Effect of the lattice misfit on the equilibrium shape of strained islands in Volmer-Weber growth

We have studied the effect of the misfit on the equilibrium shape of three-dimensional pyramidal islands grown on a foreign substrate in the case of incomplete wetting (Volmer-Weber mode of growth). We have found that tensile islands have smaller aspect ratios compared with compressed islands owing to its better adhesion to the substrate. The average strains of consecutive layers decrease faster with thickness in compressed than in tensile islands. The strains decrease rapidly with thickness, with the consequence that above a certain height, the upper layers of the pyramid become practically unstrained and does not contribute to a further reduction in the upper base. As a result, the truncated pyramids are not expected to transform into full pyramids. Our results are in good agreement with experimental observations in different systems.Comment: 6 pages, 7 figures. Accepted version, minor change

Increasing fruit and vegetable consumption in Ireland

Context: Following recommended dietary guidelines, Ireland has since 2010 implemented a  0% Value Added Tax (VAT) on fruits and vegetables to increase consumption. Eleven years after policy implementation, the Irish still do not meet recommended intake for fruit and vegetable consumption, consuming 3.9 portions a day compared to 7 daily portions recommended. Policy Options: Four alternatives for improvement were assessed and compared:            1) retain the status quo of reduced VAT for healthy foods, 2) VAT only for locally produced fruits and vegetables, 3) increased VAT for salty and sweet foods with a subsidy for fruits and vegetables, and 4) an education-based policy. Four evaluation criteria were applied for the comparison: economic feasibility, effectiveness, political feasibility, and equity. Recommendations: The status quo remains the best option for Ireland. However, further assessment of this 0% VAT policy on fruits and vegetables is warranted, pending the availability of additional data to enable an in-depth understanding of policy implementation. &nbsp

Second-layer nucleation in coherent Stranski-Krastanov growth of quantum dots

We have studied the monolayer-bilayer transformation in the case of the coherent Stranski-Krastanov growth. We have found that the energy of formation of a second layer nucleus is largest at the center of the first-layer island and smallest on its corners. Thus nucleation is expected to take place at the corners (or the edges) rather than at the center of the islands as in the case of homoepitaxy. The critical nuclei have one atom in addition to a compact shape, which is either a square of i*i or a rectangle of i*(i-1) atoms, with i>1 an integer. When the edge of the initial monolayer island is much larger than the critical nucleus size, the latter is always a rectangle plus an additional atom, adsorbed at the longer edge, which gives rise to a new atomic row in order to transform the rectangle into the equilibrium square shape.Comment: 6 pages, 4 figures. Accepted version, minor change

Relaxation of Surface Profiles by Evaporation Dynamics

We present simulations of the relaxation towards equilibrium of one dimensional steps and sinusoidal grooves imprinted on a surface below its roughening transition. We use a generalization of the hypercube stacking model of Forrest and Tang, that allows for temperature dependent next-nearest-neighbor interactions. For the step geometry the results at T=0 agree well with the t^(1/4) prediction of continuum theory for the spreading of the step. In the case of periodic profiles we modify the mobility for the tips of the profile and find the approximate solution of the resulting free boundary problem to be in reasonable agreement with the T=0 simulations.Comment: 6 pages, Revtex, 5 Postscript figures, to appear in PRB 15, October 199

Decay of one dimensional surface modulations

The relaxation process of one dimensional surface modulations is re-examined. Surface evolution is described in terms of a standard step flow model. Numerical evidence that the surface slope, D(x,t), obeys the scaling ansatz D(x,t)=alpha(t)F(x) is provided. We use the scaling ansatz to transform the discrete step model into a continuum model for surface dynamics. The model consists of differential equations for the functions alpha(t) and F(x). The solutions of these equations agree with simulation results of the discrete step model. We identify two types of possible scaling solutions. Solutions of the first type have facets at the extremum points, while in solutions of the second type the facets are replaced by cusps. Interactions between steps of opposite signs determine whether a system is of the first or second type. Finally, we relate our model to an actual experiment and find good agreement between a measured AFM snapshot and a solution of our continuum model.Comment: 18 pages, 6 figures in 9 eps file

The profile of a decaying crystalline cone

The decay of a crystalline cone below the roughening transition is studied. We consider local mass transport through surface diffusion, focusing on the two cases of diffusion limited and attachment-detachment limited step kinetics. In both cases, we describe the decay kinetics in terms of step flow models. Numerical simulations of the models indicate that in the attachment-detachment limited case the system undergoes a step bunching instability if the repulsive interactions between steps are weak. Such an instability does not occur in the diffusion limited case. In stable cases the height profile, h(r,t), is flat at radii r<R(t)\sim t^{1/4}. Outside this flat region the height profile obeys the scaling scenario \partial h/\partial r = {\cal F}(r t^{-1/4}). A scaling ansatz for the time-dependent profile of the cone yields analytical values for the scaling exponents and a differential equation for the scaling function. In the long time limit this equation provides an exact description of the discrete step dynamics. It admits a family of solutions and the mechanism responsible for the selection of a unique scaling function is discussed in detail. Finally we generalize the model and consider permeable steps by allowing direct adatom hops between neighboring terraces. We argue that step permeability does not change the scaling behavior of the system, and its only effect is a renormalization of some of the parameters.Comment: 25 pages, 18 postscript figure

