quadrupolar susceptibility has corrections that depend on the electrostatic method being utilized. One of the most important effects measured by both the static dielectric and quadrupolar susceptibility is the ability to screen charges embedded in the fluid. We use potentials of mean force between solvated ions to discuss how geometric factors can lead to distance-dependent screening in both quadrupolar and dipolar fluids.

Dielectric properties of crystalline organic molecular films in the limit of zero overlap.

We present the calculation of the static dielectric susceptibility tensor and dipole field sums in thin molecular films in the well-defined limit of zero intermolecular overlap. Microelectrostatic and charge redistribution approaches are applied to study the evolution of dielectric properties from one to a few molecular layers in films of different conjugated molecules with organic electronics applications. Because of the conditional convergence of dipolar interactions, dipole fields depend on the shape of the sample and different values are found in the middle layer of a thick film and in the bulk. The shape dependence is eliminated when depolarization is taken into account, and the dielectric tensor of molecular films converges to the bulk limit within a few molecular layers. We quantify the magnitude of surface effects and interpret general trends among different systems in terms of molecular properties, such as shape, polarizability anisotropy, and supramolecular organization. A connection between atomistic models for molecular dielectrics and simpler theories for polarizable atomic lattices is also provided.

Negative electrocaloric effect in antiferroelectric PbZrO3

The dielectric and thermal properties of a typical antiferroelectric (AFE) material are investigated by minimising numerically the free energy as given by the Kittel model of AFEs. The phase line of second-order phase transitions in the phase diagram is shown to change to a first-order line at the tricritical point T3cp, E3cp. The static dielectric susceptibility and the electrocaloric (EC) effect are calculated as a function of temperature and the applied electric field E. It is found that in a given range of electric fields and temperatures the EC effect has negative values but generally becomes positive above the AFE ordering temperature T0. The dielectric susceptibility shows characteristic peaks at the phase transitions between the field-induced polar and the AFE antipolar phase, and diverges at the tricritical point. We present experimental results for a negative EC effect, which have been obtained by direct EC measurements in PbZrO3 ceramics, and agree qualitatively with the above model.

Low-temperature anharmonicity of barium titanate: A path-integral molecular-dynamics study

We investigate the influence of quantum effects on the dielectric and piezoelectric properties of barium titanate in its (low-temperature) rhombohedral phase, and show the strongly anharmonic character of this system even at low temperature. For this purpose, we perform path-integral molecular-dynamics simulations under fixed pressure and fixed temperature, using an efficient Langevin thermostat-barostat, and an effective hamiltonian derived from first-principles calculations. The quantum fluctuations are shown to significantly enhance the static dielectric susceptibility (~ by a factor 2) and the piezoelectric constants, reflecting the strong anharmonicity of this ferroelectric system even at very low temperature. The slow temperature-evolution of the dielectric properties observed below ~ 100 K is attributed (i) to zero-point energy contributions and (ii) to harmonic behavior if quantum effects are turned off.

Quantum critical behavior of a magnetic quantum paraelectric

We discuss the interplay between anti-ferromagnetic order and polarization fluctuations in a magnetic quantum paraelectric. Using an action where anti-ferromagnetic order parameter couples to the polarization fluctuations and as well as magnetic field, we derive a set of self consistent equations to study both the temperature and the magnetic field dependence of the static dielectric susceptibility. The temperature dependence of dielectric susceptibility near both the anti-ferromagnetic quantum critical point and ferroelectric quantum critical point are described using scaling arguments. Discussions on achieving various quantum critical points in experiments are also made.

Dielectric susceptibility of a deuterated KDP crystal from experiment on Raman scattering and in the cluster approximation

The temperature dependence of the static dielectric susceptibility obtained from the experiment on Raman scattering in a deuterated KDP (DKDP) crystal has been described using the cluster approximation in terms of the microscopic mean-field theory. The optimum values obtained for parameters of the cluster approximation are in good agreement with experimental data over a wide temperature range. These values are found to be comparable to the estimates previously obtained in describing experimental data on the DKDP crystal.

Static susceptibility in a paraelectric particle

Static dielectric susceptibility for the paraelectric phase of a particle was calculated exactly. Temperature and diameter dependence are described analytically. There are two regimes of the susceptibility: for temperatures above the bulk critical ferroelectric temperature of the particle and below it. The critical transition temperature from the exact calculation is the same as that from the soft mode approach.

