Electrostatic Boundary Conditions Research Articles

Liquid-Crystal-Like Dynamic Transition in Ferroelectric-Dielectric Superlattices.

Nanostructured ferroelectrics display exotic multidomain configurations resulting from the electrostatic and elastic boundary conditions they are subject to. While the ferroelectric domains appear frozen in experimental images, atomistic second-principles studies suggest that they may become spontaneously mobile upon heating, with the polar order melting in a liquidlike fashion. Here, we run molecular dynamics simulations of model systems (PbTiO_{3}/SrTiO_{3} superlattices) to study the unique features of this transformation. Most notably, we find that the multidomain state loses its translational and orientational orders at different temperatures, resembling the behavior of liquid crystals and yielding an intermediate hexaticlike phase. Our simulations reveal the mechanism responsible for the melting and allow us to characterize the stochastic dynamics in the hexaticlike phase: we find evidence that it is thermally activated, with domain reorientation rates that grow from tens of gigahertzs to terahertzs in a narrow temperature window.

Artificial Domain Patterning in Ultrathin Ferroelectric Films via Modifying the Surface Electrostatic Boundary Conditions.

Nanoscale spatially controlled modulation of the properties of ferroelectrics via artificial domain pattering is crucial to their emerging optoelectronics applications. New patterning strategies to achieve high precision and efficiency and to link the resultant domain structures with device functionalities are being sought. Here, we present an epitaxial heterostructure of SrRuO3/PbTiO3/SrRuO3, wherein the domain configuration is delicately determined by the charge screening conditions in the SrRuO3 layer and the substrate strains. Chemical etching of the top SrRuO3 layer leads to a transition from in-plane a domains to out-of-plane c domains, accompanied by a giant (>105) modification in the second harmonic generation response. The modulation effect, coupled with the plasmonic resonance effect from SrRuO3, enables a highly flexible design of nonlinear optical devices, as demonstrated by a simulated split-ring resonator metasurface. This domain patterning strategy may be extended to more thin-film ferroelectric systems with domain stabilities amenable to electrostatic boundary conditions.

Reversible Optical Control of Polarization in Epitaxial Ferroelectric Thin Films.

Light is an effective tool to probe the polarization and domain distribution in ferroelectric materials passively, that is, non-invasively, for example, via optical second harmonic generation (SHG). With the emergence of oxide electronics, there is now a strong demand to expand the role of light toward active control of the polarization. In this work, optical control of the ferroelectric polarization is demonstratedin prototypical epitaxial PbZrx Ti1-x O3 (PZT)-based heterostructures. This is accomplished in three steps, using above-bandgap UV light, while tracking the response of the polarization with optical SHG. First, it is found that UV-light exposure induces a transient enhancement or suppression of the ferroelectric polarization in films with an upward- or downward-oriented polarization, respectively. This behavior is attributed to a modified charge screening driven by the separation of photoexcited charge carriers at the Schottky interface of the ferroelectric thin film. Second, by taking advantage of this optical handle on electrostatics, remanent optical poling from a pristine multi-domain into a single-domain configuration is accomplished. Third, via thermal annealing or engineered electrostatic boundary conditions, a complete reversibility of the optical poling is further achieved. Hence, this work paves the way for the all-optical control of the spontaneous polarization in ferroelectric thinfilms.

Extraction simulation of a carbon ion beam with particle distribution in a three-electrode system

In this study, the conventional simulations on particle beam extraction was performed using a plasma-based or particle-in-cell method. Subsequently, its validity was verified through comparisons with the conventional simulation results using the finite element method. A three-dimensional finite element method was employed as a tool to simulate the motion of charged particles that are constrained by a specific geometric structure and subjected to various specified forces. This study investigated the features of carbon-ion beam extraction and the effects of the particle beam conditions on the three-electrode extraction system of a gaseous ion source by simulating the ion beam properties using the COMSOL Multiphysics software package. The AC/DC module in this software package was used to simulate the electrostatic field, whereas the Particle Tracing Module in the package was used to extract the ion beam. This package was used to analyze the beam trajectories and their dependence on particle properties under electrostatic boundary conditions, using bidirectionally coupled particle tracing. Consequently, Twiss parameters and root mean square (rms) beam emittance of the particle beam were calculated for the particle positions and divergence in their phase space. The feasibility of the particle beam extraction performed using this approach was confirmed through comparisons with the simulation results obtained using other ion source extraction programs. Furthermore, the effectiveness of a more user-friendly multiphysics based simulation was evaluated based on the rms beam emittance.