Arthroscopic repair of the rotator cuff: Prospective study of tendon healing after 70 years of age in 145 patients

SummaryIntroductionThe level of activity of patients older than 70 years is tending to increase, as are their expectations in terms of joint function recuperation. It has not been proven that rotator cuff repair healing is satisfactory in the elderly. The main hypothesis of this study was: repair of supraspinous lesions in patients older than 70 years is reliable in terms of both clinical results and healing. The secondary hypothesis was: tendon healing is significantly correlated with the Constant, ASES, and SST scores as well as with age, tendon retraction, and fatty infiltration.Material and methodsMulticenter prospective study on 145 patients older than 70 years, with 135 patients reviewed at 1 year (93%). The mean age was 73.9 years. Full-thickness tears of the supraspinatus extended at most to the upper third of the infraspinatus and retraction limited to Patte stages 1 and 2 were included. Clinical assessment was carried out in accordance with the Constant, ASES, and SST scores. Healing was evaluated with ultrasound.ResultsA significant improvement was noted in the Constant (44/76)+31.5 (P<0.0001), ASES (35/90)+54.4 (P<0.0001), and SST (3.5/10)+6.6 (P>0.0001) scores at 1 year of follow-up. The healing rate was 89% with 15 re-tears, nine of which were stage 1 and six stage 2. The clinical result was not correlated with patient age (Constant, P=0.24; ASES, P=0.38; SST, P=0.83) nor with the retraction stage (Constant, P=0.71; ASES, P=0.35; SST, P=0.69) or the stage of fatty infiltration (P>0.7). Healing was correlated with the quality of the clinical result (Constant, P=0.02; ASES, P=0.03) and age (P=0.01) but was not correlated with retraction or the fatty infiltration stage (P>0.3).Discussion/conclusionArthroscopic repair significantly improves the clinical results, even in patients older than 70 years. The clinical results are not correlated with age (but deterioration of the result was not noted after 75 years) or frontal retraction (but the study only included retractions limited to stages 1 and 2). The healing rate is satisfactory, but this study is limited to small ruptures of the supraspinatus, and the postoperative ultrasound analysis probably inferior to CT imaging with contrast agent injection, often used as the reference. Healing proves to be correlated with the quality of the clinical result and patient age

Characterizations of how species mediate ecosystem properties require more comprehensive functional effect descriptors

The importance of individual species in mediating ecosystem process and functioning is generally accepted, but categorical descriptors that summarize species-specific contributions to ecosystems tend to reference a limited number of biological traits and underestimate the importance of how organisms interact with their environment. Here, we show how three functionally contrasting sediment-dwelling marine invertebrates affect fluid and particle transport - important processes in mediating nutrient cycling - and use high-resolution reconstructions of burrow geometry to determine the extent and nature of biogenic modification. We find that individual functional effect descriptors fall short of being able to adequately characterize how species mediate the stocks and flows of important ecosystem properties and that, in contrary to common practice and understanding, they are not substitutable with one another because they emphasize different aspects of species activity and behavior. When information derived from these metrics is combined with knowledge of how species behave and modify their environment, however, detailed mechanistic information emerges that increases the likelihood that a species functional standing will be appropriately summarized. Our study provides evidence that more comprehensive functional effect descriptors are required if they are to be of value to those tasked with projecting how altered biodiversity will influence future ecosystems

Coherent Stranski-Krastanov growth in 1+1 dimensions with anharmonic interactions: An equilibrium study

The formation of coherently strained three-dimensional islands on top of the wetting layer in Stranski-Krastanov mode of growth is considered in a model in 1+1 dimensions accounting for the anharmonicity and non-convexity of the real interatomic forces. It is shown that coherent 3D islands can be expected to form in compressed rather than in expanded overlayers beyond a critical lattice misfit. In the latter case the classical Stranski-Krastanov growth is expected to occur because the misfit dislocations can become energetically favored at smaller island sizes. The thermodynamic reason for coherent 3D islanding is the incomplete wetting owing to the weaker adhesion of the edge atoms. Monolayer height islands with a critical size appear as necessary precursors of the 3D islands. The latter explains the experimentally observed narrow size distribution of the 3D islands. The 2D-3D transformation takes place by consecutive rearrangements of mono- to bilayer, bi- to trilayer islands, etc., after exceeding the corresponding critical sizes. The rearrangements are initiated by nucleation events each next one requiring to overcome a lower energetic barrier. The model is in good qualitative agreement with available experimental observations.Comment: 12 pages text, 15 figures, Accepted in Phys.Rev.B, Vol.61, No2