Phase diagram and static dielectric susceptibility of hydrogen bonded ferro–antiferroelectric mixed compounds

A new approach for a microscopic description of dipole glass state in ferro–antiferroelectric mixed compounds is suggested. It is based on the analysis of the behaviour of single and pair (nearest neighbours) correlation functions at different values of temperature and concentration. Static dielectric susceptibility of the system investigated is obtained and discused.

Orientational Dynamics and Dielectric Response of Nanopore Water

We present numerical calculations, simulation results, and analytical considerations for the frequency-dependent dielectric constant of single-file water in narrow nanopores, described by a recently developed dipole lattice model. We find Debye relaxation over all length scales with relaxation times that strongly depend on pore length. This behavior is analyzed in terms of the dynamics of orientational defects leading to simple quantitative expressions for the static dielectric susceptibility and the relaxation time in the limits of short and long pores. Based on these formulas, we suggest how the predicted macroscopic order of nanopore water can be probed via dielectric spectroscopy and explain how the excitation energy, diffusion constant, and effective interaction of the defects that destroy the order can be extracted from such measurements.

Fluctuations and criticality in quantum paraelectrics

The temperature dependence of the static dielectric susceptibility of a system with stronglycoupled fluctuating dipoles is calculated within a self-consistent mean fluctuation fieldapproximation. Results are qualitatively in good agreement with a quantum paraelectricSrTiO3 in the low temperature regime. We identify this system as a gapped quantumparaelectric and suggest a possible experimental realization of a quantum criticalparaelectric through the application of hydrostatic pressure or doping by impurity.

The magnetic effect on static nonlinear dielectric response in magnetic relaxor ferroelectrics

The nonlinear dielectric response for the external field plays a very important role in providing useful information on relaxor ferroelectrics. Based on the spherical random-bond-random-field model and the Heisenberg model, considering the coupling interaction between the relaxor ferroelectricity and magnetism, we calculated the third-order static nonlinear dielectric susceptibility χ3 as well as the scaled nonlinear susceptibility a3=χ3/χ14 of magnetic relaxor ferroelectrics. We find that in contrast to the electric field, the external magnetic field remains relaxor state being a glass state. As the temperature passes through the magnetic phase temperature Tc, χ3, similar to linear dielectric susceptibility χ1, deviates its original low temperature behavior and shows a sharp increase. Furthermore, the position of the peak of χ3 and a3 are greatly affected by Tc and h. The temperature of maximum χ3 and a3 shift to higher temperature region with increasing Tc and h, indicating that freezing temperature Tf of magnetic relaxor ferroelectrics might be controlled by magnetism through magnetoelectric coupling.

Theoretical investigation of polarization scaling in ultrathin epitaxial PbZrxTi1−xO3 films

We present a theoretical analysis of the scaling of the polarization and the static dielectric susceptibility through a mean-polarization approach for ultrathin epitaxial PbZrxTi1−xO3 thin films. We use the traditional Euler-Lagrangian framework applied to a Landau-Ginzburg-Devonshire (LGD) nonlinear thermodynamic treatment. The novelty of our approach is that the model hinges on using experimentally measured correlation lengths and temperature scaling relationships to give the size-dependent expansion parameters of the nonlinear thermodynamic potential. These are then used in a Taylor series expansion of the polarization at the center of the film. We show that this method is able to correctly predict experimentally observed scaling without the need for the so-called extrapolation length (which is impossible to measure experimentally). Furthermore, as no implicit correlation between the correlation length (ξ) and the coefficient of the gradient term in the LGD potential (g11) is assumed, the model thus involves fully experimentally measurable parameters and their systematic temperature dependence rather than implicit assumptions. The model finds that the Curie temperature in ultrathin films is more sensitive to epitaxial strain as compared to the polarization and that the critical thickness is strongly dependent on the “temperature-epitaxial strain” parameter space. Interestingly, while it finds that at lower temperatures the depolarization field does play a strong role in the thickness dependence as well as spatial profile of the polarization, with increasing temperature, a significant weakening of the role of depolarization fields occurs. Consequently the interface-induced suppression is lower and, as a result, the polarization profile is more homogenous at higher temperatures. This indicates that systematic temperature dependent studies are fundamental to further understanding of size effects in ferroelectrics.

