Effective Medium Method Research Articles

Geometric structure and electronic properties of bilayer graphene with a Moire superlattice by interlayer asymmetric tensile strain

Abstract Using the density functional theory combined with the effective screening medium method, we investigated the energetics and electronic properties of bilayer graphene, comprising graphene layers without and with tensile strain. An interlayer interaction does not affect the structural reconstruction of each graphene layer despite the bilayer structure possessing a Moire superlattice. The structural asymmetry between the graphene layers causes a potential difference across the layers, leading to electron and hole doping in the layers without and with the tensile strain. Accumulated carriers show unique lateral distribution in each graphene layer, which depends on the interlayer atomic arrangements.

Bias-dependent surface stress by density functional theory combined with the effective screening medium method

Bias-dependent surface stress by density functional theory combined with the effective screening medium method

Effect of interlayer stacking arrangement on the dielectric properties of hexagonal boron nitride thin films

Electrostatic properties of hexagonal boron nitride (hBN) thin films with different stacking arrangements were investigated using density functional theory combined with the effective screening medium method. Our calculations showed that the dielectric properties across layers of hBN thin films are sensitive to both the interlayer stacking arrangement and the number of layers. The polarization of bilayer hBN gradually decreases with increasing lateral displacement from AB stacking, and polarity inversion occurs for particular stacking arrangements. The polarity of bilayer hBN is sensitive to twisting displacement. The polarity monotonically increases with increasing the number of layers in hBN films with rhombohedral stacking arrangement.

Energetics and electronic structure of Janus WSSe formation by continuous chalcogen substitutions

We investigated the energetics and the electronic structure of Janus WSSe when formed by continuous sulfurization and selenization of WSe2 and WS2, respectively, using density functional theory combined with the effective screening medium method. The total energy of WS2−x Se x is sensitive to the stoichiometry. The total energy increases monotonically as the substitutional surface selenization of WS2 increases. The sulfurization of WSe2 causes complex energetics with respect to the number of S atoms involved. Competition between the energy cost of polarization and the energy gain through S–W bond formation produces the compound WS0.445Se1.445, which is a metastable structure that gives the local minimum in the energy landscape. The electronic structures of the partially substituted structures are interpolated smoothly from those of Janus WS2 and WSe2.

Sensitivities of Spectral Optical Properties of Dust Aerosols to Their Mineralogical and Microphysical Properties

AbstractDust aerosol is a key component in global radiative forcing and climate modeling. We developed a mineralogy‐resolved spectral optical property model for dust aerosols. The model incorporates nine mineral groups and applies the effective medium methods to obtain the spectral complex refractive indices of inhomogeneous dust particles. This study considers 5,000 sets of dust mineral composition mixtures along with the use of the bimodal particle size distribution. We calculate the bulk properties of these samples in a wide spectral range from 0.2 to 50 μm using a comprehensive database of hexahedral dust single‐scattering properties. Through sensitivity analyses, this study reveals that the illite and hematite components substantially affect the optical properties of dust. Furthermore, the effective radii of both the fine‐mode and the coarse‐mode dust particles have significant impacts on the bulk optical properties, albeit to varying degrees.

Magnetorefractive effect in metallic Co/Pt nanostructures

Objectives. To carry out a theoretical investigation of the features of magnetorefractive effect for metal-to-metal nanostructures. This study uses the example of multilayer Co/Pt nanostructures (ferromagnetic metal–paramagnetic metal) with a different ratio of ferromagnetic and paramagnetic phases in the visible and near-infrared (IR) spectral regions.Methods. The dependence was expressed explicitly using the basic formulas for permittivity, refraction and extinction coefficients, and optical conductivity. This then confirms the common nature of these two effects. The magnetorefractive effect for s-polarization of light was calculated using Fresnel formulas for a three-layer structure. This took into account the thickness of the samples and the influence of the substrate. Effective medium methods were used to calculate the dielectric permittivity of materials. Since the average range of cobalt concentrations was being studied, the Bruggeman approximation was used to establish the effective permittivity of nanostructures. The reflection coefficient at normal incidence was calculated for all nanostructures.Results. Since the permittivity of inhomogeneous samples was replaced by a common effective parameter depending on the permittivity of each component, we were able to apply the Drude–Lorentz theory for conductors in a high-frequency alternating field and then estimate the parameters of the electronic structure of the samples being studied. Plasma and relaxation frequencies were calculated for each sample. This made it possible for the number of free electrons to be estimated and scattering in nanostructures to be investigated.Conclusions. It was shown that Langmuir shielding can be observed in the given energy range in the IR region of the spectrum. The calculated values correlate well with the experimental data.

