Interface roughness in short-period InGaAs/InP superlattices

Electron mobility was studied in lattice-matched short-period InGaAs/InP superlattices as a function of the width of the wells. The decreasing mobility with decreasing well width was shown to occur due to the interface roughness. The roughnesses of InGaAs/InP and GaAs/AlGaAs interfaces were compared. Much smoother InGaAs/InP interfaces resulted in higher electron mobility limited by interface roughness.

Exciton behavior in GaAs/AlGaAs coupled double quantum wells with interface disorder

In this work, we present a detailed study on the optical properties of two GaAs/Al(0.35)Ga(0.65)As coupled double quantum wells (CDQWs) with inter-well barriers of different thicknesses, by using photoluminescence (PL) spectroscopy. The two CDQWs were grown in a single sample, assuring very similar experimental conditions for measurements of both. The PL spectrum of each CDQW exhibits two recombination channels which can be accurately identified as the excitonic e(1)-hh(1) transitions originated from CDQWs of different effective dimensions. The PL spectra characteristics and the behavior of the emissions as a function of temperature and excitation power are interpreted in the scenario of the bimodal interface roughness model, taking into account the exciton migration between the two regions considered in this model and the difference in the potential fluctuation levels between those two regions. The details of the PL spectra behavior as a function of excitation power are explained in terms of the competition between the band gap renormalization (BGR) and the potential fluctuation effects. The results obtained for the two CDQWs, which have different degrees of potential fluctuation, are also compared and discussed. (C) 2009 Elsevier B.V. All rights reserved.

Efeitos de interface em bicamadas magnéticas

We study magnetic interface roughness in F/AF bilayers. Two kinds of roughness were considered. The first one consists of isolated defects that divide the substrate in two regions, each one with an AF sub-lattice. The interface exchange coupling is considered uniform and presents a sudden change in the defects line, favoring Neel wall nucleation. Our results show the interface field dependence of the threshold thickness for the reorientation of the magnetization in the ferromagnetic film. Angular profiles show the relaxation of the magnetization, from Neel wall, at the interface, to reoriented state, at the surface. External magnetic field, perpendicular to the easy axis of the substrate, favors the reoriented state. Depending, of the external magnetic field intensity, parallel to the easy axis of the AF, the magnetization profile at surface can be parallel or perpendicular to the field direction. The second one treats of distributed deffects, periodically. The shape hysteresis curves, exchange bias and coercivity were characterized by interface field intensity and roughness pattern. Our results show that dipolar effects decrease the exchange bias and coercivity

Morphology and bacterial colonisation of tooth/ceramic restoration interface after different cement excess removal techniques

Objectives: To evaluate the influence of different protocols for resin cement removal during cementation on biofilm formation.Methods: Twenty-eight ceramic blocks, which were injected under pressure, were placed over enamel blocks obtained from freshly extracted bovine incisors. The ceramic blocks were cemented to the enamel blocks using a dual-cured resin cement and the excess resin was removed according to the experimental group: TS: Teflon spatula; BR: brush; BR+: brush and polishing; SB+: scalpel blade and polishing. After autoclaving, the samples were colonised by incubation in a sucrose broth suspension standardised with Streptococcus mutans in microaerophilic stove. Specimens were quantitatively analysed for bacterial adherence at the adhesive interface using confocal laser scanning microscopy and counting the colony forming units, and qualitatively analysed using SEM. The roughness (Ra/Rz/RSm) was also analysed. Data were analysed by 1-way ANOVA and Tukey's test (5%).Results: The roughness values ranged from 0.96 to 1.69 mu m for Ra (p > 0.05), from 11.59 to 22.80 mu m for Rz (p = 0.02 < 0.05) and from 293.2 to 534.3 mu m for RSm (p = 0.00). Bacterial adhesion varied between 1,974,000 and 2,814,000 CFU/ml (p = 0.00). Biofilm mean thickness ranged from 0.477 and 0.556 mu m (p > 0.05), whilst the biovolume values were between 0.388 and 0.547 mu m(3)/mu m(2) (p = 0.04). Lower values for roughness, bacterial adhesion, biofilm thickness and biovolume were found with BR, whilst TS presented the highest values for most of the parameters. SEM images confirmed the quantitative values.Conclusions: The restoration margin morphology and interface roughness affects bacterial accumulation. The brush technique promoted less bacterial colonisation at the adhesive interface than did the other removal methods.Clinical significance: The brush technique seems to be a good option for removing the excess resin cement after adhesive cementation in clinical practice, as indicated by its better results with lower bacterial colonisation. (C) 2012 Elsevier Ltd. All rights reserved.

