Effective Medium Method Research Articles

Оптические и магнитооптические свойства многослойных наноразмерных плeнок [Co/TiO-=SUB=-2-=/SUB=-]-=SUB=-n-=/SUB=-

Abstract —The optical and magneto-optical properties of the metal–dielectric multilayer [Co/TiO_2]_ n structures with 2–4-nm-thick layers prepared on a silicon substrate Si(001) by ion-beam deposition have been studied. The complex permittivity of multilayer [Co/TiO_2]_ n structures has been measured by the optical ellipsometry technique in the spectral range of 0.6–5.6 eV and analyzed using the optical reflection matrices for isotropic multilayer dielectric structures taking into account the optical losses and also using the method of anisotropic effective medium. The magneto-optical Kerr effect has been measured by the polarimetric technique in the spectral range of 1.2–4.5 eV in the polar and longitudinal geometries. The magnetic anisotropy type is determined on the base of the field dependences of the magneto-optical Kerr effect. It is found that the nanosized [Co/TiO_2]_ n structures can be considered as artificial optically uniaxial media with a strong magnetic and optical anisotropy at room temperature.

Simulation of Tsunami Effect by Seismic Wave Propagation in Hypoplastic Medium at Vicinity of Free Boundary

A theoretical study of seismic waves propagation in a soil layer with a free surface has a great importance for a prediction in engineering decisions. Wave packets are radiated from an earthquake source and transfer energy. A transformation and a selection of wave packets occur in a process of wave propagating that why waves which arrive in a layer have a length considerably greater than a variation scale of heterogeneity in a medium in a layer near free surface. In the case, when the properties of different layers affect a relatively small degree on a behavior of the waves, an approximation of effective medium gives a fairly good solution. A model of a hypoplastic medium is used for a describing of some effects, which are observed in the time of seismic wave propagation. The model of hypoplastic medium allows describing many effects which are observed in granular soils. We consider a successive application of effective medium and ray methods in order to receive of approximate analytical solutions wishing to describe shear wave propagation in stratified layer, which lies on a half-space.

Low-frequency sound radiation of infinite orthogonally rib-stiffened sandwich structure with periodic subwavelength arrays of shunted piezoelectric patches

In order to minimize the low-frequency sound radiation of an infinite orthogonally rib-stiffened sandwich structure, the periodic subwavelength arrays of shunted piezoelectric patches are introduced. Based on the piezoelectric shunt technique and effective medium method, the panels and piezoelectric patches are equivalent to two homogeneous facesheets. Then, considering all the inertia terms of the rib-stiffeners, a complete theoretical model is built for harmonic point force excitation by using Kirchhoff’s thin plate theory. The vibration displacements of the facesheets in wavenumber space are solved by Fourier transform technique to complete the prediction of vibroacoustic responses. Furthermore, the correctness and effectiveness of the present model are verified by sequentially using published analytical models, simulation results and theoretical predictions in strict accordance with two prerequisites. At last, the influence laws of key influencing parameters on the research structure are investigated. All the results demonstrate that the proposed structure can effectively attenuate the structural radiation.

Herpin effective media resonant underlayers and resonant overlayer designs for ultra-high NA interference lithography.

Dielectric waveguide resonant underlayers are employed in ultra-high NA interference photolithography to effectively double the depth of field. Generally a single high refractive index waveguiding layer is employed. Here multilayer Herpin effective medium methods are explored to develop equivalent multilayer waveguiding layers. Herpin equivalent resonant underlayers are shown to be suitable replacements provided at least one layer within the Herpin trilayer supports propagating fields. In addition, a method of increasing the intensity incident upon the photoresist using resonant overlayers is also developed. This method is shown to greatly enhance the intensity within the photoresist making the use of thicker, safer, non-absorbing, low refractive index matching liquids potentially suitable for large-scale applications.

Geometric structures of Al nanoparticles adsorbed on graphene under an external electric field

Using the density functional theory combined with the effective screening medium method, we studied geometric structures of Al nanoparticles adsorbed on graphene surfaces under electron/hole injection by a counter electrode. The equilibrium spacing between graphene and an Al nanoparticle is sensitive to their mutual arrangement with respect to the electrode, carrier concentration, and carrier species. For most cases, owing to the Coulomb attractive interaction between accumulated carriers in graphene or an Al nanoparticle and the counter electrode, the Al nanoparticle is desorbed from the graphene surfaces at the carrier concentration of approximately 0.5 e or 0.5 h per unit cell.

