Lateral Fluctuations Research Articles

Effect of isotopic substitution on IR and ESR properties of mass selected ion beam deposited ta-C films

We have investigated the infrared transparency and electron spin resonance (ESR) signal of natural and 13C tetrahedral amorphous carbon (ta-C) films deposited by mass selected ion beam. In order to assess the significance of the results, which are averaged over the film, we have monitored the film through micro-Raman, showing that lateral fluctuation in the sp 3 content are limited. We show that ta-C films are transparent to IR radiation and that no hyperfine splitting can be observed in ESR spectra of 13C samples. The room-temperature line-width is the same (Δ H pp=5.5±0.1 G) in all samples. This is attributed to the strong signal narrowing caused by spin delocalization and short-range interaction at room temperature.

RBS studies of AlGaN/AlN Bragg reflectors

In this paper, the RBS phenomenon is described and experimental results concerning Bragg reflectors and microcavities grown on silicon substrates by molecular beam epitaxy are presented. This spectroscopy is not sensitive to (i) the lateral fluctuation of the alloy composition (the analyzed surface is about 1 mm(2)), (ii) to the residual strain of the layer or to (iii) the internal electric field. The measured thicknesses and compositions of the layers are very close to the nominal values. The Al concentration have been confirmed by low temperature photoluminescence experiments and the thicknesses by scanning electron microscopy studies. The straggling effect has been highlighted and plotted as function of the penetration depth of the particles.

Transient current generation during wear of high-density polyethylene by a stainless-steel stylus

Contact electrification between metals and insulators lead to dramatic transient charge transfer phenomena during sliding contact. We report simultaneous transient current and lateral force measurements as a stainless-steel stylus is drawn across a high-density polyethylene surface in vacuum. Stylus motion in this system is marked by unstable transitions between high and low velocity modes, similar to stickslip. The high velocity events coincide with falling lateral forces and high current signals. Scanning electron microscope images of the resulting wear tracks show slip-related features at intervals consistent with the lateral force and current fluctuations. Although average charge densities along the wear track ranged from 0.4−1 mC/m2, measurements at low normal forces are consistent with higher charge densities (up to 3 mC/m2) at isolated asperity contacts. Current transients as short as 60 μs were observed with total charges consistent with the detachment of 40×40 μm2 contact areas.

Polarization analysis of off-specular neutron scattering from domains and rough interfaces in a FeCoV/TiZr multilayer

Specular neutron reflectivity is commonly used as a powerful tool in determining the scattering length density (SLD) profile perpendicular to the surface of thin films. Information about the lateral fluctuations in the SLD, such as the presence of magnetic domains and rough interfaces between layers, can be obtained by studying the pattern of off-specular scattering. Advancements in neutron-source brilliance, beam-line optics and detector efficiency, as well as advances in the theoretical description of off-specular scattering, have contributed to the increasing importance of off-specular neutron-reflectometry studies. We present specular and off-specular polarized neutron reflectometry experiments on FeCoV/TiZr multilayers, with a detailed description of the orientations of the magnetic moments inside the FeCoV layers and at the FeCoV/TiZr interfaces.

Nanostructures with small- and large-scale periodic and random rough surfaces and lateral fluctuations in the permittivity and permeability-scattered near fields

The scattered electromagnetic near fields due to fluctuations in the surface height and/or lateral variations in the electromagnetic me- dium parameters are evaluated using a full-wave approach. Since the scales of the height and medium fluctuations considered could be sig- nificantly smaller or larger than the electromagnetic wavelengths, the familiar perturbation and physical/geometrical optics solutions cannot be used nor is it possible to investigate subwavelength features based on far-field measurements. The full-wave approach employs complete field expansion that includes propagating and evanescent waves as well as lateral waves and surface waves, which are not accounted for in the perturbation and physical optics solutions. Exact boundary conditions for the electric and magnetic fields are imposed at the rough interfaces. The complete modal expansions do not uniformly converge at the irregular boundaries, therefore the interchange of orders of differentiation and summation/integration of the field expansions is avoided. Large-scale and small-scale (relative to electromagnetic wavelength) approximations for the full-wave near-field solutions are expressed as integrals over the complete scattered wave number variables. Random rough surfaces as well as periodic and quasiperiodic structures are also considered. © 2002 Society of Photo-Optical Instrumentation Engineers. (DOI: 10.1117/1.1489050) Subject terms: near fields; electromagnetic wave scattering; lateral waves; sur- face waves; irregular stratified media; nanostructures.

