Opposite Tilt Research Articles

Spontaneous Enantiomeric Resolution in a Fluid Smectic Phase of a Racemate.

Right- and left-handed homochiral domains segregate in a fluid smectic phase of the racemic (R*,S*)-beta-Me-TFMHPOBC analogue shown. This CF(3)-containing liquid crystalline compound exhibits an electrooptic response in a homogeneous cell, although no macroscopic dipole is expected to exist. Moreover, the homeotropic cells of this material exhibit striped domains, which are associated with fine stripes having two opposite tilt senses with respect to the predominant stripes.

Competition between ferroelectric and flexoelectric polarization in freely suspended smectic films

We report a detailed ellipsometric study of freely suspended films of chiral liquid-crystal compounds possessing smectic-A and smectic-C phases. In the temperature region between the smectic-A - smectic-C bulk and surface transitions, a discontinuous reconstruction of the tilt profile across the film is observed in the presence of a constant d.c. electric field. Comparison of the measured ellipsometric quantities with values calculated from model tilt profiles reveals a competition between a structure possessing a homogeneous tilt direction and large ferroelectric polarization and a structure with opposite tilt direction in the two film halfs and large flexoelectric polarization.

Formation of regular step arrays on Si(111)7×7

Highly regular arrays of steps are produced on vicinal Si(111)7×7 surfaces. A tilt of the surface normal from (111) toward (1̄1̄2) produces single steps (0.3 nm high and typically 15 nm apart). The opposite tilt toward (112̄) produces bunched steps with adjustable height (1–5 nm) and a spacing of 70 nm. Preparation criteria for straight edges and regular spacings are determined, taking into account the miscut angle (azimuthal and polar), annealing sequence, current direction, and applied stress.

Synthesis, characterization, electro-optical studies and crystal structure of a chiral mesogenic compound

Abstract A new series of chiral biphenyl derivatives presenting the following general sequence of phases Cr→S∗c→N∗→I was synthesized and characterized. The mesomorphic properties were analyzed by optical microscopy, DSC and electro-optical measurements. This series presents a large temperature domain for the ferroelectric phase and large values for the spontaneous polarization (up to 200nC cm−2). For this series, the crystal structure of 4-[(2S)-2-chloro-3-methylbutanoyloxy] biphenyl-4-yl 4-undecyloxy-2,3-difluorobenzoate (C35H41O5F2Cl) is described. This compound crystallizes in the monoclinic system and P21 space group with two independent molecules per asymmetric unit (Z = 4). The cell parameters are the following: a = 6.764(5), b = 57.057(6), c = 8.597(2)Å and β = 99.08(1)°; λ(CuKα)' 1.54178 Å, R = 0.105 for 2987 observed reflections. The independent molecules A and B adopt a parallel head-to-head arrangement which gives rise to sheets parallel to the (xOz) plane with a thickness close to 28.5 Å and having a permanent dipole moment. The tilt angle within a sheet is close to 40°. Neighbouring sheets, generated by the 21 axis, are symmetric through the (xOz) plane with an opposite tilt direction and dipole moment. The molecular arrangement is antiferroelectric-like. The crystal cohesion can be attributed to dipole-dipole forces and van der Waals interactions. The structure of the present compound is compared to that of a similar compound which differs only by the replacement of one of the two terminal methyl groups with an ethyl group.

Bifurcating jets generated with stepped and sawtooth nozzles

A simple round jet is perturbed by attaching either a stepped or a sawtooth trailing edge that acts as a form of passive control. Flow visualization, laser Doppler anemometer, and hot-wire measurements are used to document the flow behavior in both water and air for Reynolds numbers of 7600 and 22 000, respectively. When the jet is forced, the flow bifurcates. The bifurcation requires a low forcing frequency that encourages the formation of tilted primary vortex rings with significant vorticity and axial spacing. In the step nozzle flow, the rings result from closed vortex loops. In the sawtooth nozzle flow, they develop from sections of a helix. A powerful secondary jet initiates in straining regions between primary ring cores, where closed-loop or helical structures with opposite tilt are present. The radial momentum in this jet is large enough to divert the path of the primary jet in the opposite direction. The bifurcating effect was found to be strongest when the forcing frequency was less than or equal to one-third of the natural roll-up frequency.