Determination of effective electromagnetic parameters of concentrated suspensions of ellipsoidal particles using Generalized Differential Effective Medium approximation

The paper presents an approach to determine the effective electromagnetic parameters of suspensions of ellipsoidal dielectric particles with surface conductivity. This approach takes into account the existence of critical porosity that corresponds to the maximum packing volume fraction of solid inclusions. The approach is based on the Generalized Differential Effective Medium (GDEM) method. We have introduced a model of suspensions containing ellipsoidal inclusions of two types. Inclusions of the first type (phase 1) represent solid grains, and inclusions of the second type (phase 2) contain material with the same physical properties as the host (phase 0). In this model, with increasing porosity the concentration of the host decreases, and it tends to zero near the critical porosity. The proposed model has been used to simulate the effective electromagnetic parameters of concentrated suspensions. We have compared the modeling results for electrical conductivity and dielectric permittivity with the empirical equations. The results obtained have shown that the GDEM model describes the effective electrical conductivity and dielectric permittivity of suspensions in a wide range of inclusion concentrations.

Review on micromechanics of nano‐ and micro‐fiber reinforced composites

This article deals with the prediction of thermomechanical properties of fiber reinforced composites using several micromechanics models. These include strength of material approach, Halpin–Tsai equations, multi‐phase mechanics of materials approaches, multi‐phase Mori–Tanaka models, composite cylindrical assemblage model, Voigt–Reuss models, modified mixture rule, Cox model, effective medium approach and method of cells. Several composite systems reinforced with short and long, aligned, random and wavy reinforcements were considered. In addition, different aspects such as fiber‐matrix interphase, fiber‐matrix interfacial thermal resistance, fiber geometry, and multiple types of reinforcements were considered to model the composites systems. The current study also presents some important preliminary concepts and application of developed micromechanics models to advanced nanocomposites such as carbon nanotube reinforced composite. Main contribution of the current work is the investigation of several analytical micromechanical models, while most of the existing studies on the subject deal with only one or two approaches considering few aspects. POLYM. COMPOS., 39:4243–4274, 2018. © 2017 Society of Plastics Engineers

Strain-induced charge transfer and polarity control of a heterosheet comprising C60 and graphene

Using density functional theory combined with the effective screening medium method, the energetics and electronic structure of a C60 molecular sheet adsorbed on graphene were studied in terms of biaxial strains. The optimum spacing and interlayer interaction monotonically decreases and increases, respectively, with an increasing biaxial tensile strain. The biaxial compressive strain induces electron transfer from the graphene to C60 at a 2% lateral compression, leading to an all-carbon charge transfer complex. The heterosheet possesses an intrinsic dipole moment along the graphene-to-C60 molecular layer direction.

Electronic structure and electric polarity of edge-functionalized graphene nanoribbons

On the basis of the density functional theory combined with the effective screening medium method, we studied the electronic structure of graphene nanoribbons with zigzag edges, which are terminated by functional groups. The work function of the nanoribbons is sensitive to the functional groups. The edge state inherent in the zigzag edges is robust against edge functionalization. OH termination causes the injection of electrons into the nearly free electron states situated alongside the nanoribbons, resulting in the formation of free electron channels outside the nanoribbons. We also demonstrated that the polarity of zigzag graphene nanoribbons is controllable by the asymmetrical functionalization of their edges.

Carrier injection in nonbonding π states of N-doped graphene by an external electric field

Using the density functional theory combined with an effective screening medium method, we studied the electronic structure of N-doped graphene under an external electric field. The electronic states near the Fermi level depend on the carrier concentration reflecting their wave function distribution. The electronic states associated with the dangling bond shift upward with increasing electron concentration, following the upward shift of the Fermi level. The electronic states associated with nonbonding π states almost retain their energy upon hole/electron doping by the external electric field.