Electrostatic Boundary Conditions and (Electro)chemical Interface Stability

AbstractInterface stability is a key factor for stable operation of electronic and electrochemical devices. This contribution introduces an approach for the operando analysis of interfaces using photoelectron spectroscopy employing a solid oxide electrochemical cell. The combined chemical and electronic information provided by the experiment reveals that not only chemical but also electrostatic boundary conditions are essential for interface stability. The approach is demonstrated using (anti‐)ferroelectric (Pb,La)(Zr,Sn,Ti)O3 dielectrics.

Transient Analysis of the Electro-Osmotic Flow of Multilayer Immiscible Maxwell Fluids in an Annular Microchannel

This work investigates the transient multilayer electro-osmotic flow of viscoelastic fluids through an annular microchannel. The dimensionless mathematical model of multilayer flow is integrated by the linearized Poisson-Boltzmann equation, the Cauchy momentum equation, the rheological Maxwell model, initial conditions, and the electrostatic and hydrodynamic boundary conditions at liquid-liquid and solid-liquid interfaces. Although the main force that drives the movement of fluids is due to electrokinetic effects, a pressure gradient can also be added to the flow. The semi-analytical solution for the electric potential distribution and velocity profiles considers analytical techniques as the Laplace transform method, with numerical procedures using the inverse matrix method for linear algebraic equations and the concentrated matrix exponential method for the inversion of the Laplace transform. The results presented for velocity profiles and velocity tracking at the transient regime reveal an interesting oscillatory behavior that depends on elastic fluid properties via relaxation times. The time required for the flow to reach steady-state is highly dependent on the viscosity ratios and the dimensionless relaxation times. In addition, the influence of other dimensionless parameters on the flow as the electrokinetic parameters, zeta potentials at the walls, permittivity ratios, ratio of pressure forces to electro-osmotic forces, number of fluid layers, and annular thickness are investigated. The findings of this study have significant implications for the precise control of parallel fluid transport in microfluidic devices for flow-focusing applications.

A theory for the stabilization of polar crystal surfaces by a liquid environment.

Polar crystal surfaces play an important role in the functionality of many materials and have been studied extensively over many decades. In this article, a theoretical framework is presented that extends existing theories by placing the surrounding solution environment on an equal footing with the crystal itself; this is advantageous, e.g., when considering processes such as crystal growth from solution. By considering the polar crystal as a stack of parallel plate capacitors immersed in a solution environment, the equilibrium adsorbed surface charge density is derived by minimizing the free energy of the system. In analogy to the well-known diverging surface energy of a polar crystal surface at zero temperature, for a crystal in solution it is shown that the "polar catastrophe" manifests as a diverging free energy cost to perturb the system from equilibrium. Going further than existing theories, the present formulation predicts that fluctuations in the adsorbed surface charge density become increasingly suppressed with increasing crystal thickness. We also show how, in the slab geometry often employed in both theoretical and computational studies of interfaces, an electric displacement field emerges as an electrostatic boundary condition, the origins of which are rooted in the slab geometry itself, rather than the use of periodic boundary conditions. This aspect of the work provides a firmer theoretical basis for the recent observation that standard "slab corrections" fail to correctly describe, even qualitatively, polar crystal surfaces in solution.

A transmission electron microscopy study of low-strain epitaxial BaTiO3 grown onto NdScO3

Ferroelectric materials exhibit a strong coupling between strain and electrical polarization. In epitaxial thin films, the strain induced by the substrate can be used to tune the domain structure. Substrates of rare-earth scandates are sometimes selected for the growth of ferroelectric oxides because of their close lattice match, which allows the growth of low-strain dislocation-free layers. Transmission electron microscopy (TEM) is a frequently used technique for investigating ferroelectric domains at the nanometer-scale. However, it requires to thin the specimen down to electron transparency, which can modify the strain and the electrostatic boundary conditions. Here, we have investigated a 320 nm thick epitaxial layer of BaTiO3 grown onto an orthorhombic substrate of NdScO3 with interfacial lattice strains of −0.45% and −0.05% along the two in-plane directions. We show that the domain structure of the layer can be significantly altered by TEM sample preparation depending on the orientation and the geometry of the lamella. In the as-grown state, the sample shows an anisotropic a/c ferroelastic domain pattern in the direction of largest strain. If a TEM lamella is cut perpendicular to this direction so that strain is released, a new domain pattern is obtained, which consists of bundles of thin horizontal stripes parallel to the interfaces. These stripe domains correspond to a sheared crystalline structure (orthorhombic or monoclinic) with inclined polarization vectors and with at least four variants of polarization. The stripe domains are distributed in triangular-shaped 180° domains where the average polarization is parallel to the growth direction. The influence of external electric fields on this domain structure was investigated using in situ biasing and dark-field imaging in TEM.

