Uniaxial Structures Research Articles

Homovalent cation substitutions leading to new lithium-containing La3CuSiS7 family members having chiral, uniaxial structures and exhibiting second- and third-harmonic generation

Eighteen isotypic, lithium-containing rare-earth sulfides, crystallizing in the chiral, polar, and noncentrosymmetric space group P63 with the La3CuSiS7-structure type have been prepared via direct combination of the elements or binary sulfides; nine of these compounds are reported for the first time. The structures of these compounds, with the formulae Ln3LiTS7 (Ln = La, Ce, Pr, Nd, Sm, Gd, and Dy for T = Si or Ge; Ln = La, Ce, Pr, and Nd for T = Sn), were determined by single crystal X-ray diffraction. According to X-ray powder diffraction data, the Ln3LiTS7 compounds are the major phase of the reaction products. All of the compounds are semiconductors with optical bandgaps spanning nearly the entire visible region. The Si- and Ge-containing analogs show high thermal stability,> 1000 °C, while the Sn-containing compounds melt in the vicinity of ~740 °C. The compounds show potentially broad regions of optical transparency in the infrared regime. Calculated bond valence sums (BVSs) and global instability index (G) values confirm the Ln3+ oxidation state and stable crystal structures with reasonable strain, respectively. Ce3LiGeS7 and Ce3LiSnS7 display non-phase-matching but significant second-harmonic generation responses with χ2 values of 21 ± 1 and 28 ± 1 pm/V, respectively at λ = 1.8 µm. The laser-induced damage threshold values for Ce3LiGeS7 and Ce3LiSnS7 are> 3 × commercial AgGaSe2, for picosecond pulses at λ = 1.064 µm. These two compounds also exhibit moderate third-harmonic generation responses. Additionally, crystal-chemical correlations are discussed.

Monolayer crumpled graphene-based mechanically and electrically tunable infrared absorbers for high-sensitivity sensing

Enhancing the absorption of infrared light in continuous monolayer graphene layer has received much attention in various optical devices. Here, based on the uniaxial and biaxial crumpled graphene structures, two mechanically and electrically tunable infrared absorbers have been proposed and investigated. We have demonstrated that the near-unity absorption spectra of proposed absorbers are ascribed to the electric resonance modes excited along the sidewalls of crumpled graphene structures, and a broad spectra tunability can be achieved through combing mechanical stretching/compression with the conventional electrostatic doping method. Furthermore, the refractive index sensing capacity of proposed biaxial absorber has been further investigated. Benefiting from the unique stereoscopic morphologies, the sensing sensitivity of each mode of the absorber can reach up to 5.546 and 8.251 µm per refractive index unit, respectively. The broad spectra tunability, high sensing sensitivity as well as flexible structures render it holding promising potentials in high-sensitivity flexible sensing applications.

Phase behavior of a lattice-gas model for biaxial nematics

We employ a lattice-gas extension of the Maier-Saupe model with discrete orientation states to study the phase behavior of a statistical model for biaxial nematogenic units in mean-field theory. The phase behavior of the system is investigated in terms of the strength of isotropic interaction between anisotropic objects, as well as the degree of biaxiality and the concentration of those units. We obtain phase diagrams with isotropic phases and stable biaxial and uniaxial nematic structures, various phase coexistences, many types of critical and multicritical behaviors, such as ordinary vapor-liquid critical points, critical end points, and tricritical points, and distinct Landau-like multicritical points. Our results widen the possibilities of relating the phenomenological coefficients of the Landau-de Gennes expansion to microscopic parameters, allowing an improved interpretation of theoretical fittings to experimental data.

Homovalent Cation Substitutions Leading to New Lithium-Containing La3cusis7 Family Members Having Chiral, Uniaxial Structures and Exhibiting Second- and Third-Harmonic Generation

Homovalent Cation Substitutions Leading to New Lithium-Containing La3cusis7 Family Members Having Chiral, Uniaxial Structures and Exhibiting Second- and Third-Harmonic Generation

Orientation control of lyotropic chromonic liquid crystals in the capillary bridge

LCLCs, PDI-HCl, are oriented in patterned capillary bridges. The given geometric confinement can control the orientation of nematic PDI-HCl to induce uniaxial, escaped, and chiral structures by varying the width of the capillary bridge.

