Layers In Unit Cell Research Articles

An atomically controlled insulator-to-metal transition in iridate/manganite heterostructures.

All-insulator heterostructures with an emerging metallicity are at the forefront of material science, which typically contain at least one band insulator while it is not necessary to be. Here we show emergent phenomena in a series of all-correlated-insulator heterostructures that composed of insulating CaIrO3 and insulating La0.67Sr0.33MnO3. We observed an intriguing insulator-to-metal transition, that depends delicately on the thickness of the iridate component. The simultaneous enhancements of magnetization, electric conductivity, and magnetoresistance effect indicate a percolation-type nature of the insulator-to-metal transition, with the percolation threshold can be reached at an exceptionally low volume fraction of the iridate. Such a drastic transition is induced by an interfacial charge transfer, which interestingly alters the electronic and crystalline structures of the bulk region rather than the limited ultrathin interface. We further showcased the central role of effective correlation in modulating the insulator-to-metal transition, by demonstrating that the critical thickness of iridate for triggering the metallic state can be systematically reduced down to a single unit-cell layer.

Bidirectional triple-band truly incident angle insensitive polarization converter using graphene-based transmissive metasurface for terahertz frequency

Abstract The design and response of a triple-band linear-to-circular polarization converter (LTCPC) in the terahertz frequency regime using a graphene-based transmission-type metasurface have been studied numerically and analytically. The unit cell of the converter is constructed using lossy silicon dioxide (SiO2) with a relative permittivity of 3.9 as the substrate. And two similar layers of graphene-based sub-wavelength structures are used at the top and bottom of it. Multiband linear to circular polarization conversion is achieved from 0.74 THz to 0.98 THz, 1.70 THz to 2.33 THz, and 4.06 THz to 5.19 THz, i.e., about 28 %, 31 %, and 24 % fractional bandwidths, respectively. As the same metasurface configuration is used at the top and bottom layers of the transmission-type unit cell, it can be used as a bidirectional polarization converter with identical responses from both sides. The geometrical optimization of the unit cell is done, and an equivalent lumped parameter circuit model is also proposed for the same, considering analogous responses. Tunability over the operating frequency is achieved by varying the chemical potential and relaxation time of graphene. Moreover, due to the ultrathin width and special types of identical configuration of the metasurfaces, the response of the system remains excellent over a wide range of incident angle variations up to 80° angle of incidence.

Second order frequency selective surface design multi-functional at two bands

This paper proposes an active frequency selective surface which furnishes three bands of higher order filtering characteristics. It controls multi-functional features, at two bands (centered at 2 GHz and 4 GHz), namely electromagnetic response and polarization of plane wave. Nine discrete filtering outputs are independently controlled through switching in bias supply to four PIN diodes embedded in two layers of AFSS unit cell. Two multifunctional bands hold stability upto 45° angles under all polarizations of wave. Passive band at around 5.9 GHz contributed by all the independent binary combinations exhibits stability upto 45° angles of TE wave only. The equivalent circuit method is used to explain and verify the designed structure. Finally, the AFSS structure is fabricated and reliability of the simulation results is verified through experiments.

Exact closed forms for the transmittance of electromagnetic waves in one-dimensional anisotropic periodic media

In this work, we obtain closed expressions for the transfer matrix and the transmittance of electromagnetic waves propagating in finite one-dimensional anisotropic periodic stratified media with an arbitrary number of cells. By invoking the Cayley–Hamilton theorem on the transfer matrix for the electromagnetic field in a periodic stratified media formed by N cells, we obtain a fourth-order recursive relation for the matrix coefficients that defines the so-called Tetranacci polynomials (TPs). In the symmetric case, corresponding to a unit-cell transfer matrix with a characteristic polynomial where the coefficients of the linear and cubic terms are equal, closed expressions for the solutions to the recursive relation, known as symmetric TPs, have recently been derived, allowing us to write the transfer matrix and transmittance in a closed form. We show as sufficient conditions that the 4×4 differential propagation matrix of each layer in the binary unit cell, Δ , a) has eigenvalues of the form ±p1 , ±p2 , with p1≠p2 , and b) its off-diagonal 2×2 block matrices possess the same symmetric structure in both layers. Otherwise, the recursive relations are still solvable for any 4×4 matrix and provide an algorithm to compute the Nth power of the transfer matrix without carrying out explicitly the matrix multiplication of N matrices. We obtain analytical expressions for the dispersion relation and transmittance, in closed form, for two finite periodic systems: the first one consists of two birefringent uniaxial media with their optical axis perpendicular to the z-axis, and the second consists of two isotropic media subject to an external magnetic field oriented along the z-axis and exhibiting the Faraday effect. Our formalism applies also to lossy media, magnetic anisotropy or optical activity.