Internal structure and deformation of an accommodation zone in the northern part of the Suez rift

Detailed structural study of the eastern (onshore) part of the Gharandal accommodation zone that separates the northern (SW-dipping) and central (NE-dipping) half grabens of the Suez rift helped\\decipher the internal structure and deformation of accommodation zones of continental rifts. This 60 km-wide zone is affected by pure normal faulting. The NE-dipping faults of the northern half graben extend southward into the accommodation zone where they interfinger with SW-dipping faults extending from the central half graben. These two sets of rift-parallel faults form several horsts and grabens in the accommodation zone. Areas dipping parallel to the northern or southern half grabens form several intermixing dip domains in the accommodation zone. Smaller-scale accommodation of dip between these dip domains proceeds by the development of rift-parallel folds (twist zones). In contrast to the southern accommodation zone of the Suez rift, the internal structure of the Gharandal accommodation zone is believed to be representative of accommodation zones in regions (a) unaffected by prerift structures lying at high angles to the rift; and (b) experiencing relatively small extension. Accommodation zones in areas having pre-rift structures lying at high angle to the rift have relatively narrow width and are characterized by transverse, strike-slip faults. Strike-slip movement on these faults is related to the torsional strain resulting from the opposite tilt directions and transport of fault blocks of adjacent half grabens.

Trigonal interactions in holmium.

The low-temperature magnetic structure of Ho has been studied using a combination of neutron-scattering and mean-field calculations. Re-examination of the cone and one-spin-slip phases has shown that the structures are distorted by interactions of trigonal symmetry, similar to those recently hypothesized for Er. In the cone phase below 18.5 K the moments have two out-of-plane tilt angles with a difference in angle of approximately 1.0\ifmmode^\circ\else\textdegree\fi{} about a mean value of 9.0\ifmmode^\circ\else\textdegree\fi{}. The spin-slip phases are more complex. The moments are modulated with a period twice that of the basic spin-slip structure, giving rise to additional peaks in the scattering. Spin-slip structures found in a c-axis field are shown to be consistent with our model. In zero field, the phase transition to the cone phase occurs in two distinct steps, as suggested by anomalies found in several bulk measurements. The intermediate phase has the wave vector locked into (1/6)${\mathbf{c}}^{\mathrm{*}}$, where any group of four moments has two tilted in one sense along c, and the other two adopt an equal and opposite tilt, resulting in no net c-axis moment. Below 18.5 K this arrangement then tilts out of the basal plane, forming the distorted cone structure.

Solitons in cell membranes.

The other equation of the set resembles an equation derived from the Nernst-Planck equation, which describes transmembrane ion transport and hence provides a mechanism for transmembrane potential variation. A more complete calculation of the velocity of the asymptotic wave form shows a lower wave speed than the estimate of Nagumo {ital et} {ital al}. The piezoelectric properties of the phase compete with its curvature elasticity to produce the soliton lattice of the cell membrane, which consists of juxtaposed regions of opposite tilt orientations. The propagation of the solitary wave requires a switching electric field, which is the form for the action potential and which moves the polarized domains by ferroelectric switching.

Selective Excitation of Tropical Atmospheric Waves in Wave-CISK: The Effect of Vertical Wind Shear

The growth of waves and the generation of potential energy in wave-CISK require unstable waves to tilt with height oppositely to their direction of propagation. This makes the structures and instability properties of these waves very sensitive to the presence of vertical shear in the basic flow. Equatorial Kelvin and Rossby-gravity waves have opposite phase tilt with height to what they have in the stratosphere, and their growth is selectively favored by basic flows with westward vertical shear and eastward vertical shear, respectively. Similar calculations are also made for gravity waves and Rossby waves. It is shown that eastward vertical shear of the basic flow promotes CISK for westward propagating Rossby-gravity, Rossby, and gravity waves and suppresses CISK for eastward propagating Kelvin and gravity waves, while westward shear of the basic flow has the reverse effects.

