Finite Electric Field Research Articles

Improved NSPSO Algorithm and Its Application in Static Insulation Optimization of Vacuum Interrupter

For optimizing the insulation performance of the 400 V/20 kA vacuum interrupter, the non-dominated sorting particle swarm optimization algorithm is improved and combined with the finite element electric field numerical simulation method. A three-dimensional electric field optimization model is proposed to minimize the peak of the electric field intensity with the contour parameters of the electrode and shielding cover as the optimized variables. The uniformity on the electric field and the macro field enhancement factor under different stroke are analyzed to realize static insulation optimization of the vacuum interrupter.

Revealing the non-adiabatic and non-Abelian multiple-band effects via anisotropic valley Hall conduction in bilayer graphene

Many quantum materials of interest, e.g. bilayer graphene, possess a number of closely spaced but not fully degenerate bands near the Fermi level, where the coupling to the far detuned remote bands can induce Berry curvatures of the non-Abelian character in this active multiple-band manifold for transport effects. Under finite electric fields, non-adiabatic interband transition processes are expected to play significant roles in the associated Hall conduction. Here through an exemplified study on the valley Hall conduction in AB-stacked bilayer graphene, we show that the contribution arising from non-adiabatic transitions around the bands near the Fermi energy to the Hall current is not only quantitatively about an order-of-magnitude larger than the contribution due to adiabatic inter-manifold transition with the non-Abelian Berry curvatures. Due to the trigonal warping, the former also displays an anisotropic response to the orientation of the applied electric field that is qualitatively distinct from that of the latter. We further show that these anisotropic responses also reveal the essential differences between the diagonal and off-diagonal elements of the non-Abelian Berry curvature matrix in terms of their the contributions to the Hall currents. We provide a physically intuitive understanding on the origin of distinct anisotropic features from different Hall current contributions, in terms of band occupations and interband coherence. This then points to the generalization beyond the specific example of bilayer graphenes.

On the concept of an effective temperature Seebeck ratchet

In disordered semiconductors, finite electric fields can heat up the charge carrier distribution to effective temperatures that can significantly exceed the lattice temperature. Here, we demonstrate that such effective temperatures can be utilized to drive a thermoelectric generator (TEG). We use this concept in a variant of the Seebeck ratchet, which was originally introduced by Büttiker, in which out-of-phase spatial modulations of the electrostatic potential and the (effective) temperature cause a DC current. In contrast to conventional TEGs, this design utilizes only a single doped semiconductor and works in a thin film geometry. We demonstrate the concept using kinetic Monte Carlo simulations and show that the proposed concept can outperform conventional TEGs of the same material. We provide an analytical model that semiquantitatively reproduces the numerical results. Finally, we propose how such a device might be realized.

Phonon thermal transport properties of GaN with symmetry-breaking and lattice deformation induced by the electric field

Electric fields commonly exist in semiconductor structures of electronics, bringing to bear on phonon thermal transport. Also, it is a popular method to tune thermal transport in solids. In this work, phonon and thermal transport properties of GaN with wurtzite and zincblende structures in the finite electric field are investigated using first-principles calculations from the perspectives of symmetry-breaking and lattice deformation. Effects of electric field on phonon transport properties including phonon dispersion and thermal conductivity from the response of electron density distribution only and response from lattice changes are studied in zincblende GaN. It is found that the former has a small but qualitative impact on phonon dispersion relations, i.e., splitting of phonon branches, since it breaks the symmetry of zincblende lattice. While the latter affects both lattice symmetry and size, causing significant changes in phonon properties and an increase in thermal conductivity. In wurtzite GaN, space-group-conserved lattice changes in the finite electric field are studied with lattice deformation only, where thermal conductivity decreases at electric fields significantly with the increase of anisotropy, much different from the changes in zincblende GaN. This work provides a comprehensive understanding of phonon thermal transport properties in GaN in the finite electric field, which promises to benefit phonon transport tuning and provide a reference for thermal management in GaN-based information and power electronics.