Large Polarization Near 50 μC/cm2 in a Single Unit Cell Layer SrTiO3.

The generally nonpolar SrTiO3 has attracted more attention recently because of its possibly induced novel polar states and related paraelectric-ferroelectric phase transitions. By using controlled pulsed laser deposition, high-quality, ultrathin, and strained SrTiO3 layers were obtained. Here, transmission electron microscopy and theoretical simulations have unveiled highly polar states in SrTiO3 films even down to one unit cell at room temperature, which were stabilized in the PbTiO3/SrTiO3/PbTiO3 sandwich structures by in-plane tensile strain and interfacial coupling, as evidenced by large tetragonality (∼1.05), notable polar ion displacement (0.019 nm), and thus ultrahigh spontaneous polarization (up to ∼50 μC/cm2). These values are nearly comparable to those of the strong ferroelectrics as the PbZrxTi1-xO3 family. Our findings provide an effective and practical approach for integrating large strain states into oxide films and inducing polarization in nonpolar materials, which may broaden the functionality of nonpolar oxides and pave the way for the discovery of new electronic materials.

Tailoring crystallisation of anatase TiO2 ultra-thin films grown by atomic layer deposition using 2D oxides as growth template

TiO2 ultrathin films are required in many material research areas. The anatase phase was found to be more stable in case of atomic layer deposition growth, nevertheless a critical thickness of around 10 nm appears necessary to obtain crystallization on native silicon oxide on Si (001). This work focuses on direct liquid injection Atomic Layer Deposition (ALD) of TiO2 films with thicknesses about some nanometers, using titanium tetra-isopropoxide as precursor and H2O as oxidant. A particular care to the treatment of ellipsometric measurements is employed. Below the threshold value, films remain amorphous unless the growth starts onto a crystalized surface, a thick TiO2 layer in our case. Here we propose and show the efficiency of 2-dimensional (2D) [Ca2Nb3O10]− and [Ti0.865O2]0.54− nanosheets as seeds at the surface for the crystallization and epitaxy of ultra-thin films of TiO2 below the critical thickness. Furthermore, we show that the structure of the 2D nanosheets determines the growth orientation of the epitaxial anatase, namely (010) for [Ti0.865O2]0.54−, and (100) for [Ca2Nb3O10]−. These results, based on the tailoring of crystal anisotropy via adapted 2D seed nanosheets and low temperature ALD, opens the way for opto-electronics applications of titanium oxide layers of a few unit cells.

Modelling of Inelastic Pentamode-Based Bridge Bearings Using Beam Elements

Metameterials have unique properties, which are mostly attributed to their geometrical configuration. Pentamodes, a subcategory of metamaterials, exhibit an almost zero shear elastic modulus while maintaining high compression stiffness, offering a behavior similar to that of a liquid, suggesting the potential application of pentamodes in seismic isolation. In this paper a real-life bridge bearing, composed of repetitive layers of pentamode unit cells in the horizontal and vertical axes is studied. The lattices are modelled using beam finite elements with an equivalent uniform diameter to ensuring a stiffness equal to that of the bi-cone rod. The importance of the chosen equivalent diameter is shown, as the assumption of an average diameter of the bi-cone may lead to significant discrepancies between the calculated stiffnesses. For small bi-cone diameters difference, and slender formulations, the error could grow up to 15% for the horizontal stiffness and up to 200% for vertical. For thick formulations the average diameter overestimates the horizontal stiffness by 3 times and the vertical by 4. These discrepancies grow exponentially as the bi-cone diameters difference increases. An elastoplastic material is selected. The bearing supporting the superstructure is subjected to a constant vertical weight load and a horizontal shear base load, due to seismic excitation. Under vertical loading plastic hinges are created in all the rods of the cell and bearing. However, under shear loading plastic hinges are rather initially created in the lowest nodes of the cell and the bearing.