Joint micromechanical model for determination of effective elastic and electromagnetic properties of porous materials

In this paper we propose an approach for calculating the effective physical properties of porous materials (for example, sedimentary rocks) which is based on the unified structure of the pore space. This approach is based on the Generalized Differential Effective Medium (GDEM) method. This method generalizes the classical differential scheme (DEM) for the case of many types of inclusions. The physical properties of a composite calculated using the GDEM depend on how the solution is constructed.A porous medium is represented by the elastic weakly conductive matrix with embedded inclusions of two types (spheroidal and cylindrical), saturated with a conductive liquid. The cylindrical inclusions appear in the system when the porosity value exceeds the void percolation. Parameters, that characterize the inclusions (the aspect ratio of spheroidal inclusions and the relative part of cylindrical inclusions), are determined in the inverse problem solving process for the experimental data approximation of the effective conductivity as a porosity function. These parameters, obtained by solving the inverse problem, were used to calculate the effective elastic moduli, electrical conductivity, and dielectric permittivity of porous media. The results obtained describe well the available experimental data for different effective physical properties.

Sound absorption properties of the metamaterial curved microperforated panel

This paper proposes a metamaterial that utilizes resonators to enhance the sound absorption capacity of the curved microperforated panel (CMPP), and reliable theoretical analyses and experimental verification are comprehensively given. A metamaterial curved microperforated panel (MCMPP) absorber is formed by attaching local resonators on the CMPP. The displacements of the midplane of the curved panel are expanded into the improved Fourier series based on Spectral-Geometry Method (SGM), and the vibration equations of the MCMPP are derived by the Rayleigh-Ritz method. The sound absorption characteristics of the MCMPP are solved by the effective medium method and the acoustic-electric analogy method. Both a structure-acoustic coupling simulation model and an impedance tube test system are established to validate the accuracy of the present theoretical model. The essential parametric analyses are carried out in conjunction with mode splitting and acoustic-vibrational coupling, and the results show that the local resonance absorption peak within 1000 Hz increases from 1 to 5 with the absorption coefficients greater than 0.8 at these peaks under the installation options of the periodic gradient array and multiple resonances, realizing the effective absorption in multiple frequency bands. This paper provides theoretical support and experimental protocols for the design and application of the metamaterial curved microperforated panel.

Anisotropic honeycomb stack metamaterials of graphene for ultrawideband terahertz absorption

Abstract Graphene aerogels have implied great potential for electromagnetic wave absorption. However, the investigation of their design for broadband absorption in the terahertz (THz) range remains insufficient. Here, we propose an anisotropic honeycomb stack metamaterial (AHSM) based on graphene to achieve ultrawideband THz absorption. The absorption mechanism is elucidated using the effective medium method, offering deeper physics insights. At low THz frequencies, the impedance matching from the air to the AHSM can be improved by reducing the chemical potential of graphene for high absorption. There is a suppression of absorption at the intermediate frequencies due to constructive interference, which can be avoided by shortening the sizes of honeycomb edges. With the aim to elevate absorption at high frequencies, one can increase the stack layer number to enhance multiple reflections and destructive interference within the metastructure. Based on the above principles, we design an AHSM that achieves a broadband absorbance of over 90 % from 1 THz to 10 THz. This absorption can tolerate a wide range of incident angles for both TE and TM wave excitations. Our research will provide a theoretical guide to future experimental exploration of graphene aerogels for THz metamaterial absorber applications.