ENDOR investigation of the polar structure of tris-sarcosine calcium chloride

1H and 13C ENDOR investigations on Mn-doped tris-sarcosine calcium chloride (TSCC) and tris-sarcosine calcium bromide (TSCB) at 4.2 K reveal the polar structure of the calcium-(sarcosine) 6 complex in the ferroelectric phase of TSCC. The sarcosine molecules of type I do not change their position in the ferroelectric phase and give no contribution to the spontaneous polarization. The molecules of type II undergo an opposite tilt of about 3° in the (100) plane at 4.2 K. This tilting angle is temperature dependent and responsible for the polar structure of TSCC below T c.

Autotuning of a TEM using minimum electron dose

A new method is proposed to measure and correct defocus, astigmatism and beam tilt misalignment of a transmission electron microscope (TEM) automatically. The method is applicable to low-dose, high-resolution electron microscopy if the specimen may be described as a weak-phase object. Defocus and astigmatism are estimated from two different pairs of images. These are obtained, respectively, by tilting the illumination in two perpendicular directions over two equal but opposite angles. Beam tilt misalignment is estimated from three images, one formed without beam tilt and two with equal but opposite tilt angles. In all cases use is made of the cross-spectra of the images involved. An optimal measuring strategy has been designed and tested with simulations for the estimation of defocus and astigmatism. The design shows that the TEM should be tuned in two steps. The first step adjusts the defocus and astigmatism to 1.5 (in Sch), with a precision of 1. The second step adjusts the TEM with high precision. The simulations show that defocus and astigmatism can be estimated with a precision of 0.02 from images with an average intensity of 6500 electrons per nm 2. First measurements show a promising agreement of measured and simulated cross spectra.

Changes in Innominate Tilt After Manipulation of the Sacroiliac Joint in Patients with Low Back Pain

The purposes of this study were to 1) propose a method to detect sacroiliac joint dysfunction (SIJD), 2) test the interrater reliability of the method on a group of patients with low back pain (LBP), and 3) document changes in innominate tilt after manipulation of the sacroiliac joint. Criteria for SIJD were established by the authors. Twenty-six patients with unilateral LBP were examined independently for presence of SIJD by two examiners. Interrater agreement for presence or absence of SIJD was found to be excellent (Cohen's Kappa = .88). Twenty of the patients who met the criteria for SIJD were randomly assigned to an Experimental Group (n = 10) or a Control Group (n = 10). The left and right innominate bones of these 20 patients were measured for tilt before and after the intervention period. The sacroiliac joint of the patients in the Experimental Group was manipulated during the intervention period, whereas the patients in the Control Group received no treatment. Data were analyzed using a mixed three-factor analysis of variance. The data analysis revealed that the manipulation procedure resulted not only in an altered innominate tilt of the same side but also in an equal and opposite tilt of the opposite side (F = 67.07; df = 1.18; p less than .05). The results indicate that SIJD can be identified reliably in patients with LBP and that a manipulative procedure purported to be specific to the sacroiliac joint changes innominate tilt bilaterally and in opposite directions.

The Formation and Cathodoluminescence Activity of Buffer Layer Edge Dislocations in In0.12Ga0.88As/GaAs Heterostructures

ABSTRACTThe defect structure of lattice-mismatched one micron In0.12 Ga0.88As epilayers on (001) GaAs was studied with scanning cathodoluminescence (CL) and transmission electron microscopy (TEM). CL examination of the GaAs buffer layer revealed the formation of a segmented network of defects below the interface. Cross-sectional TEM analysis shows that these defects are dislocation half-loops extending from the interface, and the vast majority of these loops lie on the GaAs side of the interface. The dislocations in the GaAs buffer layer were determined to be edge dislocations. Thus, CL images show that edge dislocations in this system are centers for non-radiative recombination. We propose that two 60° dislocations with opposite screw and interface tilt components can glide into the buffer layer to form edge dislocations. Potential energy plots for 60° dislocations near the interface and interacting with interface dislocations supports this model.