Enhanced transport of ions by tuning surface properties of the nanochannel.

Motivated by recent observations of anomalously large deviations of the conductivity currents in confined systems from the bulk behavior, we revisit the theory of ion transport in parallel-plate channels and also discuss how the wettability of a solid and the mobility of adsorbed surface charges impact the transport of ions. It is shown that depending on the ratio of the electrostatic disjoining pressure to the excess osmotic pressure at the walls two different regimes occur. In the thick channel regime this ratio is small and the channel effectively behaves as thick, even when the diffuse layers strongly overlap. The latter is possible for highly charged channels only. In the thin channel regime the disjoining pressure is comparable to the excess osmotic pressure at the wall, which implies relatively weakly charged walls. We derive simple expressions for the mean conductivity of the channel in these two regimes, highlighting the role of electrostatic and electrohydrodynamic boundary conditions. Our theory provides a simple explanation of the high conductivity observed experimentally in hydrophilic channels, and allows one to obtain rigorous bounds on its attainable value and scaling with salt concentration. Our results also show that further dramatic amplification of conductivity is possible if hydrophobic slip is involved, but only in the thick channel regime provided the walls are sufficiently highly charged and most of the adsorbed charges are immobile. However, for weakly charged surfaces the massive conductivity amplification due to hydrodynamic slip is impossible in both regimes. Interestingly, in this case the moderate slip-driven contribution to conductivity can monotonously decrease with the fraction of immobile adsorbed charges. These results provide a framework for tuning the conductivity of nanochannels by adjusting their surface properties and bulk electrolyte concentrations.

Theoretical study of intrinsic defects in cubic silicon carbide 3C -SiC

Using the local moment counter charge (LMCC) method to accurately represent the asymptotic electrostatic boundary conditions within density functional theory supercell calculations, we present a comprehensive analysis of the atomic structure and energy levels of point defects in cubic silicon carbide ($3C$-SiC). Finding that the classical long-range dielectric screening outside the supercell induced by a charged defect is a significant contributor to the total energy. we describe and validate a modified Jost screening model to evaluate this polarization energy. This leads to bulk-converged defect levels in finite size supercells. With the LMCC boundary conditions and a standard Perdew-Burke-Ernzerhof (PBE) exchange correlation functional, the computed defect level spectrum exhibits no band gap problem: the range of defect levels spans $\ensuremath{\sim}2.4\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$, an effective defect band gap that agrees with the experimental band gap. Comparing with previous literature, our LMCC-PBE defect results are in consistent agreement with the hybrid-exchange functional results of Oda et al. [J. Chem. Phys. 139, 124707 (2013)] rather than their PBE results. The difference with their PBE results is attributed to their use of a conventional jellium approximation rather than the more rigorous LMCC approach for handling charged supercell boundary conditions. The difference between standard dft and hybrid functional results for defect levels lies not in a band gap problem but rather in solving a boundary condition problem. The LMCC-PBE entirely mitigates the effect of the band gap problem on defect levels. The more computationally economical PBE enables a systematic exploration of $3C$-SiC defects, where, most notably, we find that the silicon vacancy undergoes Jahn-Teller-induced distortions from the previously assumed ${T}_{d}$ symmetry, and that the divacancy, like the silicon vacancy, exhibits a site-shift bistability in $p$-type conditions.

Levenberg–Marquardt Algorithm for Acousto-Electric Tomography based on the Complete Electrode Model

The inverse problem in acousto-electric tomography concerns the reconstruction of the electric conductivity in a body from knowledge of the power density function in the interior of the body. This interior power density results from currents prescribed at boundary electrodes, and it can be obtained through electro-static boundary measurements together with auxiliary acoustic probing. Previous works on acousto-electric tomography used the continuum model for the electrostatic boundary conditions; however, from Electrical Impedance Tomography, it is known that the complete electrode model is much more realistic and accurate. In this paper, the inverse problem of acousto-electric tomography is posed using the (smoothened) complete electrode model, and a reconstruction method based on the Levenberg–Marquardt iteration is formulated in appropriate function spaces. This results in a system of partial differential equations to be solved in each step. To increase the computational efficiency and stability, a strategy based on both the complete electrode model and the continuum model is proposed. The method is implemented numerically for a two-dimensional scenario, and the algorithm is tested on two different numerical phantoms, a heart and lung model and a human brain model. Several numerical experiments are carried out confirming the feasibility, accuracy and stability of the developed method.