High capacitive performance at moderate operating field in (Bi0.5Na0.5)TiO3-based dielectric ceramics via synergistic effect of site engineering strategy

Despite a great deal of research efforts on the search for high recoverable energy storage density (Wrec) at high breakdown strength, high voltage operating security and high cost of associated insulation technology are the major barriers to limiting the actual applications of advanced pulse power capacitors, and thus have increased the requirement on the electrostatic energy storage at finite electric field level. Herein, we propose an effective strategy on tailoring energy storage performance under moderate driving field via the synergistic effect of site engineering in (Bi0.5Na0.5)TiO3-based ceramics. Based on extensive experimental researches, a large Wrec (2.78–3.46 J/cm3) and a high efficiency (80–90%) under moderate electric field (<250 kV/cm) have been synchronously attainted in the optimized relaxor ferroelectric ceramics, generating a high energy storage coefficient that exceeds most previously reported dielectric bulk ceramics. Besides, the optimized ceramic possesses good thermal stability (25–200 °C), strong fatigue endurance (after 2 *105 cycles) and outstanding frequency stability (1–500 Hz). More importantly, high discharge density of 1.15 J/cm3, large power density of 65.6 MW/cm3, and ultra-short discharge time of 72 ns with excellent fatigue endurance are also simultaneously achieved. This contribution not only develops a practical lead-free candidate material for electrostatic energy storage, but also offers a feasible approach to design high-efficiency lead-free dielectrics for low electric field high-energy storage applications using synergistic effect of site engineering.

A Feasibility Study of Representing the Electric Field in Perforated Rectangular Enclosure Based on Modal Expansion Method

In this letter, the modal expansion method (MEM) based on orthogonal property of resonant modes is utilized to represent the electric field in a perforated rectangular enclosure. By applying inner product between electric field obtained by finite integration technique (FIT) and transverse electric (TE) or transverse magnetic (TM) mode field respectively, the mode coefficient for each contributing mode can be accurately obtained over the modal bandwidth. Then the electric field in a perforated rectangular enclosure can be represented by the superposition of contributing mode fields multiplying their corresponding mode coefficients. And the figures of mode coefficients' magnitude and phase give an intuitive understanding of the internal electric field. The statistical properties of internal electric fields obtained by FIT and MEM are in agreement with each other. MEM provides a better understanding of the electromagnetic environment in perforated enclosures.

Berry curvature, orbital magnetization, and Nernst effect in biased bilayer WSe2

A valley-contrasting Berry curvature in bilayer transition metal dichalcogenides with spin-orbit coupling can generate valley magnetization when the inversion symmetry is broken, for example, by an electric field, regardless of time-reversal symmetry. A nontrivial Berry curvature can also lead to anomalous transport responses, such as the anomalous Hall effect and the anomalous Nernst effect. Applied to a bilayer $\mathrm{W}{\mathrm{Se}}_{2}$, an electric field can tune the Berry curvature and orbital magnetic moment, which has important consequences for the orbital magnetization and the anomalous Nernst responses. The orbital magnetization and its two contributions, one due to the magnetic moment and one due to the Berry curvature, are calculated and interpreted in terms of opposite circulating currents of the bands in the two layers. The valley anomalous Nernst coefficient and spin Nernst coefficient are also calculated. We find that a finite electric field leads to peaks and dips in the Nernst responses that have the signs of the Berry curvatures of the bands and are proportional to their magnitudes; it also enhances the valley Nernst responses. These experimentally verifiable findings may be promising for caloritronic applications.

Electric Auxetic Effect in Piezoelectrics.

Auxetic materials are characterized by a negative Poisson's ratio that they expand laterally in the directions perpendicular to the applied stretching stress and vice versa. Piezoelectrics will change their dimensions when exposed to an external electric field. Here we introduce the concept of the "electric auxetic effect": electric auxetic materials will contract or expand in all dimensions in response to an electric field. Such unusual piezoelectric response driven by an electric field is a close analogy to the auxetic effect driven by a stress field. A key feature of electric auxetic materials is that their longitudinal and transverse piezoelectric coefficients are of the same sign. We demonstrate using first-principles calculations that the Pca2_{1} orthorhombic phase of ferroelectric HfO_{2} exhibits both the negative longitudinal piezoelectric effect and the electric auxetic effect. The unusual negative longitudinal piezoelectric effect arises unexpectedly from the domination of the negative internal-strain contribution over the positive clamped-ion contribution, a character often found in van der Waals solids. We confirm a few more electric auxetic materials with finite electric field calculations by screening through a first-principles-based database of piezoelectrics.