A design strategy to enhance the mechanical response of diamond-based lattice structures produced by laser powder bed fusion

Lattice structures have been mostly used and studied considering one unit cell with a fixed build orientation. As a consequence, these meta-materials exhibit total failures when loaded over the yield stress, resulting in localized shear bands. In this article, a design strategy to enhance the mechanical behaviour of diamond-based lattice structures is proposed. The methodology is based on the rotation of layers of unit cells around a specific axis, to generate meta-coherent twin boundaries in the lattice, that can harden the meta-material. The proposed strategy is ideal for laser powder bed fusion of diamond-based lattices as all struts in the lattice keep the same orientation hence yielding repeatable and predicable lattice quality whilst still enabling significant local variations in mechanical performance. The new design strategy has been compared with classic diamond-based lattices both numerically and experimentally. The results in both cases demonstrated the value of incorporating meta-twin boundaries in the structure to increase the mechanical response while also reducing the extent of the localized shear bands.

Performance of meta concrete panels subjected to explosive load: Numerical investigations

AbstractResearch on strengthening and retrofitting of concrete structures against explosive loading has recently received much attention. This paper proposes a new type of reinforcement for concrete panels to enhance their blast‐resistant capacity. The reinforcement structure is naturally optimized with continuous nonself‐intersecting surfaces, known as triply periodic minimal surface (TPMS)‐primitive scaffold. A numerical model is developed to investigate the performance of TPMS‐primitive reinforced concrete panels against blast loading. The results are validated by experimental data from the literature on a traditional reinforced concrete slab. The concrete slabs reinforced with one, two, and four layers of TPMS‐primitive unit cells are designed and examined, while a quarter of the panel is simulated to reduce the computational cost. A constitutive model that considers the strain‐rate effect is employed to capture the rate‐dependent impulsive behavior of the proposed panels. The numerical results indicate significant enhancements in damage resistance and substantial reductions in deflection of the concrete reinforced panels when the TPMS‐primitive scaffold replaces the rebar lattice. The proposed reinforcement with multiple layers of unit cells retains high efficiency through parametric studies, while changes in the shell thickness of the reinforcement scaffold have limited effects on filtering blast waves.

Microstructure design and optimization of multilayered piezoelectric composites with wavy architectures

In this article, the impact of poling direction and microstructure geometric parameters including PZT-7A volume fraction, PZT-7A layer number and unit cell amplitude-to-wavelength ratio on the homogenized electromechanical properties of PZT-7A/PVDF multilayered wavy composites are explored for the first time with the Multiphysics FVDAM. Then, a comprehensive parametric study is conducted toward microstructure effects of the above geometric parameters on the performance of the piezoelectric composites, evaluated by figures of merits. Further, the Particle Swarm Optimization algorithm is proposed to combine with the Multiphysics FVDAM to effectively maximize the performance indices by optimizing the microstructure geometric parameters.

New Reconfigurable Transmitarray Unit Cell Design at Ku-Band Using PIN Diodes

Beam reconfigurable Transmitarray antennas require the unit cell phase variation capability. This paper presents a new reconfigurable Transmitarray unit cell design using PIN diodes at the Ku-band. The unit cell is comprised of three layers having four PIN diodes used in every unit cell layer. This results in phase controlling by switching the PIN diodes on every layer individually. The unit cell s-parametric analysis shows a high transmission coefficient magnitude (> -3.6dB) at a design frequency of 12.5GHz and maximum wide impedance bandwidth (17.5%). Four different switching states configurations are used, which result in phase variations over the range of 150.5 degrees. The transmission coefficient magnitude remains high for all four switching states. The proposed reconfigurable unit cell can be used in designing beam switched Transmitarray antennas for satellite communications and beam steering applications.

