Small Tilt Angles Research Articles

Stable orbit bifurcations in a quantum well in a tilted magnetic field

Peak doubling regions are observed in resonant tunneling spectroscopy of a quantum well in a tilted magnetic field. They are identified with bifurcations of stable periodic orbits. The narrowness of the peak doubling region at small tilt angles arises from a “double bifurcation” which restabilizes the original periodic orbit.

Anisotropic resistivity in pulsed-laser deposited Bi2Sr2CaCu2O8+δ films

An orientational dependence of the resistivity and critical current density was observed inc-axis oriented Bi2Sr2CaCu2O8+δ films grown on MgO substrates by pulsed-laser deposition. As derived from X-ray diffraction and scanning electron microscopy, the anisotropy is caused by the small tilt angle of thec-axis with respect to the substrate surface.

Anomalous phase behavior of long chain saturated lecithin bilayers

X-ray scattering has been performed on fully hydrated unoriented multilamellar vesicles of lecithins with even chain lengths n from 16 to 24 as a function of temperature in chain ordered phases. The longer chain lengths, n ≥ 20, show anomalous behavior compared to the shorter chain lengths, n < 20. This report concentrates on n = 24. Although the history and time dependence shows that equilibrium was not always achieved, it appears that there is a second gel-like phase G2 below 40°C. The G2 phase has a small tilt angle and opposite hexagonal symmetry breaking from the usual G1 gel phase. Also, as T is raised above 45°C, the wide-angle data suggest the appearance of a phase with hexagonal chain packing and small chain tilt angle.

Selection of transverse oscillation modes in tilted ridge DFB lasers

In this paper, the effects of tilt angle on multi-transverse moded ridge-waveguide DFB lasers is presented. A small tilt angle is introduced between the waveguide orientation and the gratings to control the lasing wavelength. The authors find that although each transverse mode experiences a wavelength shift that approximately varies as 1/cos /spl theta/, the accompanying variation in /spl kappa/ dictates which transverse mode will oscillate. At small tilt angles the fundamental E/sub x,0/ mode has the lowest threshold gain; at larger angles the higher order transverse modes will be sequentially selected while all other modes are attenuated. As a result, the wavelength versus tilt angle exhibits sudden jumps due to this transverse mode selection in addition to the 1/cos /spl theta/ dependence. For the analysis, the authors develop a two-dimensional finite-difference method for modeling the multilayered ridge waveguide modes. The effect of tilt angle on the intramode coupling coefficient is derived and the behavior of lasing wavelength, threshold gain and stop-band width are studied.

Development of Two-Axis Actuator with Small Tilt Angles for One-Piece Optical Heads

An integrated one-piece optical head for optical disk drives is effective for reducing the head size and tracking error signal offset. The authors have developed a two-axis actuator which can drive a one-piece optical head with small tilt angles by compensating the torque which has two main components acting in opposite directions; one originates from the focusing driving coil and the other from the tracking coil. The tilt angle of ±0.8 mrad was achieved when the extent of actuator movement was ±0.5 mm in the focusing direction and ±0.4 mm in the tracking direction.

Precursors and transition to chaos in a quantum well in a tilted magnetic field.

The discovery of a distinct transition from integrable to chaotic electron dynamics results from resonant tunneling spectroscopy of a quantum well in a tilted magnetic field. The tunneling spectra show peak doubling regions at small tilt angles, which give way to peak tripling at larger tilt angles. The transition to chaos is mapped systematically (by varying bias voltage, magnetic field, tilt angle, and the width of the quantum well). We find that peak doubling (tripling) results from bifurcation (trifurcation) of the dominant stable periodic orbit. These are precursors of the chaos transition.

High-Q oscillator measurements of vortex properties in YBa2Cu3O7??

High-Q oscillators, vibrating with extremely small tilt angles, ∼10−7 radians, were used to study vortex behavior in single crystal YBa2Cu3O7−δ. Systematic changes in the dissipation and frequency of the oscillator were explored for various fields both parallel and perpendicular to twin boundaries. At the small tilt oscillation amplitudes employed here, a number of features were observed which are not evident at the more typically utilized large oscillation amplitudes.

