Uniaxial Structures Research Articles

Epitaxial growth of erbium silicide nanowires on silicon(001)

Submonolayer amounts of Er deposited onto Si(001) react with the substrate to form epitaxial nanowires of crystalline ErSi 2. The ErSi 2 nanowires are <1 nm high, a few nanometers wide, close to a micron long, and crystallographically aligned to Si〈110〉 directions. The growth of uniaxial structures occurs because the different crystal structures of ErSi 2 and Si have a good lattice match along one Si〈110〉 crystallographic axis (−1.3%) but a significant mismatch along the perpendicular Si〈110〉 axis (+6.5%). An energetic model explains the formation of the ErSi 2 nanowires.

Desmin integrates the three-dimensional mechanical properties of muscles

Striated muscle is a linear motor whose properties have been defined in terms of uniaxial structures. The question addressed here is what contribution is made to the properties of this motor by extramyofilament cytoskeletal structures that are not aligned in parallel with the myofilaments. This question arose from observations that transverse loads increase muscle force production in diaphragm but not in the hindlimb muscle, thereby indicating the presence of structures that couple longitudinal and transverse properties of diaphragmatic muscle. Furthermore, we find that the diaphragms of null mutants for the cytoskeletal protein desmin show 1) significant reductions in coupling between the longitudinal and transverse properties, indicating for the first time a role for a specific protein in integrating the three-dimensional mechanical properties of muscle, 2) significant reductions in the stiffness and viscoelasticity of muscle, and 3) significant increases in tetanic force production. Thus desmin serves a complex mechanical function in diaphragm muscle by contributing both to passive stiffness and viscoelasticity and to modulation of active force production in a three-dimensional structural network. Our finding changes the paradigm of force transmission among cells by placing our understanding of the function of the cytoskeleton in the context of the structural and mechanical complexity of muscles.

Laser polarimetry of the orientation structure of bone tissue osteons

A matrix method is proposed for describing polarization properties of biological tissues as an aggregate of uniaxial crystal structures. We investigated 25 µm-thick histological sections of compact bone tissue which provided single light scattering. It is shown that in this case there exists a unique relationship between the polarization parameters of the boundary laser field and the parameters of anisotropy and orientation of the structural elements of biological tissue. On this basis, contactless methods of laser polarimetry of histological sections of bone tissue have been proposed.

Thermal coarsening of uniaxial and biaxial field-structured composites

When a suspension of colloidal particles is subjected to a strong electric or magnetic field, the induced dipolar interactions will cause the particles to form organized structures, provided a sufficient permittivity or permeability mismatch exists, respectively, between the particles and the suspending liquid. A uniaxial field will produce uniaxial structures, and a biaxial field, such as a rotating field, will produce biaxial structures, and either of these structures can be pinned by polymerizing the continuous phase to produce field-structured composites. We have previously reported on the coarsening of field-structured composites in the absence of thermal effects, i.e., Brownian motion. Athermal simulations are primarily valid in describing the deep quenches that occur when the induced dipolar interactions between particles greatly exceed kBT. However, deep quenches can lead to kinetic structures that are far from equilibrium. By introducing Brownian motion we have shown that structures with significantly greater anisotropy and crystallinity can form. These structures have enhanced material properties, such as the conductivity, permittivity, and optical attenuation. Careful anneals at certain fixed fields, or at continuously increasing fields, should produce more anisotropic structures than the deep quenches we have used to synthesize real materials.

Octupolar versus dipolar crystalline structures for nonlinear optics: A dual crystal and propagative engineering approach