Deep brain stimulation of terminating axons

BackgroundDeep brain stimulation (DBS) of the subthalamic region is an established treatment for the motor symptoms of Parkinson's disease. Several types of neural elements reside in the subthalamic region, including subthalamic nucleus (STN) neurons, fibers of passage, and terminating afferents. Recent studies suggest that direct activation of a specific population of subthalamic afferents, known as the hyperdirect pathway, may be responsible for some of the therapeutic effects of subthalamic DBS. ObjectiveThe goal of this study was to quantify how axon termination affects neural excitability from DBS. We evaluated how adjusting different stimulation parameters influenced the relative excitability of terminating axons (TAs) compared to fibers of passage (FOPs). MethodsWe used finite element electric field models of DBS, coupled to multi-compartment cable models of axons, to calculate activation thresholds for populations of TAs and FOPs. These generalized models were used to evaluate the response to anodic vs. cathodic stimulation, with short vs. long stimulus pulses.Results: Terminating axons generally exhibited lower thresholds than fibers of passage across all tested parameters. Short pulse widths accentuated the relative excitability of TAs over FOPs.Conclusion(s): Our computational results demonstrate a hyperexcitability of terminating axons to DBS that is robust to variation in the stimulation parameters, as well as the axon model parameters.

Theory of wave-packet transport under narrow gaps and spatial textures: Nonadiabaticity and semiclassicality

We generalise the celebrated semiclassical wavepacket approach from the adiabatic to the non-adiabatic regime. A unified description covering both of these regimes is particularly desired for systems with spatially varying band structures where band gaps of various sizes are simultaneously present, e.g. in moir\'{e} patterns. For a single wavepacket, alternative to the previous derivation by Lagrangian variational approach, we show that the same semiclassical equations of motion can be obtained by introducing a spatial-texture-induced force operator similar to the Ehrenfest theorem. For semiclassically computing the current, the ensemble of wavepackets based on adiabatic dynamics is shown to well correspond to a phase-space fluid for which the fluid's mass and velocity are two distinguishable properties. This distinction is not inherited to the ensemble of wavepackets with the non-adiabatic dynamics. We extend the adiabatic kinetic theory to the non-adiabatic regime by taking into account decoherence, whose joint action with electric field favours certain form of inter-band coherence. The steady-state density matrix as a function of the phase-space variables is then phenomenologically obtained for calculating the transport current. The result, applicable with a finite electric field, expectedly reproduces the known adiabatic limit by taking the electric field to be infinitesimal, and therefore attains a unified description from the adiabatic to the non-adiabatic situations.

Low-Frequency Dielectric Response of Tetragonal Perovskite CH3NH3PbI3

The dielectric properties of tetragonal hybrid perovskite CH3NH3PbI3 are studied through molecular dynamics at a temperature of 300 K in the presence of a finite electric field. The high-frequency dielectric constant ε∞ is found to be 4.5 along the a axis and 4.7 along the c axis. The values of the respective static dielectric constants ε0 are 43 ± 1 and 53 ± 3, much larger than the value of ∼25 pertaining to the orthorhombic phase, in which the organic cations cannot rotate. At frequencies below 3 cm-1, we observe a significant increase in ε0 by ∼23 (a axis) and ∼30 (c axis) compared to a vibrational approach, which does not account for the reorientation of the molecular units. The decomposition shows that the reorientation of the organic cations accounts for an increase of only ∼10. An increase of similar size results from the displacement of the cations within the cages of the lattice. The dominant contribution is found to arise from lattice vibrations coupled to the motion of the organic cations.

Linear Versus Nonlinear Electro-Optic Effects in Materials.

Two schemes are proposed to compute the nonlinear electro-optic (EO) tensor for the first time. In the first scheme, we compute the linear EO tensor of the structure under a finite electric field, while we compute the refractive index of the structure under a finite electric field in the second scheme. Such schemes are applied to Pb(Zr,Ti)O_{3} and BaTiO_{3} ferroelectric oxides. It is found to reproduce a recently observed feature, namely, why Pb(Zr_{0.52}Ti_{0.48})O_{3} adopts a mostly linear EO response while BaTiO_{3} exhibits a strongly nonlinear conversion between electric and optical properties. Furthermore, the atomistic insight provided by the proposed abinitio scheme reveals the origin of such qualitatively different responses, in terms of the field-induced behavior of the frequencies of some phonon modes and of some force constants.