95/04074 Stack effect in tall buildings

An investigation of the wind field around roof-mounted solar arrays has been undertaken, utilizing synchronized time-resolved particle image velocimetry and pressure measurements, in order to better understand the flow structures and aerodynamic mechanisms which cause the peak wind loads. The study focused on wall normal wind directions, which result in critical loads on panels within the separation bubble. The mean flow is not significantly altered above the array as compared to that for a bare roof. However, the array has a significant effect on the turbulence above and around the panels in the separation bubble. When panels are installed on the roof, both Reynolds normal and Reynolds shear stresses are markedly reduced when compared to the flow over the bare roof in the region of the separated shear layer. Ensemble averaged flow fields, conditioned on peak panel uplift, were used to investigate the mechanisms associated with peak uplifts on the array. As the tilt angle of the solar array is increased, a progression from purely vortex driven suctions, which cause peak uplifts on a bare roof, towards local flow driven uplifts is observed. For small tilt angle (2°) arrays this local flow is established by large-scale building generated vortices, while for the larger tilt angle (20°) arrays the instantaneous reattachment of the (building generated) separated shear layer sets up the local flow. For the larger tilt angles, south wind peak uplifts are driven by large vertical gusts, while peaks for northern winds are the result of streamwise gusts. The interaction of these large-scale features with the panels, in the form of locally oriented drag, results in the peak uplifting loads.

Simulations of Self-Assembled Monolayers with the Same Surface Density but Different Grafting Patterns

Molecular dynamics simulations have been carried out for self-assembled monolayers of alkylsiloxane molecules on planar silica substrates. The simulation system consisted of 36 alkylsiloxane molecules terminated by hydroxyl groups arranged in a simulation cell with periodic boundary conditions in the plane of the surface. Two models have been studied as alternative representations of the attachment of the alkylsiloxanes to the surface : a square lattice (model I) and a triangular lattice (model II), formed by the head groups of the chains with the same surface density. After equilibration at 300 or 350 K, the two models yield monolayers in which the chains exhibit a different orientation, which suggests that the tilt angle is not determined by surface density alone. The average thickness of the monolayers for model I is smaller than that for model II because the chains in model II have a smaller tilt angle. When the tilt angle of a chain is large, the chain is found to favor an orientation in which the plane containing the carbon backbone is perpendicular to the plane formed by the chain axis and the surface normal. The fraction of gauche conformation is 2.6%, and most of the gauche defect occurs at the last C-C bond near the tail of the chains. The surface pattern formed by the tail oxygen atoms is disordered, and the approach between these oxygen atoms may be the result of the attraction between oxygen and hydrogen atoms.

Orientation Model for Particles in Turbulence

Abstract The problem of predicting the orientations of falling nonspherical particles has been addressed by the construction of a heuristic model that assumes the particles are subject to isotropic turbulence within or below the inertial subrange, that is, the Kolmogorov spectrum of eddies, depending on the particle dimensions. The rms tilt angle of a spheroidal particle of small eccentricity is determined by Langevin-type averaging over its equation of motion, taking into account the first-order restoring torque that arises when the stable fall mode is perturbed by either thermal or turbulent fluctuations. By invoking dimensional constraints concerning the nature of the main flow and turbulent stresses and by assuming the thermal and turbulent fluctuations are uncorrelated, an approximate expression for the variance of an assumed Gaussian orientation distribution for small tilt angles and small flow Reynolds numbers is obtained. The expression is then generalized to provide a semiquantitative, nearly Gau...

Asymmetric adsorption of CS 2 on Cu(100) studied by S K-edge X-ray absorption fine structure spectroscopy

The surface structure and the electronic property of submonolayer carbon disulfide CS 2 adsorbed molecularly on Cu(100) at 106 K were investigated by means of S K-edge surface extended X-ray absorption fine structure (SEXAFS), near edge X-ray absorption fine structure (NEXAFS) and X-ray photoelectron spectroscopy (XPS) measurements. The CS 2 molecule is found by NEXAFS to lie nearly flat on the surface with a small tilt angle of the molecular axis from the surface. The π∗ resonance intensity in NEXAFS suggests that the adsorbed species is in the state of CS 1.3− 2. SEXAFS has clarified the adsorption geometry of the CS 2 molecule; the molecule adsorbs asymmetrically with the two S atoms respectively locating at the fourfold hollow site and closely to the bridge site. Such an adsorption structure is quite similar to submonolayer CS 2 on a Cu(111) surface.