Dielectric susceptibility and heat capacity of ultra-cold glasses in magnetic fields

Recent experiments demonstrated unexpected, even intriguing properties of certain glassymaterials in magnetic fields at low temperatures. We have studied the magnetic fielddependence of the static dielectric susceptibility and the heat capacity of glasses at lowtemperatures. We present a theory in which we consider the coupling of the tunnellingmotion to nuclear quadrupoles in order to evaluate the static dielectric susceptibility. In thelimit of weak magnetic field we find the resonant part of the susceptibility increasing likeB2 while for large magneticfield it behaves as 1/B. In the same manner we consider the coupling of the tunnelling motion to nuclearquadrupoles and angular momentum of tunnelling particles in order to find the heatcapacity. Our results show the Schottky peak for the angular momentum part, andB2 dependence for the nuclear quadrupole part of the heat capacity, respectively. We discusswhether or not this approach can provide a suitable explanation for such magneticproperties.

Role of the spin-pair correlation in magnetic relaxor ferroelectrics

In magnetic relaxor ferroelectrics, for the coupling interaction between the relaxor ferroelectricity and magnetic order, the dielectric susceptibility exhibits a sharp increase around the magnetic phase transition temperature. By adding an appropriate coupling term between the electrical and magnetic subsystems, which is related to the interaction of the Heisenberg spin and the order parameter field, the replica theory based on the spherical random-bond-random-field model and the mean-field theory based on the Heisenberg model are successfully applied to the magnetic relaxor ferroelectrics. We find that the fluctuation of the spin-pair correlation plays an important role in the change of dielectric susceptibility around the magnetic phase transition temperature. The obtained static dielectric susceptibility and the magnetocapacitance are in good agreement with the experimental results.

Enhanced ferroelectric phase-transition temperature in perovskite-based solid solutions

We explain the phenomena of ferroelectric phase transition temperature $T_c$ enhancement beyond the end members in perovskite solid solutions like BiMeO$_3$-PbTiO$_3$ (Me=Sc, In, etc.) is related to nonlinear and spatial correlation effects. The explanation is based on the calculation of $T_c$ in the framework of our random field theory with additional account for nonlinear effects in the above substances. We show that the maximum of $T_c$ for certain PbTiO$_3$ content takes place when coefficient of nonlinearity is positive, the value of this coefficient is found from best fit between theory and experiment. This nonlinearity coefficient is the only adjustable parameter of the theory. We show that enhancement of positive nonlinearity coefficients enhances greatly the $T_c$ maximum over its value for end members. The theory lays the foundation to calculate not only $T_c$ for above solid solutions but virtually any equilibrium and/or nonequilibrium thermodynamic characteristics such as static and dynamic dielectric susceptibility, specific heat etc as a functions of PbTiO$_3$ content, temperature, electric field and other external parameters.

Dielectric and Thermal Study of the Influence of Aerosils on the Ferroelectric SmA-SmC* Phase Transition

The influence of the dispersed aerosil particle concentration on the SmA-SmC* phase transition in CE8 liquid crystal was studied by using dielectric spectroscopy and high-resolution calorimetry. With increasing aerosil concentration x the soft mode critical slowing down anomaly is strongly suppressed, while the static dielectric susceptibility exhibits strong rounding effects. Both dielectric and heat capacity measurements show appearance of the pretransitional critical wing above the TC with increasing x.

Coulomb correlations in semiconductors

We use tight-binding theory to demonstrate how $k\ensuremath{\cdot}p$ theory and local-density approximation (LDA) energies should be corrected to incorporate Coulomb correlation corrections. Applications to the enhanced band gap for the creation of quasiparticles, to the effective mass of carriers, and to the static dielectric susceptibility are given. We find that, in the $k\ensuremath{\cdot}p$ calculations of effective masses, use of the enhanced gap is only accurate for small-gap semiconductors. In the expression for the static dielectric constant, one should use the unenhanced LDA gap for the leading term and the enhanced gap for the metallization term.

Origin of giant dielectric response in ferroelectric thin film multilayers

A thermodynamic theory of dielectric response in ferroelectric thin film multilayers is developed. The solution of Lame equation for static dielectric susceptibility has shown that susceptibility diverges at the transition temperature of the thickness induced ferroelectric phase. This divergence is shown to be the origin of the giant dielectric response observed in some multilayers. The theory gives an excellent fit to the temperature dependence of the giant susceptibility observed recently in multilayers of PbTiO3-Pb1-xLaxTiO3 (x = 0.28).