Electroelastic Response of Isotropic Dielectric Elastomer Composites with Deformation-Dependent Apparent-Permittivity Matrix

AbstractThis paper puts forth an approximate yet accurate free energy for the elastic dielectric response—under finite deformations and finite electric fields—of non-percolative dielectric elastomer composites made out of a non-Gaussian dielectric elastomer matrix with deformation-dependent apparent permittivity isotropically filled with nonlinear elastic dielectric particles that may exhibit polarization saturation. While the proposed free energy applies in its most general form to arbitrary isotropic non-percolative microstructures, closed-form specializations are recorded for the practically relevant cases of rigid or liquid-like spherical particles. The proposed free energy is exact by construction in the asymptotic context of small deformations and moderate electric fields and is shown to remain accurate for arbitrary large deformations and electric fields via comparisons with full-field finite-element simulations. The proposed constitutive model is deployed to probe the electrostriction response of these dielectric elastomer composites and corresponding results reveal that their elastic dielectric response strongly depends on the deformation-dependent apparent permittivity of the matrix they comprise.

The electromagnetic field effects in in-out and in-in formalisms

In this work, we compare the effects of electromagnetic (EM) fields on strong coupling systems in two formalisms: in-out and in-in, which are different from each other when a finite electric field is present. The chiral effective Nambu–Jona-Lasinio model is adopted for this study, and two kinds of EM field distributions are considered: pure electric field and parallel EM (PEM) field with equal electric and magnetic components. For both distributions, we find that the results of in-out and in-in formalisms start to diverge when the electric field (qE)1/2 is larger than the chiral effective mass M=[m2+(π0)2]1/2, that is, when the Schwinger pair production mechanism becomes important. Besides, the chiral restorations are stiffer in the in-in formalism, especially the transition shifts to first-order instead of the second-order for the PEM field. The neutral collective modes are also explored accordingly: For the PEM field, more precise calculations show nonmonotonic features of their pole masses due to parity mixing, and then the Goldstone-like mode is found to be noneffective at the end of chiral rotation because of chiral anomaly.

Switchable two-dimensional electron gas based on ferroelectric Ca: SrTiO3

Two-dimensional electron gases (2DEGs) can form at the surface of oxides and semiconductors or in carefully designed quantum wells and interfaces. Depending on the shape of the confining potential, 2DEGs may experience a finite electric field, which gives rise to relativistic effects such as the Rashba spin-orbit coupling. Although the amplitude of this electric field can be modulated by an external gate voltage, which in turn tunes the 2DEG carrier density, sheet resistance and other related properties, this modulation is volatile. Here, we report the design of a ''ferroelectric'' 2DEG whose transport properties can be electrostatically switched in a non-volatile way. We generate a 2DEG by depositing a thin Al layer onto a SrTiO$_3$ single crystal in which 1 percent of Sr is substituted by Ca to make it ferroelectric. Signatures of the ferroelectric phase transition at 25 K are visible in the Raman response and in the temperature dependences of the carrier density and sheet resistance that shows a hysteretic dependence on electric field as a consequence of ferroelectricity. We suggest that this behavior may be extended to other oxide 2DEGs, leading to novel types of ferromagnet-free spintronic architectures.

ABINIT: Overview and focus on selected capabilities.

abinit is probably the first electronic-structure package to have been released under an open-source license about 20 years ago. It implements density functional theory, density-functional perturbation theory (DFPT), many-body perturbation theory (GW approximation and Bethe-Salpeter equation), and more specific or advanced formalisms, such as dynamical mean-field theory (DMFT) and the "temperature-dependent effective potential" approach for anharmonic effects. Relying on planewaves for the representation of wavefunctions, density, and other space-dependent quantities, with pseudopotentials or projector-augmented waves (PAWs), it is well suited for the study of periodic materials, although nanostructures and molecules can be treated with the supercell technique. The present article starts with a brief description of the project, a summary of the theories upon which abinit relies, and a list of the associated capabilities. It then focuses on selected capabilities that might not be present in the majority of electronic structure packages either among planewave codes or, in general, treatment of strongly correlated materials using DMFT; materials under finite electric fields; properties at nuclei (electric field gradient, Mössbauer shifts, and orbital magnetization); positron annihilation; Raman intensities and electro-optic effect; and DFPT calculations of response to strain perturbation (elastic constants and piezoelectricity), spatial dispersion (flexoelectricity), electronic mobility, temperature dependence of the gap, and spin-magnetic-field perturbation. The abinit DFPT implementation is very general, including systems with van der Waals interaction or with noncollinear magnetism. Community projects are also described: generation of pseudopotential and PAW datasets, high-throughput calculations (databases of phonon band structure, second-harmonic generation, and GW computations of bandgaps), and the library libpaw. abinit has strong links with many other software projects that are briefly mentioned.