Eye movements and motion perception induced by off-vertical axis rotation (OVAR) at small angles of tilt after spaceflight.

The nystagmus and motion perception of two astronauts were recorded during Earth-vertical axis rotation and during off-vertical axis rotation (OVAR) before and after 7 days of spaceflight. Postflight, the peak velocity and duration of per- and postrotatory nystagmus during velocity steps about the Earth-vertical axis were the same as preflight values. During OVAR at constant velocity (45/s, tilt angles successively 5, 10, and 15 degrees), the mean horizontal slow-phase eye velocity (bias), produced by the 'velocity storage mechanism' in the vestibular system, and the peak-to-peak amplitude (modulation) in horizontal eye velocity and position, generated from the output of otolith afferents, were also the same before as after flight. There were, however, changes in the vertical eve position and in the perceived body motion during OVAR. The angle of the perceived body path described as a cone was larger in both astronauts postflight. One astronaut experienced either a large cone angle with its axis upright, or a smaller cone angle with its axis tilted backwards, accompanied by an upward vertical eye drift. These results suggest an increase in the sensitivity of the otolithic system after spaceflight and a longer period of readaptation to Earth's gravity for otolith-induced responses than for canal-induced responses. Our data support the hypothesis that just after spaceflight the CNS generally interprets changes in the otolith signals to be due to translation rather than to tilt.

MATCHED FIELD SOURCE LOCALIZATION WITH A HORIZONTAL LINE ARRAY

Matched field source localization performance is investigated for a source in aft endfire of a horizontal line array operating in a deep water environment. Simulations are used to investigate the performance for ideal conditions and in the presence of noise. In particular, the effects of uncertainty in experimental geometry and mismatch in sound speed profile are considered. It is shown that the performance is sensitive to the vertical tilt of the array for relatively small tilt angles of less than 5º. However, the horizontal array is robust to mismatch in the gradient of the sound speed profile at the bottom of the water column. The results of the simulations are interpreted in terms of the characteristics of acoustic propagation in the environment.

Ordering effects in MOCVD grown Ga 0.5In 0.5P on misoriented (100) GaAs

The effects of substrate misorientation on the structural and optical properties of Ga 0.5In 0.5P on GaAs grown by low pressure metalorganic vapor phase epitaxy have been studied. The highest degree of ordering was found on the (100) exact substrates. The degree of ordering was found to decrease with the increase of misorientation angle toward all four directions, [111], [111], [110] and [101]; the most effective misorientation to disorder is tilted toward [111]. The band gap energy increases monotonically with increasing angle of misorientation. The value of the full width at half maximum (FWHM) is 14 meV for GaInP on 6° off toward [111] sample; this is among the narrowest FWHM ever reported at low temperature. For three other tilt directions, the FWHM initially increase at a small tilt angle and then decrease with increasing tilt angle. From these, we found the best misorientation is tilted toward [111] which has the disordered structure to achieve the high energy band gap and narrow FWHM.

The formation and characterization of Langmuir-Blodgett films of dipalmitoylphosphatidic acid

Comparison of small-angle X-ray diffraction patterns of Langmuir-Blodgett layers of 1,2-dipalmitoyl-sn-glycero-3-phosphatidic acid on different substrates reveals a small range of values for the monolayer thickness. These are close to the length of the lipid molecule. Polarized attenuated-total-internal-reflection Fourier transform IR spectroscopy shows a high degree of order in the multilayer film, with the alkyl chains approximately parallel to the substrate normal. Together, these results suggest either a very small molecular tilt angle or slight interdigitation of the layers.