A new type of crystalline structure for nonlinear optics whereby octupolar symmetry features are displayed at both molecular and crystalline levels is exemplified by the prototype 2,4,6-triaryloxy-1,3,5-triazine (TPOT) crystal and analyzed in terms of both individual molecular responses and crystal packing features. Polarized harmonic light scattering permits the full determination of the molecular β hyperpolarizability tensor and confirms the octupolar trigonal symmetry of the TPOT molecule. An oriented gas model is used to infer therefrom an estimate of the crystalline nonlinear d tensor which is predicted to be of the same order as that of the reference dipolar N-4-nitrophenyl-(L)-prolinol crystal. The concept of optimal packing toward quadratic nonlinear optics, which had been initially introduced in the realm of quasi-one-dimensional structures, is revisited and enlarged to encompass more isotropic uniaxial structures potentially amenable, in the case of octupoles, to larger optimal values than in the one-dimensional case. Moreover, considerations pertaining to phase matching which had been left aside in the earlier one-dimensional optimization framework are now considered and the various type I and type II configurations compared for both one-dimensional and octupolar uniaxial structures. Application perspectives of octupolar structures toward short pulse nonlinear optics are discussed: their structurally built-in polarization independence is outlined as a major asset in contrast with the more traditional one-dimensional structures.

The role of stress in the heteroepitaxy of Au on W(110)

The role of stress in forming a variety of Au film structures on W(110) is examined with scanning tunneling microscopy (STM). The structural manifestations are different from those previously observed with STM because this system involves both an fcc(111) bcc(110) interface that has mixed (tensile and compressive) strain and the Au(111) surface which reconstructs through strain relaxation. Whereas sub-monolayer films already show isotropic strain relaxation through a combination of three uniaxial expanded structures, the complete monolayer is pseudomorphic. Strain relaxation then leads to a two-dimensional dislocation structure in the bilayer film and a non-bulk-like yet fully strain-relaxed trilayer film. Stress is reintroduced for subsequently thicker films; this leads to the growth of flat-topped three-dimensional crystallites, none of which are terminated by the fourth layer, and to the eventual formation of the uniaxially compressed surface reconstruction of Au(111).

Hyper-elastic model analysis of anterior cruciate ligament

The objectives of this study were to theoretically analyze changes in shape and stress distribution which occur over the entire surface of the anterior cruciate ligament (ACL). While many studies on the cruciate ligaments' tensile characteristics have been conducted, the length patterns and the differential function of the ACL are still controversial issues for two reasons. Highly variable deformations of the ligaments (which are not uniaxial structures such as a bundle of separate fibres) cannot be adequately quantified by one-dimensional and/or localized measurements. Furthermore, it is impossible to directly measure in situ, non-uniform distribution of biaxial strain over the entire surface of the intra-articular ligaments. We employed an alternative approach which may have the potential to solve some of these difficulties. Using the finite element method, in which the ligament was treated as an incompressive hyper-elastic membrane, the finite deformation of the ligament was theoretically analyzed. Boundary conditions were applied by prescribing the displacements at the boundary nodes corresponding to the insertions. Special attention was paid to the distribution of longitudinal strain. The following values were obtained as a function of knee flexion: three-dimensional change in shape and strain distribution, length change of each fibre, length changes in positions along a fibre, resultant pull force on the tibial insertion, and change of strain pattern by anterior–posterior displacement of the tibia. The results demonstrated that strain distribution varied, even along the fibre run, and large strain gradients were observed in the regions near the insertions. It was found that distance between insertions introduced noticeable discrepancies from the length along a curved fibre. The net resultant force at the tibial insertion of the ligament was maximum at 0° of flexion, decreased until 50° of flexion, then slightly increased up to 120° of flexion. Predominantly, the anterior and the posterior displacements of the tibia, respectively, increased and decreased strain in the ligament, however, some portions were even more stretched by the posterior displacement.

Artificial nonreciprocal uniaxial magnetoelectric composites

In this communication we explore the possibilities of using magnetostatic resonators as inclusions for artificial magnetoelectric composites (as suggested by Kamenetskii). The appropriate form of effective constitutive relations is identified; the properties of eigenwaves and reflection and transmission coefficients in slabs are analyzed for uniaxial structures. Potential applications are briefly addressed. © 1997 John Wiley & Sons, Inc. Microwave Opt Technol Lett 15: 260–263, 1997.

ChemInform Abstract: Observation of Uniaxial Structures of Underpotentially Deposited Cadmium on Au(111) with in situ Scanning Tunneling Microscopy.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Observation of Uniaxial Structures of Underpotentially Deposited Cadmium on Au(111) with in Situ Scanning Tunneling Microscopy

The atomic structures of several distinct adlayers of Cd underpotentially deposited onto a Au(111) surface in sulfuric acid electrolyte were revealed by in situ scanning tunneling microscopy (STM). Three ordered adlattices were observed, all of which have a long-range linear morphology rotated by 30° from the substrate's lattice directions. Complete geometric descriptions and proposed structures are presented for each. A purely electrostatic model is insufficient to explain the banded morphology which is observed; therefore, other structure-forming interactions, including lattice strain, are discussed as possible contributors.