Advanced recursive modal integral technique for accurate hybrid mode characterization of symmetrical/asymmetrical multilayered uniaxial anisotropic planar structures

This paper introduces, for the first time, an extended modal integral technique for reliable characterization of multilayered symmetrical/asymmetrical planar structures built on arbitrary number of uniaxial electric/magnetic anisotropic layers. Based on a simple but efficient recursive algorithm devoted to composite substrates, the proposed hybrid mode analysis involves the Galerkin’s procedure combined with the formalism concept of mathematical operators to more conveniently formulate the boundary conditions. By properly selecting adequate trial functions, the technique was demonstrated through a close agreement between the obtained results and published data.

Review of Recent Progress in Robotic Knee Prosthesis Related Techniques: Structure, Actuation and Control

As the essential technology of human-robotics interactive wearable devices, the robotic knee prosthesis can provide above-knee amputations with functional knee compensations to realize their physical and psychological social regression. With the development of mechanical and mechatronic science and technology, the fully active knee prosthesis that can provide subjects with actuating torques has demonstrated a better wearing performance in slope walking and stair ascent when compared with the passive and the semi-active ones. Additionally, with intelligent human-robotics control strategies and algorithms, the wearing effect of the knee prosthesis has been greatly enhanced in terms of stance stability and swing mobility. Therefore, to help readers to obtain an overview of recent progress in robotic knee prosthesis, this paper systematically categorized knee prostheses according to their integrated functions and introduced related research in the past ten years (2010–2020) regarding (1) mechanical design, including uniaxial, four-bar, and multi-bar knee structures, (2) actuating technology, including rigid and elastic actuation, and (3) control method, including mode identification, motion prediction, and automatic control. Quantitative and qualitative analysis and comparison of robotic knee prosthesis-related techniques are conducted. The development trends are concluded as follows: (1) bionic and lightweight structures with better mechanical performance, (2) bionic elastic actuation with energy-saving effect, (3) artificial intelligence-based bionic prosthetic control. Besides, challenges and innovative insights of customized lightweight bionic knee joint structure, highly efficient compact bionic actuation, and personalized daily multi-mode gait adaptation are also discussed in-depth to facilitate the future development of the robotic knee prosthesis.

Multiaxis three-dimensional (3D) glass fiber preform/cementitious matrix concrete composites: Experimental characterizations by panel test

Multiaxis three-dimensional (3D) alkali-resistant continuous glass fiber/cementitious matrix concrete composites were developed using the “winding and molding” method. The panel properties of the concretes were experimentally investigated and compared with the neat concrete. The experiment revealed that the load-bearing capability, strength and energy absorption properties of the multiaxis three-dimensional concretes were significantly influenced by the placement and orientation of the reinforcement fiber. The newly developed concrete composite was found to have the strain hardening behavior which attributed to an improved energy absorption performance of the concrete. It was also identified that four directional glass structure (ARG-4D) demonstrated somewhat better energy absorption performance compared to that of the biaxial (ARG-2D) and uniaxial (ARG-1D) structures because of the fiber orientation in the multiaxial direction in the concrete. Further investigation of the fractured four directional glass fiber/cementitious matrix concrete (ARG-4D) showed that the fiber-matrix interfacial debonding in each yarn set, local matrix breakages, yarn bridging due to tensile strengthening, tensile pull-out of outer filament and stick-slip frictions between the filament TOWs and the cementitious matrix were the principal fracture mechanisms. The micro-cracks occurred in multiaxis concretes were restricted by the multiaxis 3D fiber architecture. The multiaxis three-dimensional alkali-resistant continuous glass fiber/cementitious concrete can be considered as the damage tolerant structures with regard to neat concrete.

Convective meniscus splitting of polysaccharide microparticles on various surfaces

In contrast to convective self-assembly methods for colloidal crystals etc., “convective meniscus splitting method” was developed to fabricate three-dimensionally ordered polymeric structures. By controlling the geometry of evaporative interface of polymer solution, a deposited membrane with uniaxial orientation and layered structures can be prepared. Here it is demonstrated that xanthan gum polysaccharide microparticles with diameter ~ 1 µm can bridge a millimeter-scale gap to form such a membrane because the capillary force among the particles is more dominant than the gravitational force on the evaporative interface. This method is applicable for various substrates with a wide range of wettability (water contact angle, 11°–111°), such as glass, metals, and plastics. The specific deposition can be also confirmed between frosted glasses, functional-molecules-modified glasses, and gold-sputtered substrates. By using such a universal method, the membrane formed on a polydimethylsiloxane surface using this method will provide a new strategy to design a functional polysaccharide wall in microfluidic devices, such as mass-separators.