Tailoring interfacial properties in CaVO3 thin films and heterostructures with SrTiO3 and LaAlO3 : A DFT+DMFT study

In this paper we use density functional theory combined with dynamical mean-field theory (DFT+DMFT) to study interface effects between thin films of the correlated metal CaVO$_3$ and the two typical substrate materials SrTiO$_3$ and LaAlO$_3$. We find that the CaVO$_3$/SrTiO$_3$ interface has only a marginal influence on the CaVO$_3$ thin film, with the dominant effect being the (bulklike) epitaxial strain imposed by the large lattice mismatch, rendering the CaVO$_3$ film insulating due to the enhanced orbital polarization related to the strong level splitting between the t$_{\mathrm{2g}}$ orbitals. In contrast, at the polar CaVO$_3$/LaAlO$_3$ interface, the presence of the interface can have a huge effect on the thin film properties, depending both on the specific interface termination as well as the specific boundary conditions imposed by the multilayer geometry. We compare three different approaches to model the interface between the correlated metal CaVO$_3$ and the band insulator LaAlO$_3$, which all impose a different set of (electrostatic) boundary conditions on the electronic structure. The spectral properties obtained from our calculations reveal a strong influence of the supercell geometry, ranging from bulklike to highly doped and structurally distorted phases, indicating a potential tunability of the interfacial properties via multilayer engineering.

A perspective on electrode engineering in ultrathin ferroelectric heterostructures for enhanced tunneling electroresistance

The combination of ferroelectricity and quantum tunneling enables the tantalizing possibility of next-generation nonvolatile memories based on ferroelectric tunnel junctions (FTJs). In the last two decades, significant progress has been achieved in the understanding of FTJs in terms of the role of the critical thickness for ferroelectricity, interface-related factors that yield an enhanced tunneling electroresistance effect, as well exploiting the combination of magnetism and ferroelectricity to realize multiferroic or magnetoelectric tunnel junctions. One key ingredient in the successful design of FTJs is the type and nature of the electrode used—indeed device performance strongly hinges on the ability to precisely tune and modulate the electrostatic boundary conditions. This perspective presents an overview of the experimental state of the art in electrode engineering for FTJs. We discuss related governing factors and methods for various electrode-FTJ combinations, highlighting and comparing the advantages and weaknesses for each system. Finally, we also reveal the challenges and identify the opportunities for the future development of FTJs. In summary, we aim to provide significant insights into electrode engineering of high-quality FTJs with excellent tunneling electroresistance performance.

Full Control of Polarization in Ferroelectric Thin Films Using Growth Temperature to Modulate Defects

AbstractDeterministic control of the intrinsic polarization state of ferroelectric thin films is essential for device applications. Independently of the well‐established role of electrostatic boundary conditions and epitaxial strain, the importance of growth temperature as a tool to stabilize a target polarization state during thin film growth is shown here. Full control of the intrinsic polarization orientation of PbTiO3 thin films is demonstrated—from monodomain up, through polydomain, to monodomain down as imaged by piezoresponse force microscopy—using changes in the film growth temperature. X‐ray diffraction and scanning transmission electron microscopy reveal a variation of c‐axis related to out‐of‐plane strain gradients. These measurements, supported by Ginzburg–Landau–Devonshire free energy calculations and Rutherford backscattering spectroscopy, point to a defect mediated polarization gradient initiated by a temperature dependent effective built‐in field during growth, allowing polarization control not only under specific growth conditions, but ex‐situ, for subsequent processing and device applications.

Soft Matter Mechanics and the Mechanisms Underpinning the Infrared Vision of Snakes

Summary Pit-bearing snakes (vipers, pythons, and boas) have the extraordinary ability to “see” and accurately locate their prey and predators in total darkness. These animals use the infrared radiation emanating from objects that are warmer relative to the background environment to form a thermal image. Although enormous progress has been made to identify the key physiological features that enable the infrared vision of these snakes and a few other animals, the precise thermoelectric transduction mechanism that mediates the conversion of infrared heat to processable electrical signals has remained elusive. In this work, we quantitatively outline how cells in the snake's pit membrane organ act as apparent pyroelectric materials and convert infrared radiation into electrical signals. Despite the exceptional simplicity of our proposed mechanism and model, we are able to explain many central experimental results pertaining to the transduction process.

Experimental studies on the effect of electrostatic boundary conditions and frequency on the performance of a trigrid electrostatic coalescer

In this work, we explore the effect of electric field distribution and incorporating two different frequencies in two different regions of a trigrid coalescer on the electrocoalescer efficiency. Specifically grounding water at the bottom of the coalescer is found to be effective. The effect of frequency on emulsion breakup is essentially due to chaining tendency of water droplets at high frequencies. The reason for this could be reduced electrostatic force between drops at higher frequencies due to higher capacitance of interstitial oil film between two droplets. Moreover, at low frequencies significant bouncing of drops from interface is observed, leading to lower separation rates.