Maxwell Garnett modeling of uniaxial chiral composites with bianisotropic inclusions

A generalized Maxwell Garnett model for artificial composites formed of small bianisotropic particles arranged in three-dimensional arrays is developed. Explicit expressions for averaged dyadic constitutive parameters are given. The approach is suitable for the case of incomplete magnetoelectric coupling dyadics, since it leads to expressions which do not involve any inverses of these dyadics. Based on an analytical antenna model of single chiral particles, the averaged parameters can be easily evaluated. The theory is illustrated by numerical examples for uniaxial chiral structures. Reflection coefficients from an ideal conductor covered by uniaxial chiral slabs are calculated for different types of axially symmetric distributions of the inclusions. It is demonstrated that for different frequency ranges different geometries correspond to better reduction of reflection.

Study of the iron contribution to the 3d-sublattice anisotropy in some uniaxial YCoFe structures derived from the CaCu 5 unit cell

The effects of the Fe substitution for Co on the anisotropy of uniaxial structures (YCo 3, Y 2Co 7, Y 5Co 19, YCo 5) all derived from the parent CaCu 5 unit cell were studied. The unique Singular Point Detection (SPD) technique was used to measure temperature and composition dependence of the anisotropy. A characteristic behaviour of both Co and Fe anisotropies was individuated. The overall Fe sublattice anisotropy is always opposite and 2–3 times larger than that of Co. From the temperature behaviour of the anisotropy constant ( K 1), spin reorientation transitions (SRT) were predicted at temperature values which depend on Fe content. ac susceptibility measurements seem to confirm the presence of SRT. The magnetic phase diagrams of mixed Y-Co-Fe compounds were derived.

2 × 2 propagation matrix for electromagnetic waves propagating obliquely in layered inhomogeneous uniaxial media

A 2 × 2 propagation-matrix formalism for finding the obliquely propagating electromagnetic fields in layered inhomogeneous uniaxial structures is presented. The 2 × 2 propagation matrix was produced by our generalized geometrical-optics approximation method. A 2 × 1 field vector for the transverse electric field is defined together with a 2 × 2 propagation matrix, which describes the spatial propagation of the 2 × 1 field vector. Eight different equivalent forms for the propagation matrix are presented. The propagation matrix depends on the dielectric parameter and other optical parameters of the medium, the thickness of the medium, and the propagation vector of the incident light. The inhomogeneous medium is divided into sublayers, and the problem is then solved as if each layer has spatially invariant optical parameters. The propagation matrix for the inhomogeneous medium is equal to the matrix product of the propagation matrices for any complete series of layers that subdivide the inhomogeneous medium. The 2 × 2 propagation matrix reduces to the conventional Jones matrix for the case of normally incident light. The results are applied in studying the electro-optics of various liquid-crystal displays. The computation results are the same as those produced by the generalized geometrical-optics approximation and are in good agreement with those computed by the 4 × 4 propagation matrix and with experimental results.

First-principles calculation of the magnetocrystalline anisotropy energy of the pnictide MnSb

The magnetocrystalline anisotropy energy (MAE) of the pnictide MnSb is investigated by means of the augmented spherical wave method. The spin-orbit coupling enters the variational step of the procedure and the force theorem is applied to determine the dependence of the MAE on the directions of magnetization. The authors numerical results agree well with the first-order development of anisotropy energy in uniaxial crystal structures. The easy axis of magnetization is then found to be in the basal plane of the hexagonal structure in agreement with experiment, and the first-order anisotropy constant is given.