Thickness dependence of dielectric constant of alumina films based on first-principles calculations

Optoelectronic properties of devices made of two-dimensional materials depend largely on the dielectric constant and thickness of a substrate. To systematically investigate the thickness dependence of dielectric constant from first principles, we have implemented a double-cell method based on a theoretical framework by Martyna and Tuckerman [J. Chem. Phys. 110, 2810 (1999)] and therewith developed a general and robust procedure to calculate dielectric constants of slab systems from electric displacement and electric field, which is free from material-specific adjustable parameters. We have applied the procedure to a prototypical substrate, Al2O3, thereby computing high-frequency and static dielectric constants of a finite slab as a function of the number of crystalline unit-cell layers. We find that two and four layers are sufficient for the high-frequency and static dielectric constants of (0001) Al2O3 slabs to recover 90% of the respective bulk values computed by a Berry-phase method. This method allows one to estimate the thickness dependence of dielectric constants for various materials used in emerging two-dimensional nanophotonics, while providing an analytic formula that can be incorporated into photonics simulations.

σh symmetry and electron-phonon interaction in two-dimensional crystalline systems

The coupling of electrons and phonons is governed wisely by the symmetry properties of the crystal structures. In particular, for two-dimensional (2D) systems, it has been suggested that the electrons do not couple to phonons with pure out-of-plane distortion, as long as there is a $\sigma_h$ symmetry. We show that such a statement is correct when constituents of the unit-cell layer are only located in the $\sigma_h$ symmetric plane; a prominent example of such a system is graphene. For those 2D crystals in which atoms are vertically located away from the horizontal symmetric plane (e.g., 1H transition metal dichalcogenides), acoustic flexural modes do not couple to the electrons up to linear order, while optical flexural phonons, which preserve $\sigma_h$ symmetry, do couple with the electrons. Our conclusions are supported by an analytic argument together with numerical calculations using density functional perturbation theory.

Excited Electron‐Rich Bi(3–x)+ Sites: A Quantum Well‐Like Structure for Highly Promoted Selective Photocatalytic CO2 Reduction Performance

AbstractMost of the current research on the photocatalytic mechanism of semiconductors is still on the simulation and evaluation of ground‐state active sites. Insights into photogenerated electron transition paths and excited‐state active sites during photocatalysis are still insufficient. Herein, combining femtosecond time‐resolved transient absorption spectroscopy, in situ Fourier‐transform infrared spectroscopy, synchronous illumination X‐ray photoelectron spectroscopy, and theoretical calculation results rationally reveal that in complex bimetallic oxyhalides the ultrathin rich oxygen vacancies (ROV) PbBiO2Cl (PBOC) double unit cell (DUC) layers facilitate migration and separation of photogenerated electrons from the bulk to Bi sites near the surface oxygen vacancies (OVs), then form the excited electron‐rich Bi(3–x)+ sites like quantum well structure. The excited Bi(3–x)+ sites act as wells for photogenerated electrons leading to lower energy barrier in the rate determining step for the formation of *CO from *COOH intermediate. Without photosensitizers and sacrificial agents, ROV DUC PBOC exhibit high CO generation rate (16.02 µmol h–1 g–1) that is 18 times higher than that of bulk PBOC. In situ characterization combined with theoretical calculation provides effective insight into the photocatalytic mechanism of photoexcited semiconductor materials.

Experimental Study and Numerical Analysis of the Tensile Behavior of 3D Woven Ceramic Composites

In this work, the tensile responses of 3D woven quartz fiber silica matrix composites were experimentally and numerically investigated. The ceramic composites reinforced by 3D layer-to-layer angle interlock woven preforms were manufactured and tested under warp direction tension. A numerical method is proposed to model the mechanical response of the ceramic composites under tension. The method is based on a mesoscopic single layer unit cell for the composites, using a progressive damage analysis approach to account for damage evolution. The predicted results are compared with experimental data, and good agreement in the stress–strain response up to the ultimate tensile strength of the composites is obtained. It has been demonstrated that the proposed numerical model based on a simple single layer unit cell is both efficient and effective in characterization of the mechanical behavior of the 3D layer-to-layer woven ceramic composites.