Photonic management of silicon nanocylinder arrays to enhance photovoltaic performance

The light–matter interaction of subwavelength and periodic silicon (Si) nanostructures strongly correlates with their geometrical features, resulting in them being highly unsuitable for the practical development of Si-based photovoltaic applications. In this study, the concepts of effective medium and retrieval methods are needed to deal with the subwavelength periodic dielectric structure. Using finite-difference time-domain simulations, we study the interactions of electromagnetic radiation with a square array of dielectric rods parallel to the incident light, and the effective optical properties such as refractive index, permittivity, and permeability are calculated. Furthermore, the electric field distributions are also plotted for a deeper understanding of the energy changes within Si nanocylinder arrays (SiNCAs) under different incident wavelengths of radiation. By employing calculated optimized SiNCAs for the construction of hybrid solar cells, improved cell performances showing a conversion efficiency of 13.79% are demonstrated, with further estimation by electrical chemical measurements for a better understanding of the carrier transition. These are numerically and experimentally interpreted by the involvement of excellent light-trapping effects, delivering a method to design correlated photovoltaic devices.

Extinction and attenuation by voids in absorbing host media.

Extinction and attenuation by particles in an absorbing host have suffered a long-lasting controversy, which has impeded the physical insights on the radiative transfer in the voids dispersed composite. In this paper, we outline the existing extinction definitions, including an equivalence theorem neglecting the host absorption, the near-field analytical definition neglecting the far-field effects, and the operational way which simulates the actual detector readings. It is shown that, under the independent scattering approximation, the generalized operational definition is equivalent to a recent effective medium method according to the rigorous theory of multiple scattering. Using this generalized extinction, we show the important influences of the host absorption on the void extinction. Specifically, at the void resonance, the extinction cross sections of the small voids can be positive, zero, and even negative, which is regulated quantitively by host absorption. Considering the voids in SiC or Ag, the intriguing properties are verified through the attenuation coefficient calculated by the Maxwell-Garnett effective medium theory. In contrast, the equivalent theorem cannot describe any void resonance structures in the absorbing media. Also, the near-field definition fails to generate negative extinction and cannot thus describe the diminished total absorption by the voids. Our results might provide a better understanding of complex scattering theory in absorbing media.

Energetics and electronic structure of bilayer Janus WSSe

Employing density functional theory along with the effective screening medium method, we investigated the energetics and electronic structure of bilayer Janus WSSe in terms of their interlayer stacking arrangement. Through the orbital hybridization between chalcogen atoms at interfaces, the energetics are sensitive to the interlayer stacking orientation and interface atomic arrangements. This interface atomic arrangement creates the unique electronic structure of bilayer Janus WSSe determined by the dipole moment arrangement of the constituent WSSe layers. The net polarity of thin films of Janus transition-metal dichalcogenides is a simple superposition of the dipole moments of the constituent layers.

Effective medium method in solution of the elastic and thermo-elastic homogenization problems for polycrystals with spheroidal transverse isotropic grains

Self-consistent effective medium method is used for determination of the effective elastic and thermo-elastic properties of polycrystalline materials with spheroidal transverse isotropic grains. The grains with arbitrary aspect ratios including very oblate (disks) and very prolate (fibers) spheroids are considered. The function of the Owens-March type is used for description of distribution of grains over orientations. The equations for the effective elastic stiffness tensor and the tensor of effective coefficients of thermo-expansion of polycrystals are derived using integral equations for elastic and thermo-elastic fields in heterogeneous media and the hypotheses of the effective medium method. The numerical algorithm of the method is described in detail. Numerical examples are presented for polycrystalline clay rock materials with strongly anisotropic grains and for polycrystalline zinc. Graphs of the dependences of the effective elastic constants and effective coefficients of thermo-expansion of the polycrystals on the parameters of the orientation distribution function and aspect ratios of grains are presented.