Solid-liquid interface of a 2-propanol-perfluoromethylcyclohexane mixture: from adsorption to wetting.

The liquid-solid interface between a silicon substrate and the binary mixture perfluoromethylcyclohexane (PFMC) and 2-propanol (IP) is examined by x-ray specular reflectivity and diffuse scattering under grazing angles. The wetting films between the PFMC-rich phase and the substrate are characterized with respect to the density profile and lateral fluctuations. We find that the liquid-liquid interface of the film is anomalously broadened as compared to capillary wave theory. This broadening is caused by a locally slow variation of the density between the liquid phases and marks an adsorption profile that does not reflect the bulk properties of the film phase. Essentially the same behavior is present for a fused silica substrate.

Small-scale turbulence characteristics of two-dimensional bluff body wakes

Measurements have been made in nominally two-dimensional turbulent wakes generated by five different bluff bodies. Each wake has a different level of large-scale organization which is reflected in different amounts of large-scale anisotropy. Structure functions of streamwise (u) and lateral (v) velocity fluctuations at approximately the same value of Rλ, the Taylor microscale Reynolds number, indicate that inertial-range scales are significantly affected by the large-scale anisotropy. The effect is greater on v than u and more pronounced for the porous-body wakes than the solid-body wakes. In particular, ‘relative’ values of the scaling (or power-law) exponents indicate that the magnitude of the transverse exponents can exceed that of the longitudinal ones in the porous-body wakes. This is supported by the inertial-range behaviour of the spectra of u and v. The difference between the transverse and longitudinal exponents appears to depend on the large-scale anisotropy of the flow, as measured by the ratio of the variances of v and u and ratio of the integral length scales of v and u. The spanwise vorticity spectra are much less affected by the anisotropy than the spectra of u and v.

Rapid thermal annealing in GaNxAs1−x/GaAs structures: Effect of nitrogen reorganization on optical properties

We report on the effect of rapid thermal annealing (RTA) on the photoluminescence (PL) properties of GaNxAs1−x/GaAs structures. In particular, a blueshift of the PL peak energy is observed when annealing the samples. The results are examined as a consequence of a RTA-induced nitrogen diffusion inside the GaNxAs1−x material rather than diffusion out of the alloy, which homogenizes initial nitrogen composition fluctuations. We propose a simple model that describes the RTA-induced blueshift of the low temperature PL peak energy. This model is in good agreement with experimental results and is consistent with recent studies in which lateral composition fluctuations in the GaNAs alloy were reported.

Analysis of groundwater behaviour in a farmer controlled subsurface tile drainage system in Mardan, Pakistan

Extensive subsurface drainage system was installed in districtMardan in the North West Frontier Provinceof Pakistan in 1987 to control increasingwater logging and salinity problems due tocanal irrigation. Several recentlycompleted fields studies have indicatedthat subsurface drainage system hasenormously lowered watertable in certainareas due to extensive drainage network. Therefore, a study of controlled subsurfacedrainage technique was initiated in MardanScarp area to observe the temporal andspatial variations in water table depths ofthis specific case under various modes ofcanal irrigation and monsoon rains. Twoartificially drained areas, consisting of40 ha and 160 ha respectively, werecontrolled and selected for extensivemonitoring. A total of 98 observationswells (7.6 cm dia. and 4.1 m depth) wereinstalled in between lateral drains toobserve water table fluctuation. Theresults of this study are very interesting.Each of the two areas monitored in thestudy behaved differently. It was observedthat in one of the areas design water tabledepth at 1.1 m was maintained with properfunctioning of the controlled techniqueapplied to the subsurface drainage system. The results from this area showed that 25to 55% of the time throughout the yearachieved this objective whereas in thesecond area desired water table could notbe maintained and water table depth in thisarea remained between 2.0 to 2.7 m causingunnecessary water stress to plants. Alsoit was observed that watertable in theformer area is mostly controlled by thefunctional behavior of the irrigationcanal. In addition, the proper functioningof controlled techniques in subsurfacedrainage system supplemented veryefficiently to retain the groundwater levelto the optimal limits in dry season and tothe design ones in the others for timelyneeds of the crops. Also rainfalls havesignificant impact on the spatial andtemporal behaviors of water table depths inboth the areas during the monsoon season.