STUDIES OF DOMAINS, WALLS AND DISCLINATIONS IN THE SMECTIC C PHASE

Domains of opposite tilt are generated and studied in smectic C samples with boundary conditions in principle giving a Schlieren type, although more regularly ordered, texture. A number of features characteristic of the C phase as a vector medium and differing from the analoguous nematic case are pointed out and investigated experimentally : a closed wall is equivalent to a family of point singularities and transforms to such a structure on annealing. This is a particularly illustrating example of the disclinations being created pair-wise, obeying a kind of conservation law for disclination strength, analoguous to charge conservation.

Depth adjacency and the rod-and-frame illusion

In Experiment I, the tactile adjustment of a comparison bar was used to measure the change in the rod-and-frame illusion as a function of the stereoscopic position of the rod relative to the frame. A vertical rod was presented at a near, a middle, or a far distance, with a single frame tilted counterclockwise at the near distance, a single frame tilted clockwise at the far distance, or two frames of opposite tilt presented simultaneously at the near and far distances. When only one frame was present, displacement of the rod in front of the far frame but not behind the near frame significantly reduced the illusion. When both frames were present, the illusion was determined mainly by the frame at the apparent distance of the rod. In Experiment II, the tactile method of measuring perceived tilt was validated for the range of perceived tilts encountered in Experiment I. The results of Experiment I are discussed in terms of the perceptual resolution of the conflict between relative and absolute cues of tilt.

The crystal structure of triacetylsphingosine.

The crystal structure of triacetylsphingosine (D-erythro-l,3-diacetoxy-2-acetamido-4-trans-octadecene, C24H43OsN) has been determined by direct methods. The crystals are orthorhombic, P212121, with a = 5.002, b = 8709 and c = 60.62/~. Positional and isotropic thermal parameters of the non-hydrogen atoms were refined to give a final R index of 0109. The molecules are arranged head-to-tail in layers within which the carbon chains pack according to the common orthorhombic subcell, O A.. The chain axis forms an angle of 58 ° with the end group planes. Adjacent layers show opposite tilt of the chains. In spite of the bulky acetyl branches the molecules adopt a very effective packing (Din = 1.07 g.cm-3). The molecules are connected by a continuous system of N-H--O hydrogen bonds parallel to a, and there is also evidence for two weaker C-H---O type interactions.

The interaction of postural systems in visual orientation

Visual orientation judgments were made during right head tilt, left body tilt, and left trunk tilt, each of 30 deg. In these postures the otolith, neck, and trunk systems are stimulated in pairs. The hypothesis that the visual vertical during trunk tilt is equal to the algebraic sum ofthose in the same direction ofbody tilt and the opposite head tilt was tested and accepted. This result demonstrates the interaction of these postural systems in visual orientation, and complements the confirmation of the same hypothesis for the visual aftereffect following prolonged tilt.

Electron Microscopic Images of Single and Intersecting Stacking Faults in Thick Foils II. Intersecting Faults

AbstractThe dynamic theory of electron transmission microscopic images of intersecting stacking faults is developed and discussed. Intersecting faults of the same type in f.c.c. alloys are considered, the fault planes having opposite tilt. The expected images are compared with observations on thick Cu‐15% Ga foils.The aspects of the bright‐field images are different under different diffraction conditions: (i) “streaked” fringes parallel to the projection of the intersection line for large values of the excitation error s; (ii) for small values of s the images consist of two distinct parts: the central one and the outer one. The central part consists of a few nearly unmodulated straight fringes parallel to the intersection. The outer part is a more or less deformed superposition of two fringe patterns. A pure superposition of the fringe patterns due to each fault separately is found for a particular value of s and for the outer part of the image.The qualitative features of the image for small s in thick foils are due to anomalous absorption effects.