Asymmetric carrier accumulation in double-walled carbon nanotube by an external electric field

We studied the electronic structure of a double-walled carbon nanotube (DWCNT) in a field-effect transistor structure under an external electric field. Our calculations using density functional theory combined with the effective screening medium method showed that the carrier accumulation in the DWCNT is sensitive to the carrier species and concentration. Injected holes are distributed primarily on the outer wall of the DWCNT, whereas the electrons are accommodated on both its outer and inner walls. The extended nature of the accumulated electron density leads to an interwall electric field from the outer to the inner wall.

Energetics and electronic structures of thin films and heterostructures of a hexagonal GaN sheet

Using the density functional theory (DFT) combined with van der Waals correction and effective screening medium methods, we study the geometric and electronic structures of GaN thin films, each atomic layer of which exhibits a hexagonally bonded two-dimensional (2D) network. Our DFT calculations containing the van der Walls correction showed that the hexagonal GaN (h-GaN) sheets in the thin films are tightly bound to each other owing to the small interlayer spacing, such that their electronic structures are sensitive to the number of layers. We also investigate the energetics and electronic structures of hybrid structures of h-GaN with other layered materials, graphene and h-BN: For both hybrids, the optimum interlayer spacing is 3.4 Å, indicating that the h-GaN sheet is bound to graphene or h-BN via a weak van der Waal interaction. Owing to the weak interlayer interaction, graphene and h-BN retain their characteristic electronic structures. We further found that GaN thin films with a wurtzite structure undergo a structural phase transition into the layered structure of h-GaN when a biaxial tensile strain is applied.

Electronic properties of electron-doped [6,6]-phenyl-C61-butyric acid methyl ester and silylmethylfullerene

Electronic properties of electron-doped chemically decorated C60 fullerenes, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and silylmethylfullerene (SIMEF), by a planar electrode were studied using density functional theory combined with the effective screening medium method to simulate the heterointerface between the chemically decorated C60 and cationic counter materials. We find that the distribution of accumulated electrons and induced electric field depend on the molecular arrangement with respect to the external electric field of the electrode. We also show that the quantum capacitance of the molecule is sensitive to molecular arrangement owing to the asymmetric distribution of the accumulated electrons.

Erratum: Improved modeling of electrified interfaces using the effective screening medium method [Phys. Rev. B 88 , 155427 (2013)

Erratum: Improved modeling of electrified interfaces using the effective screening medium method [Phys. Rev. B <b>88</b> , 155427 (2013)

First-principles molecular dynamics simulation for electrochemical hydrogen production by 4,4′-bipyridine molecular catalyst on silver electrode

Electrochemical hydrogen evolution reaction (HER) on silver is significantly accelerated by adding 4,4′-bipyridine (BiPy) into neutral or alkaline aqueous solution (T. Uchida et al. J. Am. Chem. Soc., 130 (2008) 10826). The mechanism of the accelerated HER by BiPy is studied by the first-principles molecular dynamics simulation with the constant Fermi energy scheme of the effective screening medium (ESM) method. The simulation shows that H2 is evolved through the following three steps: First, BiPy adsorbed on the electrode via one N-end directing another N-end toward the solution is reduced to N-hydro BiPy radical (BiPy-H⋅), in which H is bonded to N atom of the solution side. BiPy-H⋅ is further reduced to N,C(3)-dihydro BiPy with an activation free energy of 0.5eV by the second proton-coupled electron transfer. Finally, H2 is released from the two H atoms on N and C(3) of N,C(3)-dihydro BiPy by the assist of H2O or OH−. BiPy is reproduced by the H2 evolution and the processes are catalytic-cycled.

Sound transmission loss through metamaterial plate with lateral local resonators in the presence of external mean flow.