On the use of combined geophysical methods to assess water content and water conductivity of near-surface formations

We propose to deepen the interpretation of combined geophysical methods (georadar, seismic and electric) to assess physical properties characterizing the near-surface porous formations, especially the influence of water. Velocity analysis of multioffset georadar data are used together with seismic methods to estimate lateral and vertical ground water fluctuations. This enables us to identify transitions from non-saturated to fully saturated porous layers with certainty. Furthermore, the accurate knowledge of seismic velocities helps to estimate the porosity of the ground water formations. Finally, we show how the radar technique may be useful in solving the problem of trade-off between bulk resistivity, which is deduced from electrical measurements, and that of water ionic conductivity and water content. These theoretical considerations are illustrated using various measurements conducted at the same test site. Our interpretation is compared with a few laboratory measurements on water and soil samples. This study displays the impact of combined geophysical approaches for providing models of water and ionic transfers down to a depth of several metres.

Structural characterization of InGaAs/InP heterostructures grown under compressive and tensile stress

A systematic study of strain relaxation mechanisms by TEM, XRD, RBS-channelling, SEM-CL and AFM in In x Ga 1− x As/InP heterostructures grown by MOCVD under tensile and compressive initial misfit is reported. It is found that the layers under compression (0.61< x<0.74) relax nearly symmetrically along the two 〈1 1 0〉 directions. The residual strain vs. the layer thickness follows the same law experimentally determined for compressive MBE-grown In x Ga 1−x As/GaAs (x<0.2) specimens showing no composition, growth technique or temperature effect. The tensile layers (0.2< x<0.36) relax asymmetrically with a critical thickness larger than for the compressive case. The relaxation is faster along the [1 1 0]-direction with the asymmetry that increases by increasing the tensile misfit. Cracks, formed after the growth, are found to present a different density along [1 1 0] and [1 −1 0] depending on the residual strain. Grooves also develop along the two 〈1 1 0〉 directions as soon as a measurable strain relaxation appears. A possible correlation between grooves, planar defects and the mechanism of strain release is discussed. Strain-balanced In x Ga 1− x As/In y Ga 1− y As multi-quantum wells (MQWs) grown on a compositionally graded In x Ga 1− x As/GaAs buffer to extend the absorption edge of photovoltaic devices to 1 eV are finally investigated. The MD network results in a marked cross-hatched morphology that affects the lateral strain distribution in the whole structure. Lateral composition fluctuations in the MQW are discussed in terms of step bunching mechanism occurring at the valleys of the cross-hatched surface.

‘Intrinsic’ profiles and capillary waves at interfaces between coexisting phases in polymer blends

Lateral fluctuations in the local position of the center of the interface between coexisting phases in unmixed polymer blends lead to a broadening of interfacial widths; comparing self-consistent field predictions for the ‘intrinsic’ profile to simulations (or experiments), this ‘capillary wave’ broadening needs consideration. This problem has been studied by extensive Monte Carlo simulations of the bond fluctuation model for symmetrical polymer mixtures, both for free interfaces (between bulk phases) and for confined interfaces (in thin films between parallel walls). While the capillary wave predictions at large length scales are confirmed, the extraction of the ‘intrinsic’ profile remains a problem. Related experiments are also briefly discussed.