Multiaxis three dimensional (3D) carbon and basalt preforms/cementitious matrix concretes: Experimental study on fiber orientation and placement by panel test

Multiaxis three-dimensional (3D) continuous basalt and carbon fiber/cementitious matrix concretes were developed and their panel properties were experimentally investigated. It was found that the fiber placement and orientation in concrete remarkably affected its load carrying, strength, and energy absorption performance. It was identified that the multiaxis 3D fiber concrete had strain hardening behavior. The strain hardening increased the energy absorbing capacity of the concrete. Four directional basalt and carbon structures exhibited slightly better energy absorption compared to the biaxial and uniaxial structures due to the multiaxis fiber orientation. Failed fiber concretes had local matrix breakages, matrix-filament bundle debonding in each yarn set, yarn bridging due to tensile strengthening, outer filament tensile pull-out and stick–slip phenomena between filaments in the yarn, and a cementitious matrix. This led to the in-plane angularly radial aligned multiaxis micro cracks, which were probably controlled by the multiaxis 3D fiber architecture, and they were considered damage tolerant materials.

2 × 2 anisotropic transfer matrix approach for optical propagation in uniaxial transmission filter structures.

Multi-layered metamaterial structures show promise in a wide variety of optical applications such as superlenses, electromagnetic cloaking, tunable filters, sensors, and spatial light modulators. Optical transmission analysis of multilayer metallo-dielectric stacks with overall thickness less than the wavelength of light can be modeled using effective medium theory and the Berreman matrix method. For multilayer anisotropic stacks of arbitrary thickness, a rigorous 4 × 4 transfer matrix embodiment is typically used. In this work, a 2 × 2 anisotropic transfer matrix method is developed to analyze optical propagation through multilayer uniaxial stacks of arbitrary thicknesses. Optical transmission of a multilayer silver-zinc oxide stack deposited on a quartz substrate is modeled with this 2 × 2 anisotropic transfer matrix method and reconciled with experimental observations. Results indicate that this numerical approach is applicable to in situ assessment of the complex refractive indices of constituent metal and dielectric layers. Additionally, the anisotropic 2 × 2 transfer matrix method enables the possibility of modeling the transmission of the same metallo-dielectric structure deposited on an electro-optic, uniaxial substrate. Simulation results predict that adjusting the bias field across the substrate results in an electrically tunable transmission filter.

Graphical Representation and Explanation of the Conductivity Tensor of Anisotropic Media

Electrical anisotropy is a property of the Earth materials that can be studied through electromagnetic geophysical methods, such as magnetotellurics. It consists of the electrical conductivity changing with the orientation and being mathematically characterized by the conductivity tensor. In order to better understand the conductivity tensor and provide more effective tools for quantitatively analyzing the conductivity tensor of anisotropic structures, three graphical representations for symmetric tensors using ellipsoids, Mohr circles and geometric forms are presented. The ellipsoid representation can be applied to indicate the strength of the anisotropy in different directions. The Mohr circle provides a graphic representation of a tensor as a function of the rotation of the coordinate system. For the geometric forms, one-dimensional (1-D), two-dimensional (2-D) and three-dimensional (3-D) sheet models with given parameters (sizes and resistivities of the constituent prisms), the macroscopic anisotropic conductivity may be calculated using the closed-form mathematical formulas. These three graphical representations have different abilities for revealing information on the conductivity tensor. Four synthetic examples involving uniaxial or biaxial anisotropic conductivity structures are examined in the principal axis coordinate system to investigate the advantages and disadvantages of the graphical displays.