In the context of sound incident upon a metamaterial plate, explicit formulas for sound transmission loss (STL) are derived in the presence of external mean flow. Metamaterial plate, consisting of homogeneous plate and lateral local resonators (LLRs), is homogenized by using effective medium method to obtain the effective mass density and facilitate the calculation of STL. Results show that (a) vigorously oscillating LLRs lead to higher STL compared with bare plate, (b) increasing Mach number of the external mean flow helps obtain higher STL below the coincidence frequency but decreases STL above the coincidence frequency due to the added mass effect of light fluid loading and aerodynamic damping effect, (c) the coincidence frequency shifts to higher frequency range for the refracted effect of the external mean flow. However, effects of the flow on STL within negative mass density range can be neglected because of the lateral local resonance occurring. Moreover, hysteretic damping from metamaterial can only smooth the transmission curves by lowering higher peaks and filling dips. Effects of incident angles on STL are also examined. It is demonstrated that increasing elevation angle can improve the sound insulation, while the azimuth angle does not.

Low-Frequency Broadband Sound Transmission Loss of Infinite Orthogonally Rib-Stiffened Sandwich Structure with Periodic Subwavelength Arrays of Shunted Piezoelectric Patches

This paper studies low-frequency sound transmission loss (STL) of an infinite orthogonally rib-stiffened sandwich structure flexibly connected with periodic subwavelength arrays of finite shunted piezoelectric patches. A complete theoretical model is proposed by three steps. First, the panels and piezoelectric patches on both sides are equivalent to two homogeneous facesheets by effective medium method. Second, we take into account all inertia terms of the rib-stiffeners to establish the governing equations by space harmonic method, separating the amplitude coefficients of the equivalent facesheets through virtual work principle. Third, the expression of STL is reduced. Based on the two prerequisites of subwavelength assumption and convergence criterion, the accuracy and validity of the model are verified by finite element simulations, cited experiments, and theoretical values. In the end, parameters affecting the STL performance of the structure are studied. All of these results show that the sandwich structure can improve the low-frequency STL effectively and broaden the sound insulation bandwidth.

A Tunable Ultrabroadband Ultrathin Terahertz Absorber Using Graphene Stacks

The concept of an ultrabroadband ultrathin terahertz (THz) absorber based on graphene stacks is presented. To realize dynamic tuning of the absorption band, we combine two concentric circular split-rings encircling a split circular resonator with graphene stacks embedded in a SiO 2 dielectric. The transmission line model for the proposed absorber has been theoretically investigated via the effective-medium method. The simulated results show a 0.9 absorption coefficient over the range 2.58-3.56 THz, and the central frequency of the absorption band can be tuned without absorption reduction by changing the gate voltage on the graphene stack. Furthermore, the absorption performance exhibits insensitivity to the angle of incidence. Thus, the proposed absorber is very promising for various THz applications.

First-principles studies of electric field effects on the electronic structure of trilayer graphene

A gate electric field is a powerful way to manipulate the physical properties of nanojunctions made of two-dimensional crystals. To simulate field effects on the electronic structure of trilayer graphene, we used density functional theory in combination with the effective screening medium method, which enables us to understand the field-dependent layer-layer interactions and the fundamental physics underlying band gap variations and the resulting band modifications. Two different graphene stacking orders, Bernal (or ABC) and rhombohedral (or ABA), were considered. In addition to confirming the experimentally observed band gap opening in ABC-stacked and the band overlap in ABA-stacked trilayer systems, our results reveal rich physics in these fascinating systems, where layer-layer couplings are present but some characteristics features of single-layer graphene are partially preserved. For ABC stacking, the electric-field-induced band gap size can be tuned by charge doping, while for ABA band the tunable quantity is the band overlap. Our calculations show that the electronic structures of the two stacking orders respond very differently to charge doping. We find that in the ABA stacking hole doping can reopen a band gap in the band-overlapping region, a phenomenon distinctly different from electron doping. The physical origins of the observed behaviors were fully analyzed, and we conclude that the dual-gate configuration greatly enhances the tunability of the trilayer systems.

Electronic properties of pentaorgano[60]fullerenes under an external electric field

The electronic properties of pentaorgano[60]fullerene under an external electric field were studied by combining the density functional theory with the effective screening medium method. Pentaorgano[60]fullerene possess a dipole moment because of their asymmetric molecular form owing to their five functionalized groups. When electrons and holes are injected into the molecule, the magnetic states of the molecule change from S = 1/2 to nonmagnetic and S = 1 triplet states for electron and hole doping, respectively. The asymmetric molecular shape causes the unusual distribution of the accumulated carriers depending on their mutual molecular arrangement in the electric field.