Suppression of lateral fluctuations in CdSe-based quantum wells

We report a reduction of inhomogeneous broadening in CdSe-related quantum wells in ZnSe by employing a growth technique that uses a CdS-compound source instead of the standard Cd elemental source for molecular-beam epitaxy. Assisted by the low sticking coefficient of sulfur and possibly an exchange reaction between S and Se, only a small S contamination is observed. A comparison with standard layers reveals an increase in quality and homogeneity by a strong reduction of the photoluminescence (PL) linewidth. Samples obtained by our method show extremely little lateral confinement as indicated by a lack of sharp single dot emission lines in micro PL and the absence of the extensive redshift observed in temperature dependent PL of fluctuating well potentials.

CoFe/Ir/CoFe artificial antiferromagnetic sandwich as a hard magnetic layer in hard-soft GMR sensors

We report on the magnetic and transport properties of CoFe/sub 3 nm//Ir/sub 0.5 nm//CoFe/sub 3 nm/ and CoFe/sub 1.5 nm//Ir/sub 0.5 nm//CoFe/sub 3 nm/ sandwiches, grown by ion beam sputtering. The sandwich with identical CoFe thicknesses presents a small rigidity and no plateau around zero field. For the same coupling strength, this rigidity and the plateau are strongly improved for the sandwich with asymmetrical CoFe thicknesses. In both cases, when the thicker layer is on the top or on the bottom of the AAF, coercive fields H/sub c/ of 600 Oe are obtained. However, the GMR plateau is only slightly improved for asymmetric sandwiches, probably due to lateral coupling fluctuations. Roughness analysis and TEM observations have been made in order to confirm the last hypothesis.

Time-resolved spectra and kinetics of the exciton photoluminescence in different types of GaAs/AlAs superlattices

The temperature dependence of the recombination dynamics of excitons is investigated by time-resolved photoluminescence spectroscopy in(GaAs)n/(AlAs)n superlattices, where n denotes the layer thickness in monolayers, for different types of band structures. In direct-gap superlattices with a layer thickness of the order or larger than the exciton Bohr radius, the carrier dynamics is dominated by the transfer from light-hole to heavy-hole excitons. When the layer thickness becomes smaller than the exciton radius, the dynamics of free excitons is controlled by localization. vb In quasi-direct and indirect-gap superlattices, the influence of lateral potential fluctuations due to interface roughness completely governs exciton recombination.

Smectic Polymorphism and Fluctuations in a Polar Liquid-Crystal Binary Mixture

High-resolution x-ray diffraction studies have been performed on the polar liquid-crystal binary mixture of octyl- and decyl-oxyphenyl nitrobenxoyloxy benzoate (47.4 mole% DB8ONO2 + 52.6% DB10ONO2). At temperatures below 127 °C this mixture exhibits the phase sequence (with decreasing temperature) smectic-Ad—reentrant nematic Nd—reentrant nematic N1—smectic-A1—tilted smectic antiphase smectic- [Ctilde] -smectic-A2—smectic-C2. The development of lateral fluid antiphase fluctuations from the partial bilayer fluctuations in the smectic-A1 phase have been probed and analyzed as well as the transition from the tilted smectic antiphase to the tilted bilayer smectic-C2 phase that is accompanied by the occurrence of an intermediate smectic-A2 phase.

COMPUTER SIMULATION OF PROFILES OF INTERFACES BETWEEN COEXISTING PHASES: DO WE UNDERSTAND THEIR FINITE SIZE EFFECTS?