Interface-mediated ferroelectric patterning and Mn valency in nano-structured PbTiO3/La0.7Sr0.3MnO3

We employed a multitechnique approach using piezo-force response microscopy and photoemission microscopy to investigate a self-organizing polarization domain pattern in PbTiO3/La0.7Sr0.3MnO3 (PTO/LSMO) nanostructures. The polarization is correlated with the nanostructure morphology as well as with the thickness and Mn valence of the LSMO template layer. On the LSMO dots, the PTO is upwards polarized, whereas outside the nanodots, the polarization appears both strain and interface roughness dependent. The results suggest that the electronic structure and strain of the PTO/LSMO interface contribute to determining the internal bias of the ferroelectric layer.

Backside contact effect on the morphological and optical features of porous silicon photonic crystals

The present work reports on the effect of the type of backside contact used in the electrochemical process and their relation with the structural features and optical responses of the one-dimensional photonic crystal (PC) anodized in simple and double electrochemical cell. The PC, obtained in the single cell, showed to have thicker layers than of the PC obtained in double electrochemical cell. Additionally, the PC obtained in double cell showed highest reflectance in the band gap region than of the PCs obtained in single cell. These results suggest that the interface roughness between adjacent layers in the PC devices obtained in double electrochemical cell is minimized. (C) 2008 Elsevier Ltd. All rights reserved.

Porous silicon optical cavity structure applied to high sensitivity organic solvent sensor

The present work reports the thermal annealing process, the number of layer and electrochemical process effect in the optical response quality of Bragg and microcavity devices that were applied as organic solvent sensors. These devices have been obtained by using porous silicon (PS) technology. The optical characterization of the Bragg reflector, before annealing, showed a broad photonic band-gap structure with blue shifted and narrowed after annealing process. The electrochemical process used to obtain the PS-based device imposes the limit in the number of layers because of the chemical dissolution effect. The interface roughness minimizations in the devices have been achieved by using the double electrochemical cell setup. The microcavity devices showed to have a good sensibility for organic solvent detection. The thermal annealed device showed better sensibility feature and this result was attributed to passivation of the surface devices. (c) 2007 Elsevier Ltd. All rights reserved.

Propriedades magnéticas de nanopartículas de ferro em substratos antiferromagnéticos

The effect of finite size on the magnetic properties of ferromagnetic particles systems is a recurrent subject. One of the aspects wide investigated is the superparamagnetic limit where the temperature destroys the magnetic order of ferromagnetic small particles. Above the block temperature the thermal value of the magnetic moment of the particle vanishes, due to thermal fluctuations. The value of the blocking temperature diminishes when the size of the particle is reduced, reflecting the reduction of the anisotropy energy barrier between the uniform states along the uniaxial axis. The increasing demand for high density magnetic media has recently attracted great research interest in periodic arrangements of nanometric ferromagnetics particles, approach in the superparamagnetic limit. An interesting conjecture is the possibility of stabilization of the magnetic order of small ferromagnetic particles (F) by interface coupling with antiferromagnetic (AF) substrate. These F/AF systems may also help to elucidate some details of the effect of exchange bias, because the effect of interface roughness and the paper of domain walls, either in the substrate or the particle, are significantly reduced. We investigate the magnetic phases of small ferromagnetic particles on a antiferromagnetic substrate. We use a self-consistent local field method, incorporating the interface field and the dipole interaction between the spins of the ferromagnetic particle. Our results indicate that increasing the area of the interface favors the formation of the uniform state. Howere above a critical height value appears a state non-uniform is formed where the spins of in the particle s free surface are rotated with respect to the interface spins direction. We discuss the impact of the competition between the dipolar and interface field on the magnetic charge, that controls the field of flux leakage of the particle, and on the format of the hysteresis curves. Our results indicate that the liquid magnetic charge is not a monotonically increasing function of the height of the particle. The exchange bias may display anomalous features, induced for the dipolar field of the spins near the F/AF interface