Equivalent Linearization for Seismic Responses. I: Formulation and Error Analysis

A random‐vibration approach for uniaxial and biaxial nonlinear structures is presented based on the stochastic equivalent‐linearization method. By modifying and combining existing modeling techniques, a unified hysteretic modeling scheme is proposed. The method is applicable both to conventional uniaxial systems and general orthotropic systems, i.e., structural systems having different nonlinear characteristics in different directions. A significant attention is directed to the numerical errors associated with the equivalent‐linearization approximation. Numerical errors are classified as stationary and nonstationary errors. The mechanism of each error is carefully studied and quantitatively evaluated through a series of parametric studies. The postyield stiffness ratio, a, has been singled out as a key parameter in the error analysis. A practical error‐correction scheme is presented based on a large number of simulations.

Linear theory of magnetocrystalline anisotropy and magnetostriction in exchange-dominated 3d-4f intermetallics.

Temperature dependence of the rare-earth contribution to magnetocrystalline anisotropy constants and magnetorestriction of 3d-4f intermetallic compounds is calculated in an approximation linear in the crystal-field and single-ion magnetoelastic interactions, which are regarded as perturbations with respect to a stronger 3d-4f exchange interaction. The results for an arbitrary quantum number J are compactly formulated in terms of new generalized Brillouin functions which coincide with the known reduced hyperbolic Bessel functions in the classical limit, J\ensuremath{\rightarrow}\ensuremath{\infty}. It is shown, however, that the quasiclassical approximation is inapplicable to the systems concerned. Specific cases of some uniaxial crystal structures are considered, being of interest for permanent magnet applications.

Dislocation dynamics. II. Applications to the formation of persistent slip bands, planar arrays, and dislocation cells.

The dynamic organization of dislocations into spatially heterogeneous substructures is demonstrated by applying the principles of dislocation dynamics that were outlined in the preceding paper. Here it is shown that the formation of persistent slip bands is a consequence of the competition between dipole formation and annihilation of dislocations of opposite Burgers vectors in the absence of temperature-enhanced climb recovery under cyclic stress conditions. Planar arrays, which are also uniaxial structures, are shown to arise from enhanced dislocation multiplication and the formation of stable dipole configurations along a slip plane at lower temperatures where climb is unimportant. Biaxial dislocation systems experience additional degrees of freedom compared with uniaxial systems because of available motion along additional slip systems. It is demonstrated that for a system of orthogonal slip directions at high temperatures in which climb and glide motion are competitive, dislocation cellular structures form as a result of immobile dipole and junction formation and by the internal elastic strain energy minimization caused by long-range self-shielding. It is shown that the internal elastic strain energy is reduced by the self-organization process. However, the short-range nonlinear processes (i.e., dipole and junction formation) are shown not to allow absolute elastic energy minimization.

Uniaxial incommensurate overlayers with two-component displacement vector

Uniaxial structures which consist of commensurate adatom rows are considered. According to experiment these structures are in the incommensurate state compressed normal to the rows and frequently tilted due to a shift of the rows parallel to each other. A simple theoretical model of these structures of constructed. For the ground state of this model both tilted and untilted phases are found. The possible phase transitions are considered. Upon heating at fixed coverage there can be phase transitions from untilted incommensurate to commensurate or disordered phases, and from tilted incommensurate to untilted incommensurate or disordered phases. Upon a change of coverage at constant temperature a commensurate crystal can undergo phase transformations to a commensurate structure with vacancies, to a fluid phase, or to an incommensurate untilted phase, depending on the system parameters. Possible phase diagrams are constructed.

Films formed from co-polymers containing azobenzene side groups

Co-polymers of styrene-maleic anhydride have been synthesised with a number of different loadings of azobenzene-containing side groups and formed into thin films by casting and by the Langmuir-Blodgett technique. By illumination with polarised white light, appropriately loaded materials cast as films could be transformed from isotropic to uniaxial structures that exhibited dichroism and birefringence of Delta n=0.21.

Pseudo-Stokes parameter representation of light propagation in layered inhomogeneous uniaxial media in the geometric optics approximation

We study the propagation of normally incident, elliptically polarized light in layered inhomogeneous uniaxial structures by using the geometric optics approximation (GOA). We show that, in the GOA, light propagation can be described by the evolution of four real-valued pseudo-Stokes parameters. These parameters are linearly related to the familiar Stokes parameters and obey a simple first-order differential equation. Conservation of energy and angular momentum is preserved under this approximation.