Interfaces between coexisting phases are very common in condensed matter physics, and thus many simulations attempt to characterize their properties, in particular, the interfacial tension and the interfacial profile. However, while theory usually deals with the "intrinsic profile", the latter is not a straightforward output of a simulation: The actual profile (observed in simulations and/or experiments!) is broadened by lateral fluctuations. Therefore, in the usual simulation geometry of L × L × L (in three dimensions), where one chooses suitable boundary conditions to stabilize one or two interfaces of (minimal) area L × L, the profile (and in particular the interfacial width) depends on both linear dimensions L and D (parallel and perpendicular to the interface). Choosing recent simulations of interfaces between coexisting phases of unmixed binary polymer mixtures as an example, we show that this interfacial broadening is not a small correction, but has pronounced effects; for a reliable data analysis, it is (unfortunately!) necessary to vary L and D over a wide range. We present counterexamples to the widespread belief that for small linear dimensions, the intrinsic profile is straightforwardly recovered and speculate about conditions where this belief may be valid.

An Attempt To Separate Roughness from Interdiffusion in the Interfacial Broadening between Two Immiscible Polymers

A detailed study of the interface formation between polystyrene (PS) and chlorinated polybutadiene (CPB) at 150 °C is carried out on bilayer samples by X-ray reflectometry (XRR) and X-ray diffuse scattering (XDS). An equilibrium melt interfacial width σm(150 °C) = 18.8 ± 0.3 Å is determined by XRR. Lateral fluctuations being averaged over the coherence area in this method, σm must be considered as originating from both homopolymer interdiffusion (σCPB/PS) and interfacial roughness (σr): = + . Interfacial roughness is generally described in the literature to be due to either a residual roughness from sample preparation (hypothesis 1) or capillary fluctuations building up at the interface (hypothesis 2). The nature and importance of the interfacial roughness are assessed by XDS scans, which include the detection of the diffuse (off-specular) scattering arising from lateral correlations in the roughness profile. While hypothesis 1 can be unambiguously rejected from the analysis of our XDS data, at least two equally valuable fits of theory to data are found under the hypothesis that the interfacial roughness changes during annealing. In the first one, interdiffusion is restricted, and the interfacial width is dominated by roughness (σr > σCPB/PS). In the second one, the reverse is true: σCPB/PS ≫ σr. The former picture is closer to predictions from existing molecular theories of capillarity and gives for the Flory−Huggins interaction parameter computed through mean-field arguments a better agreement with the interaction parameter obtained from solubility parameters of CPB and PS.

Dissipation Measurement with a Moored Instrument in a Swift Tidal Channel

Abstract A moored and autonomous instrument that measures velocity and temperature fluctuations in the inertial subrange using shear probes and FP07 thermistors has been deployed in a swift [O(1 m s−1)] tidal channel for eight days. The measured velocity signals are free from body vibrations for frequencies below 16 Hz in flows faster than 0.5 m s−1 and below 8 Hz for slower flows. At lower frequencies, fluctuations of torque on the instrument, due mainly to fluctuations of the ambient current, produce large pitching and rolling motions (≈4° peak) that can easily be reduced by minor mechanical changes. Vibrations at higher frequencies do not scale with flow speed and stem mainly from mechanical structures. The velocity spectrum is free from contamination by body motions between 0.2 and 20 cpm (the inertial subrange), and consistent estimates of the rate of dissipation of kinetic energy are obtained from the spectral levels of vertical and lateral velocity fluctuations within this range.

Interface Applications of Scanning Near-Field Optical Microscopy with a Free Electron Laser

The study of semiconductor interfaces and of solid interfaces in general requires novel instrument capable to investigate the lateral fluctuations of properties on a microscopic scale. We present the first result of a major effort in that framework, whose main objective is the exploitation of the unique characteristics of free electron laser (FEL) infrared sources. The background was provided by our previous development of FEL-based techniques to measure interface barriers with high accuracy and reliability. Quite recently, we were able to implement similar investigations with high lateral resolution. The key elements were the use of a small-tip optics fiber and its coupling with a scanning module; in this way, we achieved and verified the condition of near-field microscopy — including a lateral resolution much below the wavelength value. Our discussion includes a presentation of the first scanning near-field optical microscopy (SNOM) images obtained with an FEL and data on small and microscopic-scale fluctuations of semiconductor interface barriers. The likely development of this exciting new field will also be discussed, in particular considering the new proposals for powerful FELs in the low-wavelength spectral range.