Nucleação de vortices e paredes de domínio em nanoestruturas magnéticas

We report a theoretical investigation of the magnetic phases and hysteresis of exchange biased ferromagnetic (F) nanoelements for three di erent systems: exchange biased nanoparticles, exchange biased narrow ferromagnetic stripes and exchange biased thin ferromagnetic lms. In all cases the focus is on the new e ects produced by suitable patterns of the exchange energy coupling the ferromagnetic nanoelement with a large anisotropy antiferromagnetic (AF) substrate. We investigate the hysteresis of iron and permalloy nanoparticles with a square basis, with lateral dimensions between 45 nm and 120 nm and thickness between 12 nm and 21 nm. Interface bias is aimed at producing large domains in thin lms. Our results show that, contrary to intuition, the interface exchange coupling may generate vortex states along the hysteresis loop. Also, the threshold value of the interface eld strength for vortex nucleation is smaller for iron nanoelements. We investigate the nucleation and depinning of an array of domain walls pinned at interface defects of a vicinal stripe/AF bilayer. The interface exchange eld displays a periodic pattern corresponding to the topology of the AF vicinal substrate. The vicinal AF substrate consists of a sequence of terraces, each with spins from one AF subalattice, alternating one another. As a result the interface eld of neighboring terraces point in opposite direction, leading to the nucleation of a sequence of domain walls in the ferromagnetic stripe. We investigated iron an permalloy micrometric stripes, with width ranging from 100 nm and 300 nm and thickness of 5 nm. We focused in domain wall sequences with same chirality and alternate chirality. We have found that for 100nm terraces the same chiraility sequence is more stable, requiring a larger value of the external eld for depinning. The third system consists of an iron lm with a thickness of 5 nm, exchange coupled to an AF substrate with a periodic distribution of islands where the AF spins have the opposite direction of the spins in the background. This corresponds to a two-sublattice noncompensated AF plane (such as the surface of a (100) FeF2 lm), with monolayer-height islands containing spins of one sublattice on a surface containing spins of the opposite sublattice. The interface eld acting in the ferromagnetic spins over the islands points in the opposite direction of that in the spins over the background. This a model system for the investigation of interface roughness e ects. We have studied the coercicivity an exchange bias hysteresis shift as a function of the distance between the islands and the degree of interface roughness. We have found a relevant reduction of coercivity for nearly compensated interfaces. Also the e ective hysteresis shift is not proportional to the liquid moment of the AF plane. We also developed an analytical model which reproduces qualitatively the results of numerical simulations

Structural development of silicated films self-assembled at the air-water interface

The development of structure perpendicular to and in the plane of the interface has been studied for mesoporous silicate films self-assembled at the air/water interface. The use of constrained X-ray and neutron specular reflectometry has enabled a detailed study of the structural development perpendicular to the interface during the pre-growth phase. Off-specular neutron reflectometry and grazing incidence X-ray diffraction has enabled the in-plane structure to be probed with excellent time resolution. The growth mechanism under the surfactant to silicate source ratios used in this work is clearly due to the self-assembly of micellar and molecular species at the air/liquid interface, resulting in the formation of a planar mesoporous film that is tens of microns thick. (C) 2003 Elsevier Science B.V. All rights reserved.

The fixation of the cemented femoral component. Effects of stem stiffness, cement thickness and roughness of the cement-bone surface.

After cemented total hip arthroplasty (THA) there may be failure at either the cement-stem or the cement-bone interface. This results from the occurrence of abnormally high shear and compressive stresses within the cement and excessive relative micromovement. We therefore evaluated micromovement and stress at the cement-bone and cement-stem interfaces for a titanium and a chromium-cobalt stem. The behaviour of both implants was similar and no substantial differences were found in the size and distribution of micromovement on either interface with respect to the stiffness of the stem. Micromovement was minimal with a cement mantle 3 to 4 mm thick but then increased with greater thickness of the cement. Abnormally high micromovement occurred when the cement was thinner than 2 mm and the stem was made of titanium. The relative decrease in surface roughness augmented slipping but decreased debonding at the cement-bone interface. Shear stress at this site did not vary significantly for the different coefficients of cement-bone friction while compressive and hoop stresses within the cement increased slightly.