Electron Transport through Nanoconfined Ferrocene Solution: Density Functional Theory─Nonequilibrium Green Function Approach

We investigate the transport of electrons in a ferrocene aqueous solution nanoconfined between two Pt electrodes using density functional theory and a nonequilibrium Green function method. The system consists of three characteristic phases: metal electrodes, the electrode–solution interface, and the nanoconfined solution phase. By performing the geometry optimization of such systems, it is found that the molecular configuration of water molecules at the Pt surface is adjusted due to the Pt–water interaction, and the charges at the Pt surface are redistributed. Next, by applying the external bias potential via the effective screening medium method during ab initio molecular dynamics simulations, it is revealed that water molecules are packed at the Pt–water interfacial region, and the electrostatic potential profile in the water phase is significantly changed due to aligned water layers. From the analysis of the electron transport characteristics, it is discovered that the ferrocene molecule generates a strong transmission function near the Fermi level and high electron density of states spatially connecting both electrodes through the water phase. These features explain the drastic enhancement in electrical current–voltage characteristics. Therefore, it is concluded that ferrocene enhances electron transport through nanoconfined solutions, indicating that it can be used as a signaling molecule in nanoscale systems for electrochemical sensor applications.

Broadband Rayleigh wave attenuation utilizing an inertant seismic metamaterial

In this work, a concept of constructing inertant seismic metamaterials for Rayleigh wave attenuation using inerter-based vibration absorbers (IDVAs) is proposed. The dynamic behaviors of the vibration absorbers are controlled by an inertant mechanical network, while three typical oscillator configurations are considered in the design of the proposed inertant seismic metamaterial. An expanded effective medium method is developed to obtain the dispersion of Rayleigh waves, which is separated into three branches by two broad Rayleigh wave band gaps at low frequencies. The influences of the inertance-to-mass ratio as well as the stiffness ratio on the Rayleigh wave propagation are thoroughly discussed. The results show that the multiple-resonance IDVAs can efficiently open low frequency band gaps of the inertant seismic metamaterial without having to increase the mass of the resonators. Band gaps of the low-frequency Rayleigh waves can be further merged to form an extra wide one by adding dissipative mechanisms, which further enhance the attenuation of the Rayleigh waves. Specifically, by analyzing the corresponding displacement fields, it is observed that the seismic energy propagating as Rayleigh surface waves is converted to that propagating as bulk waves not concentrated to the surface, which offers a protected area behind the inertant seismic metamaterials for the facilities located at the surface of the half space. This work provides a new approach for seismic vibration isolations utilizing inertant seismic metamaterials that can be applied in protecting large infrastructures or civil engineering architectures.

First-Principles Investigation of Charged Germagraphene as a Cathode Material for Dual-Carbon Batteries.

As part of the concerted effort for development of energy storage technologies, dual-ion batteries (DIBs) or dual-carbon batteries (DCBs) are attracting interest, owing primarily to their eco-friendly active materials. The use of carbon as the active materials of DCBs brings about several challenges involving capacity and stability. This contribution aims to provide an in-depth understanding of the structural and electronic properties of Ge-doped graphene (Germagraphene) as a novel cathode material for DCBs. Density functional theory (DFT) calculations are combined with the effective screening medium (ESM) method for analyzing the electronic and band structure of PF6 - anion-adsorbed Germagraphene under a potential bias. These theoretical investigations indicate that the use of Ge as a dopant for graphene has a positive impact on the adsorption of the anion on the cathode under both neutral and electrically biased conditions.