Design and Development of Artificial Spinal Ligaments for Paediatric Synthetic Spine

A synthetic spine is a model fabricated from artificial materials consisting of the vertebrae, intervertebral discs and ligaments for spinal testing. The synthetic spine overcomes many difficulties associated with biological specimens such as handling, biohazard concerns, high costs, and limited availability of specimens, quality and large inter-specimen variability. This paper presents the design and development of spinal ligaments to mimic the stiffness of the paediatric ligaments for use in the synthetic spine. Spinal ligaments are uniaxial structures in the spine that carry tensile loads along the direction of the fibres. Early in the research, silicone materials were used to cover the whole spinal unit, but it became apparent that the material responses were inadequate. The synthetic spine was revised to use fibreglass tape to more closely simulate the natural spinal structures. The composite design applied in this paper consisted of soft silicone rubber and fibreglass tape to obtain the natural stiffness that normally occurred in the spinal ligaments.

Orientational ordering and layering of hard plates in narrow slitlike pores

We examine the ordering behavior of hard platelike particles in a very narrow, slitlike pore using the Parsons-Lee density functional theory and the restricted orientation approximation. We observe that the plates are orientationally ordered and align perpendicularly (face-on) to the walls at low densities, a first-order layering transition occurs between uniaxial nematic structures having n and n+1 layers at intermediate densities, and even a phase transition between a monolayer with parallel (edge-on) orientational order and n layers with a perpendicular one can be detected at high densities. In addition to this, the edge-on monolayer is usually biaxial nematic, and a uniaxial-biaxial nematic phase transition can be also seen at very high densities.

Uniaxial and biaxial structures in the elastic Maier-Saupe model.

We perform statistical mechanics calculations to analyze the global phase diagram of a fully connected version of a Maier-Saupe-Zwanzig lattice model with the inclusion of couplings to an elastic strain field. We point out the presence of uniaxial and biaxial nematic structures, depending on temperature T and on the applied stress σ. Under uniaxial extensive tension, applied stress favors uniaxial orientation, and we obtain a first-order boundary along which there is a coexistence of two uniaxial paranematic phases, and which ends at a simple critical point. Under uniaxial compressive tension, stress favors biaxial orientation; for small values of the coupling parameters, the first-order boundary ends at a tricritical point, beyond which there is a continuous transition between a paranematic and a biaxially ordered structure. For some representative choices of the model parameters, we obtain a number of analytic results, including the location of critical and tricritical points and the line of stability of the biaxial phase.

Field-induced uniaxial and biaxial nematic phases in the Maier–Saupe–Zwanzig (MSZ) lattice model

ABSTRACTWe report detailed statistical mechanics calculations to analyse the field-temperature phase diagram of a fully connected Maier–Saupe–Zwanzig (MSZ) lattice model for the nematic transitions in liquid-crystalline systems. If the field is applied along the uniaxial direction, with positive anisotropy, there is a first-order boundary between distinct uniaxial paranematic phases, which ends at a simple critical point. For negative values of the anisotropy, the first-order boundary becomes continuous at a tricritical point, and the transition is between field-induced uniaxial and biaxial structures. This elementary MSZ lattice model provides a way to systematically regain all of the main features of earlier phenomenological and model calculations. Due to the simplicity of the calculations, critical and tricritical points, as well as critical lines, can be located analytically, which leads to a possibility of comparisons with values for real experimental systems. Also, we formulate a generalised six-state MSZ model, which takes into account intrinsic molecular biaxiality, and perform calculations to determine the location of the critical point in the field-temperature phase diagram.

Uniaxial strain-modulated electronic structures of CdX (X = S, Se, Te) from first-principles calculations: A comparison between bulk and nanowires**Project supported by the National Natural Science Foundation of China (Grant Nos. 11204296 and 61427901) and the National Basic Research Program of China (Grant Nos. 2014CB643902 and 2013CB933304).

Using first-principles calculations based on density functional theory, we systematically study the structural deformation and electronic properties of wurtzite CdX (X = S, Se, Te) bulk and nanowires (NWs) under uniaxial [0001] strain. Due to the intrinsic shrinking strain induced by surface contraction, large NWs with {100} facets have heavy hole (HH)-like valence band maximum (VBM) states, while NWs with {110} facets have crystal hole (CH)-like VBM states. The external uniaxial strain induces an HH–CH band crossing at a critical strain for both bulk and NWs, resulting in nonlinear variations in band gap and hole effective mass at VBM. Unlike the bulk phase, the critical strain of NWs highly depends on the character of the VBM state in the unstrained case, which is closely related to the size and facet of NWs. The critical strain of bulk is at compressive range, while the critical strain of NWs with HH-like and CH-like VBM appears at compressive and tensile strain, respectively. Due to the HH–CH band crossing, the charge distribution of the VBM state in NWs can also be tuned by the external uniaxial strain. Despite the complication of the VBM state, the electron effective mass at conduction band minimum (CBM) of NWs shows a linear relation with the CBM–HH energy difference, the same as the bulk material.