Omnidirectional transport and navigation of Janus particles through a nematic liquid crystal film

We create controllable active particles in the form of metal-dielectric Janus colloids which acquire motility through a nematic liquid crystal film by transducing the energy of an imposed perpendicular AC electric field. We achieve complete command over trajectories by varying field amplitude and frequency, piloting the colloids at will in the plane spanned by the axes of the particle and the nematic. The underlying mechanism exploits the sensitivity of electro-osmotic flow to the asymmetries of the particle surface and the liquid-crystal defect structure. We present a calculation of the dipolar force density produced by the interplay of the electric field with director anchoring and the contrasting electrostatic boundary conditions on the two hemispheres, that accounts for the dielectric-forward (metal-forward) motion of the colloids due to induced puller (pusher) force dipoles. These findings open unexplored directions for the use of colloids and liquid crystals in controlled transport, assembly and collective dynamics.

Stability of charged sulfur vacancies in 2D and bulk MoS2 from plane-wave density functional theory with electrostatic corrections

Two-dimensional (2D) semiconducting transition metal dichalcogenides such as MoS$_2$ have attracted extensive research interests for potential applications in optoelectronics, spintronics, photovoltaics, and catalysis. To harness the potential of these materials for electronic devices requires a better understanding of how defects control the carrier concentration, character, and mobility. Utilizing a correction scheme developed by Freysoldt and Neugebauer to ensure the appropriate electrostatic boundary conditions for charged defects in 2D materials, we perform density functional theory calculations to compute formation energies and charge transition levels associated with sulfur vacancies in monolayer and layered bulk MoS$_2$. We investigate the convergence of these defect properties with respect to vacuum spacing, in-plane supercell dimensions, and different levels of theory. We also analyze the electronic structures of the defects in different charge states to gain insights into the effect of defects on bonding and magnetism. We predict that both vacancy structures undergo a Jahn-Teller distortion, which helps stabilize the sulfur vacancy in the $-1$ charged state.

Experimental studies on the performance and analysis of an electrostatic coalescer under different electrostatic boundary conditions

The rate of coalescence in an electrocoalescer, which governs the residence time, sensitively depends upon the electrostatic conditions as well as the electrodes arrangement. An experimental study of a cylindrical electrocoalescer vessel for two different modes of operation is reported in this work. The design of the coalescer is optimized for the residence time and water separation. To justify the experimental findings and to unravel more physical insights, the single phase electrostatics is simulated using COMSOL Multiphysics software. The volume of the emulsion under the influence of 0.3–1.0kV/cm electric field, calculated using COMSOL, correlates well with the water separation. Moreover, the simultaneous measurement of current suggests an ordering, that leads to an increase in size of the droplets and settling of drops in the normal operation. On the other hand, chaining and shorting leads to surge in current at high fields and seems to prevent water separation.

Microwave multichannel tunable filter based on transmission and reflection properties of 1D magnetized plasma photonic crystal heterostructures

The transmission and reflection properties of one-dimensional magnetized plasma photonic crystal heterostructures have been theoretically investigated. The proposed structure is composed of two sub-PCs containing magnetized cold plasma and lossless dielectric materials. The optical properties of the structure are suitable for multichannel tunable transmission filter and omnidirectional band-stop filters applications. The investigations have been carried out by applying transfer matrix method and employing electrostatic boundary conditions for TE and TM wave, respectively, in microwave region. The transmission spectra of the proposed structure possess external magnetic field-dependent 3N − 3 comb-like resonant peaks called as transmission channels for period number (N) > 1. Due to multiple interactions between forward and backward decaying evanescent waves in plasma and dielectric layers, respectively, 3N − 3 transmission channels are found in defect-free magnetized plasma photonic crystal heterostructure, enabling the structure to work as a multichannel filter. Next, the filter properties have been made tunable, i.e., channel frequency of each channel can either be red or blue shifted, depending upon the RHP and LHP configurations of external magnetic field under magneto-optical Faraday effect, respectively. The number of channels and their positions can also be modulated by changing the number of periods (N) and the incident angle (θ0) for TE and TM waves both besides other plasma parameters. Apart from the transmission properties of the proposed structure, we have also studied the reflection properties to obtain multichannel tunable omnidirectional photonic band gaps under the influence of external magnetic field. These results may be utilized to develop new kind of externally tunable single to multichannel omnidirectional band-stop filters.