Analysis of transmission spectra in one-dimensional ternary photonic crystals with complex unit cell

We investigate the light transmission spectra of a one-dimensional semiconductor ternary photonic crystal with a complex unit cell composed of different combinations of two three-layers (symmetric and asymmetric) using the transfer matrix method. The transmittance properties of such photonic crystals are analyzed numerically for the normal and oblique incidence angles of electromagnetic radiation. The complete transmission spectra for the complex ternary photonic structures have been verified from the Brewster angle. Special attention is paid to the study of light transmission spectra dependencies as a function of the thicknesses of layers in the complex unit cell. The proposed structure can be used to design the optical wide-band filters, omnidirectional reflectors at the infrared and terahertz regimes.

A high gain antenna utilizing Mu-near-zero metasurface structures for 5G applications

AbstractA high gain antenna with Mu-near-zero metasurface (MNZ-MS) is introduced in this work. The proposed antenna can be utilized for future 5G applications. The patch antenna is designed on the upper face of the square substrate with a ground plane at the bottom face. To improve the antenna gain, a layer of MNZ-MS unit cells is placed above the antenna substrate with 0.48λ0 air space between the two layers, where λ0 is the wavelength in free space at the resonance frequency of 26.3 GHz. The MNZ-MS layer consists of four double-sided split square resonators. The overall thickness of the proposed antenna is 0.66λ0. To approve the execution of the proposed antenna, a prototype model is manufactured and tested. The outcomes fulfill a reflection coefficient lower than −10 dB over the frequency spectrum of 26–28 GHz. A gain improvement better than 5 dB is obtained compared to the gain of the reference patch at the center frequency of 26.3 GHz.

Searching effective indicators of microstructural changes in bitumens during aging: A multi-technique approach

A study on the effect of artificial aging process applied to asphalt binders characterized by different degrees of penetration was carried out by performing rheological characterization, thermal analysis by means of differential scanning calorimetry and optical microscopy, X-Ray powder diffraction, FT-IR and Raman spectroscopy. The microstructural information such as dimension of unit cell (crystalline parameters) layer diameter, inter-layer distance, height of the unit cell, and the aromaticity were calculated and compared between unaged and aged asphaltenes. The parameter that quantifies the aromaticity proved to be a good indicator of the extent of aging to which the bituminous materials under study were subjected. It turned out that more rigid bitumens possess more polar fractions which can be oxidized by aging, therefore bringing to a more pronounced increase in rigidity upon aging.

A tunable metasurface based on Vanadium dioxide for Broadband RCS reduction

In this paper, a novel chessboard-like of metasurface based on two kinds of unit cell, using Vanadium dioxide (VO2) is proposed, designed, and demonstrated for radar cross-section (RCS) reduction. By accurately designing the geometric parameters of two units, the 180 ∘ ± 37 ∘ reflection phase difference between the reflected waves can be obtained over a broadband region, thereby causing the interference cancellation and leading to the RCS reduction. In addition, the utilization of VO2 enables this metasurface to realize the reconfiguration of the working band while under different temperature conditions. Based on the above points, simulation results show that the proposed metasurface is provided with a −10 dB RCS reduction in 6.40–7.23 GHz and 14.97–23.97 GHz at a low-temperature state (300 K), and can be shifted to 7.10–14.13 GHz and 24.10–25.87 GHz at a high-temperature state (350 K), actualizing full coverage of the working frequency band from 6.40–25.87 GHz. Besides, the influences brought by different periodic arrangements of the unit cell and dielectric layer thickness on the reduction of RCS are also discussed in detail.

Multiband ultrathin chiral metasurface for polarization conversion, symmetric and asymmetric transmission

This paper presents a planar chiral metasurface with two layers of omega-shaped unit cells placed on opposite sides of a substrate. The metasurface achieves linear-to-circular polarization conversion within a single band and cross-polarized transmission of linearly polarized incident waves in five bands. Due to the asymmetry and anisotropy of the metasurface in its plane, the linear-to-circular and cross-polarization conversion transmission bands are asymmetric in nature. The structure also exhibits a symmetric co-polarized transmission band for both x- and y-directed incident E-field orientations. The proposed structure gives good response for incident angles up to 600 from 3 GHz to 11.5 GHz and up to 300 from 11.5 GHz to 13.5 GHz for both transverse-electric and transverse-magnetic incidence. It is also ultrathin and has a thickness of λ0/64.7, where λ0 is the free space wavelength at the lowest frequency of operation. The proposed MS can be used for S, C, X as well as Ku microwave band applications.