Analysis of a mode III interface crack in a piezoelectric bimaterial based on the dielectric breakdown model

A mode III electrically conductive crack between two different piezoelectric materials under the action of anti-plane mechanical and in-plane electric loadings is analyzed. The strip dielectric breakdown (DB) model, which is free from the electric field singularity, is developed for this crack. According to this model, the electric field along a DB-zone situated in continuation of a crack is assumed to be equal to the electric breakdown strength. The DB-zone lengths are found from the condition of a finite electric field at the end point of such a zone. Using special representations of field variables via sectionally analytic functions, an inhomogeneous combined Dirichlet–Riemann boundary value problem is formulated and solved analytically. Explicit expressions for the shear stress, the electric field and the crack faces’ sliding displacement jump are derived. The stress intensity factor is determined as well. The dependencies of the mentioned values on the magnitude of the external electromechanical loading are presented.

High energy-storage density under low electric field in lead-free relaxor ferroelectric film based on synergistic effect of multiple polar structures

Although dielectric energy-storage devices are frequently used in high voltage level, high voltage risk and large cost of insulation technology have increased the demand on the energy-storage devices at finite electric field strength. Here, we propose an approach on enhancing energy-storage density (Wrec) of dielectric materials under low electric field by utilizing the synergistic effect of multiple polar structures. An excellent Wrec of 40.8 J/cm3 under only 1500 kV/cm is obtained in 0.5(Bi0.5Na0.5)TiO3-0.5Bi(Zn0.5Zr0.5)O3 (BNT-0.5BZZ) relaxor ferroelectric film, resulting in a large energy-storage coefficient that exceeds most of the dielectric materials. It is revealed that the addition of BZZ to BNT matrix induces the distortion of the octahedral [TiO6] in the lattice and establishes a coexistence structure of ferroelectric domains and polar nanoregions (PNRs), which enlarges the discrepancy between the maximum polarization and the remnant polarization. Besides, the BNT-0.5BZZ film shows strong fatigue endurance after 5 × 107 cycles and good thermal stability in a wide temperature range (25–145 °C). This work not only develops a promising lead-free candidate for low electric field electrostatic energy storage, but also, more importantly, opens up a new door to systematically design lead-free dielectrics for high-energy-storage applications at low electric field via microstructural engineering.

Simulating Electrochemical Systems by Combining the Finite Field Method with a Constant Potential Electrode.

A better understanding of interfacial mechanisms is needed to improve the performances of electrochemical devices. Yet, simulating an electrode surface at fixed electrolyte composition remains a challenge. Here, we apply a finite electric field to a single electrode held at constant potential and in contact with an aqueous ionic solution, using classical molecular dynamics. The polarization yields two electrochemical interfaces on opposite sides of the same metal slab. The net charge on one electrode surface is the opposite of the net charge on the other, maintaining overall charge neutrality of the metal. The electrode surface charge fluctuations are compensated by the adsorption of ions from the electrolyte, forming a pair of electric double layers with aligned dipoles. This opens the way towards the efficient simulation of electrochemical interfaces using any flavor of molecular dynamics, from classical to first-principles-based methods.

Asymptotically-flat Black Hole in EiBI-Born-Infeld Theory

We study static spherically-symmetric solutions of charged Eddington-inspired Born-Infeld (EiBI) theory of gravity in asymptotically-flat D-dimensional spacetime. We consider nonlinear electrodynamics for the matter fields. In this particular work, the nonlinear theory we specifically consider is the Born-Infeld electrodynamics. We found that metrics can not be integrated in general dimensions. In order to solve it, we focus on studying on the case α ≡ 4κb2 = 1. We study the electric fields. We find that this theory produces finite electric fields for any dimension. In Ricci scalar calculations, we find only point singularity.