Thin Injection-Molded Substrate for High Density Recording Phase-Change Rewritable Optical Disk

A thin disk substrate was examined for use as a high-density recording optical disk. An injection-molded polycarbonate thin disk substrate of 0.6 mm thickness showed birefringence under 60 nm and small tilt angle under 5 mrad. The track pitch was 1.0 µm and the diameter was 86 mm. The laser source was visible light at wavelength 680 nm, and the numerical aperture (NA) of the optical head was as large as 0.6. This new high-density disk and drive system had the characteristics of wide tilt angle tolerance, equivalent to conventional thick substrates of 1.2 mm, and high-density small mark length of 0.52 µm, giving a carrier-to-noise (C/N) ratio beyond 48 dB. We found that for new high-density disk generation, thin disk substrates were suitable.

Molecular packing in water supported monolayers of F(CF2)11COOH and F(CF2)10CH2COOH

We report the results of a grazing incidence x-ray diffraction study of monolayers of F(CF2)11COOH and F(CF2)10CH2COOH on water at several temperatures. It is found that in both monolayers the amphiphile molecules have a very small collective tilt angle (relative to the normal to the surface). For the temperature range studied (∼4–30 °C) the data for both monolayers indicate the existence of only a first-order transition between an ordered condensed phase and a disordered dilute phase; there is no evidence, in this temperature range, for the existence of a continuous tilt transition such as has been observed in monolayers of arachidic acid. The experimental data suggest that the critical point for the transition observed is at a lower temperature for a monolayer of F(CF2)11COOH than for a monolayer of F(CF2)10CH2COOH.

From vicinal to rough crystal surfaces

One generally expects the properties of a vicinal surface to be independent of the existence of steps as soon as these steps overlap, i.e. when their mutual distance is smaller than their width. By using the roughening theory by Nozieres and Gallet [1], we show that, at least for surfaces weakly coupled to the lattice, this overlap occurs for distances significantly larger than the commonly defined width. Our prediction is supported by an analysis of the various measurements of the angular variation of the surface stiffness of helium crystals, which were performed by Wolf et al. [2], Andreeva et al. [3] and Babkin et al. [4]. As a consequence, the interaction between crystal steps should be studied on vicinal surfaces with a much smaller tilt angle than previously thought. This article is also an opportunity to return to the relation between the step width and the correlation length on smooth surfaces, as well as to the treatment of the various finite size effects which occur in the problem of roughening. We finally reconsider how the weak coupling hypothesis applies to the case of helium crystals

Structural properties of Co/Au superlattices investigated by high-resolution transmission electron microscopy

Ion-beam-sputtered and electron-beam-evaporated Co/Au multilayers have been investigated with high-resolution transmission electron microscopy. The multilayers are found to exhibit a strong {111} texture and to contain Co predominantly in the cubic (fcc) phase. Misfit dislocations were directly observed and found to be located near the Co/Au interfaces. Structural features correlate with the magnetic anisotropy. Large crystalline units (vertical columns with ∼50 nm diameter reaching throughout the superlattice) are composed of small regions, a few nanometers in size (nanocrystallites), which are tilted towards each other by a few degrees. The microstructure of the large units remains unchanged for all investigated samples. The average size of the nanocrystallites and their mutual tilting angle vary systematically and are obviously correlated to the magnetic anisotropy. A high-crystalline quality (large nanocrystallites and small tilt angles) increases the value of the magnetic anisotropy constant Keffu. This behavior is unique for the presently investigated Co/Au multilayers independent of the fabrication technique. Moreover, these magnetostructural correlations may as well be valid for other magnetic/nonmagnetic metal superlattices.

Assessment of coupled-mode theory for tapered optical coupler

The accuracy and the scope of validity of different coupled-mode formulations for a tapered optical waveguide coupler are assessed by comparison with the beam propagation method (BPM). It is demonstrated that the coupled-mode theory (CMT) based on local modes is valid only for small tilt angles, whereas an improved formulation that takes into account the wavefront-tilt effect yields highly accurate results for relatively large tilt angles. The radiation loss at the input and the output junctions can also be estimated by a simple mode-matching method combined with the improved CMT. The coherent coupling between two junctions,which is neglected in the coupled-mode formulations, is observed in the BPM simulations.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>