Creep behavior of submicron copper films under irradiation

The creep behavior under heavy ion irradiation of 200 nm and 500 nm thick annealed Cu films is characterized using on chip uniaxial microtensile test structures. The tests are performed at room temperature with an applied stress between 100 and 250 MPa and a damage rate of 5 × 10−4 and 6.3 × 10−4 dpa s−1. The deformation rates produced under irradiation are several orders of magnitude larger than when measured out of flux. The main advantage of this method is that it allows the simultaneous measurement of several tens of specimens fully irradiated over their entire thickness. The plasticity mechanisms appear remarkably more homogeneous during irradiation creep than under static loading. The creep power law involves a stress exponent equal to 5 depending only weakly on the microstructure of the films. The SEM and TEM microstructural observations suggest that the creep mechanism results from climb-assisted glide of dislocations as rationalized by a simple closed-form model.

Lattice Statistical Models for the Nematic Transitions in Liquid-Crystalline Systems

We investigate the connections between some simple Maier-Saupe lattice models, with a discrete choice of orientations of the microscopic directors, and a recent proposal of a two-tensor formalism to describe the phase diagrams of nematic liquid-crystalline systems. This two-tensor proposal is used to formulate the statistical problem in terms of fully-connected lattice Hamiltonians, with the local nematic directors restricted to the Cartesian axes. Depending on the choice of interaction parameters, we regain all of the main features of the original mean-field two-tensor calculations. With a standard choice of parameters, we obtain the well-known sequence of isotropic, uniaxial, and biaxial nematic structures, with a Landau multicritical point. With another suitably chosen set of parameters, we obtain two tricritical points, according to some recent predictions of the two-tensor calculations. The simple statistical lattice models are quite easy to work with, for all values of parameters, and the present calculations can be carried out beyond the mean-field level.

Anisotropic AC Magnetic Susceptibility in (Ga,Mn)As Films

Ferromagnetic semiconductor (Ga, Mn)As has emerged as the most thoroughly studied material for semiconductor spintronic applications, showing excellent magnetic properties that are usually described by the single-domain model [1]. However, certain uncertainties about magnetic phase structure and their dynamic properties of this system still remain unresolved. In this study, we systematically carry out both dc and ac magnetic measurements as a function of temperature and applied magnetic field in (Ga, Mn)As films with nominal Mn concentration of 5.0% grown by molecular beam epitaxy. The ac susceptibility vs. temperature shows similar results to those reported previously [2,3]: two peaks are observed with magnetic field H along [100], one close to T C and one close to T C /2, while one peak close to T C is observed with H along [1-10], and one peak close to T C /2 is seen for H along [110]. These results are consistent with dc magnetization measurements along these three axes. Careful analysis of hysteresis loops and of frequency-dependent ac susceptibility leads to the unambiguous conclusion that the ac susceptibility peak near T C occurs because of transition from paramagnetic to ferromagnetic phase; and the peak near T C /2 occurs because of the onset of a biaxial domain structure [4]. Specifically, ac susceptibility near T C involves a 180 degree magnetization flip along the [1-10] axis (i.e., the uniaxial easy axis), while the peak of the ac susceptibility near T C /2 originates from a magnetization flip between two biaxial easy axes with an angular separation of θ(T), arising from the competition between uniaxial and cubic anisotropy. Due to the difference between these two mechanisms, frequency dependences of ac susceptibilities in these two temperature regions are also quite different, providing additional information on both biaxial and uniaxial domain structures in (Ga, Mn)As films. In this connection, magnetic dynamic properties (such as the relaxation times) are analyzed and discussed using various dynamic models in order to allow us to get better insight into the evolution of magnetic domain transitions in (Ga, Mn)As films. The present observations on magnetic domain transitions and their dynamics provide new information relevant to spintronic applications such as memory and logic devices. This work was supported by the NSF Grant DMR14-00432.