Structural analysis of PEO-PBO copolymer monolayers at the air-water interface

X-ray reflectivity (XR) and grazing incidence X-ray diffraction (GIXD) have been used to examine an oxyethylene-b-oxybutylene (E23B8) copolymer film at the air-water interface. The XR data were fitted using both a one- and a two-layer model that outputted the film thickness, roughness, and electron density. The best fit to the experimental data was obtained using a two-layer model (representing the oxyethylene and oxybutylene blocks, respectively), which showed a rapid thickening of the copolymer film at pressures above 7 mN/m. The large roughness values found indicate a significant degree of intermixing between the blocks and back up the GIXD data, which showed no long range lateral ordering within the layer. It was found from the electron density model results that there is a large film densification at 7 mN/m, possibly suggesting conformational changes within the film, even though no such change occurs on the pressure-area isotherm at the same surface pressure.

Studying the contact point and interface moving in a sinusoidal tube with lattice Boltzmann method

The multicomponent nonideal gas lattice Boltzmann model by Shan and Chen (S-C) is used to study the immiscible displacement in a sinusoidal tube. The movement of interface and the contact point (contact line in three-dimension) is studied. Due to the roughness of the boundary, the contact point shows "stick-slip" mechanics. The "stick-slip" effect decreases as the speed of the interface increases. For fluids that are nonwetting, the interface is almost perpendicular to the boundaries at most time, although its shapes at different position of the tube are rather different. When the tube becomes narrow, the interface turns a complex curves rather than remains simple menisci. The velocity is found to vary considerably between the neighbor nodes close to the contact point, consistent with the experimental observation that the velocity is multi-values on the contact line. Finally, the effect of three boundary conditions is discussed. The average speed is found different for different boundary conditions. The simple bounce-back rule makes the contact point move fastest. Both the simple bounce-back and the no-slip bounce-back rules are more sensitive to the roughness of the boundary in comparison with the half-way bounce-back rule. The simulation results suggest that the S-C model may be a promising tool in simulating the displacement behaviour of two immiscible fluids in complex geometry.

A review of urban roughness sublayer turbulence

It is becoming increasingly important that we can understand and model flow processes in urban areas. Applications such as weather forecasting, air quality and sustainable urban development rely on accurate modelling of the interface between an urban surface and the atmosphere above. This review gives an overview of current understanding of turbulence generated by an urban surface up to a few building heights, the layer called the roughness sublayer (RSL). High quality datasets are also identified which can be used in the development of suitable parameterisations of the urban RSL. Datasets derived from physical and numerical modelling, and full-scale observations in urban areas now exist across a range of urban-type morphologies (e.g. street canyons, cubes, idealised and realistic building layouts). Results show that the urban RSL depth falls within 2 – 5 times mean building height and is not easily related to morphology. Systematic perturbations away from uniform layouts (e.g. varying building heights) have a significant impact on RSL structure and depth. Considerable fetch is required to develop an overlying inertial sublayer, where turbulence is more homogeneous, and some authors have suggested that the “patchiness” of urban areas may prevent inertial sublayers from developing at all. Turbulence statistics suggest similarities between vegetation and urban canopies but key differences are emerging. There is no consensus as to suitable scaling variables, e.g. friction velocity above canopy vs. square root of maximum Reynolds stress, mean vs. maximum building height. The review includes a summary of existing modelling practices and highlights research priorities.

The impact of variable sea ice roughness on changes in Arctic Ocean surface stress: a model study

The Arctic sea ice cover is thinning and retreating, causing changes in surface roughness that in turn modify the momentum flux from the atmosphere through the ice into the ocean. New model simulations comprising variable sea ice drag coefficients for both the air and water interface demonstrate that the heterogeneity in sea ice surface roughness significantly impacts the spatial distribution and trends of ocean surface stress during the last decades. Simulations with constant sea ice drag coefficients as used in most climate models show an increase in annual mean ocean surface stress (0.003 N/m2 per decade, 4.6%) due to the reduction of ice thickness leading to a weakening of the ice and accelerated ice drift. In contrast, with variable drag coefficients our simulations show annual mean ocean surface stress is declining at a rate of -0.002 N/m2 per decade (3.1%) over the period 1980-2013 because of a significant reduction in surface roughness associated with an increasingly thinner and younger sea ice cover. The effectiveness of sea ice in transferring momentum does not only depend on its resistive strength against the wind forcing but is also set by its top and bottom surface roughness varying with ice types and ice conditions. This reveals the need to account for sea ice surface roughness variations in climate simulations in order to correctly represent the implications of sea ice loss under global warming.