Sound transmission of active elastic wave metamaterial with double locally resonant substructures surrounded by external mean flow

The elastic wave metamaterial with active feedback control is presented to discuss its dynamic effective parameter and sound transmission properties. Both studies are initiated from the local resonance with the active feedback control and dynamic structural response. The acoustic-structure coupling of the metamaterial with resonators and orthogonal stiffeners are analyzed with effects of the external mean flow. Different analytical methods are used to derive the effective mass density and sound transmission loss (STL) of the elastic wave metamaterial. The effective density is obtained by the dynamic effective medium method firstly. And the second one applies the spatial harmonic expansion method with the principle of virtual work for the STL based on the Bloch–Floquet theorem and Poisson summations. The effective mass density shows frequency-dependent properties and appears positive and negative values in a certain frequency region simultaneously. This research also illustrates both characteristics can be tuned by the active feedback control with the acceleration and displacement feedback control actions. Numerical results of coupled structures show that the changes of transmission characteristics in the certain frequency region are consistent with the effective mass density properties of uncoupled unit cell structure. In addition, influences of the lattice constants, incident angles and Mach number on the STL are illustrated and discussed.

Elasticity of randomly distributed sheet networks

Plate-like sheet networks assembled by randomly distributed sheets are commonly found in artificial nacre-like materials. However, the effects of spatial randomness and porosity of the network on its mechanical properties still remain largely unrevealed. To make a comprehensive understanding on the elasticity of these networks, a mechanics model is established based on the effective medium method where the intralayer spatial randomness of the sheets and porosity of the network are considered. An analytical expression of the effective Young’s modulus of random sheet networks is derived and verified by FE simulation. The effective elastic modulus of the network is a quadratic function of the effective sheet area fraction that can be obtained by the real sheet area fraction minus a threshold. The threshold is ineffective sheet area fraction that cannot contribute to load bearing, and found to be a constant as 0.25. Besides, the effects of sheet size dispersion and sheet shape on the effective Young’s modulus are investigated. The modulus can be improved by increasing the diameter variance or replacing the circular sheets with square sheets. This work provides a method to estimate the effect of randomness of arrangement, size and shape of sheets on the elasticity of network materials.

Characterization of elastic moduli with anisotropic rock physics templates considering mineralogy, fluid, porosity, and pore-structure: A case study in Volve field, North Sea

A practical methodology was designed to characterize mineralogy and the rock microstructure by constructing anisotropic rock physics templates (ARPTs) based on the Effective Medium Method (EMM). The ARPTs consider parameters such as pore aspect ratio (α), porosity, fluid saturation, and elastic properties of rock minerals. A three-stage methodology was applied to an oil well for a sandstone formation in the Volve field, North Sea. The first stage is the petrophysical evaluation, which is validated with core data. The second, the inversion scheme with differential effective medium (DEM) was used to predict aspect ratios in depth. The obtained results characterize the sandy formation setting intergranular porosity with 0.12 < α < 0.3, although there is also the presence of stylolites caused by the dissolution mechanism between quartz, feldspar, mica, calcite, and clay. The validation of the pore-structure is accomplished with thin sections. The last stage comprises the construction of ARPTs, which includes the axial and two transversal templates of Young's modulus versus Poisson's ratio. These templates are used to characterize lithology, porosity, pore-structure, and the mechanical behavior of the sandstone formation from well logs and gave invaluable information about the pay zone. In addition, because the lithotype interpretation proposed is merely borne by rock's elastic properties, it has the potential to be scaled to field scale by using seismic inversion data.

Development of a dielectrically consistent reference interaction site model combined with the density functional theory for electrochemical interface simulations

The dielectrically consistent reference interaction site model (DRISM) is one of the methods used to solve the well-known drawback of the reference interaction site model (RISM) theory: That it underestimates the dielectric constant of the solutions. Recently, Nishihara and Otani developed the first-principles effective screening medium (ESM) method combined with RISM theory, called ESM-RISM, to simulate physical properties at the electrode/electrolyte interface [Phys. Rev. B 96, 115429 (2017)]. In this study, we combined DRISM with the ESM-RISM framework to increase the accuracy of electrochemical interface simulations, which was applied to the Pt(111)/aqueous water interface as a benchmark calculation. The electrochemical properties at the interface, such as the potential of zero charge, standard hydrogen electrode potential, and double-layer capacitance, all reasonably agree with previous experiments. The thickness of the contact layer was found to correlate well with double-layer capacitance. We believe that the present method is a useful tool to better model the physical properties at electrochemical interfaces.