Thermal Re-emission Model

Starting from a continuum description, we study the non-equilibrium roughening of a thermal re-emission model for etching in one and two spatial dimensions. Using standard analytical techniques, we map our problem to a generalized version of an earlier non-local KPZ (Kardar-Parisi-Zhang) model. In 2+1 dimensions, the values of the roughness and the dynamic exponents calculated from our theory go like $\alpha \approx z \approx 1$ and in 1+1 dimensions, the exponents resemble the KPZ values for low vapor pressure, supporting experimental results. Interestingly, Galilean invariance is maintained althrough.Comment: 4 pages, minor textual corrections and typos, accepted in Physical Review B (rapid

Epitaxial Growth Kinetics with Interacting Coherent Islands

The Stranski-Krastanov growth kinetics of undislocated (coherent) 3-dimensional islands is studied with a self-consistent mean field rate theory that takes account of elastic interactions between the islands. The latter are presumed to facilitate the detachment of atoms from the islands with a consequent decrease in their average size. Semi-quantitative agreement with experiment is found for the time evolution of the total island density and the mean island size. When combined with scaling ideas, these results provide a natural way to understand the often-observed initial increase and subsequent decrease in the width of the coherent island size distribution.Comment: 4 pages, 4 figure

Mass-Transport Models with Multiple-Chipping Processes

We study mass-transport models with multiple-chipping processes. The rates of these processes are dependent on the chip size and mass of the fragmenting site. In this context, we consider k-chip moves (where k = 1, 2, 3, ....); and combinations of 1-chip, 2-chip and 3-chip moves. The corresponding mean-field (MF) equations are solved to obtain the steady-state probability distributions, P (m) vs. m. We also undertake Monte Carlo (MC) simulations of these models. The MC results are in excellent agreement with the corresponding MF results, demonstrating that MF theory is exact for these models.Comment: 18 pages, 4 figures, To appear in European Physical Journal

Aggregation Number-Based Degrees of Counterion Dissociation in Sodium n-Alkyl Sulfate Micelles

Values of the degree of counterion dissociation, R, for sodium n-alkyl sulfate micelles, denoted by SN c S, where N c is the number of carbon atoms in the alkyl chain, are defined by asserting that the aggregation number, N, is dependent only on the concentration, C aq , of counterions in the aqueous pseudophase. By using different combinations of surfactant and added salt concentrations to yield the same value of N, R can be determined, independent of the experimental method. Electron paramagnetic resonance measurements of the hyperfine spacings of two nitroxide spin probes, 16-and 5-doxylstearic acid methyl ester (16DSE and 5DSE, respectively), are employed to determine whether micelles from two samples have the same value of N to high precision. The EPR spectra are different for the two spin probes, but the values of R are the same, within experimental error, as they must be. In agreement with recent work on S12S and with prevailing thought in the literature, values of R are constant as a function of N. This implies that the value of R is constant whether the surfactant or added electrolyte concentrations are varied. Interestingly, R varies with chain length as follows: However, the theory also predicts that, for a given value of N c , R decreases as N increases. Moreover, this decrease is predicted to be different if N is increased by adding salt or by increasing the surfactant concentration. A modification to the theory in which dissociated counterions contribute to the ionic strength while added co-ions (Cl -) do not, brings theory and experiment into closer accord. Assuming R to be constant versus N permits a direct application of the aggregation number-based definition of R using time-resolved fluorescence quenching to measure values of N as well as other experimental parameters that vary monotonically with N, such as the microviscosity measured with spin probes and the quenching rate constant. For S13S micelles at 40°C, R ) 0.20 ( 0.02 is derived from N; R ) 0.21 ( 0.02 from the microvisicosity, and R ) 0.21 ( 0.02 from the quenching rate constants, in agreement with the hyperfine spacing results. The aggregation numbers for S13S are well described by the power law N ) N°(C aq /cmc 0 ) γ , where cmc 0 is the critical micelle concentration in the absence of added salt, N°) 67, and γ ) 0.26

Droplet Fluctuations in the Morphology and Kinetics of Martensites

We derive a coarse grained, free-energy functional which describes droplet configurations arising on nucleation of a product crystal within a parent. This involves a new `slow' vacancy mode that lives at the parent-product interface. A mode-coupling theory suggests that a {\it slow} quench from the parent phase produces an equilibrium product, while a {\it fast} quench produces a metastable martensite. In two dimensions, the martensite nuclei grow as `lens-shaped' strips having alternating twin domains, with well-defined front velocities. Several empirically known structural and kinetic relations drop out naturally from our theory.Comment: 4 pages, REVTEX, and 3 .eps figures, compressed and uuencoded, Submitted to Phys. Rev. Let

Current-Induced Step Bending Instability on Vicinal Surfaces

We model an apparent instability seen in recent experiments on current induced step bunching on Si(111) surfaces using a generalized 2D BCF model, where adatoms have a diffusion bias parallel to the step edges and there is an attachment barrier at the step edge. We find a new linear instability with novel step patterns. Monte Carlo simulations on a solid-on-solid model are used to study the instability beyond the linear regime.Comment: 4 pages, 4 figure

Stress distribution and the fragility of supercooled melts

We formulate a minimal ansatz for local stress distribution in a solid that includes the possibility of strongly anharmonic short-length motions. We discover a broken-symmetry metastable phase that exhibits an aperiodic, frozen-in stress distribution. This aperiodic metastable phase is characterized by many distinct, nearly degenerate configurations. The activated transitions between the configurations are mapped onto the dynamics of a long range classical Heisenberg model with 6-component spins and anisotropic couplings. We argue the metastable phase corresponds to a deeply supercooled non-polymeric, non-metallic liquid, and further establish an order parameter for the glass-to-crystal transition. The spin model itself exhibits a continuous range of behaviors between two limits corresponding to frozen-in shear and uniform compression/dilation respectively. The two regimes are separated by a continuous transition controlled by the anisotropy in the spin-spin interaction, which is directly related to the Poisson ratio $\sigma$ of the material. The latter ratio and the ultra-violet cutoff of the theory determine the liquid configurational entropy. Our results suggest that liquid's fragility depends on the Poisson ratio in a non-monotonic way. The present ansatz provides a microscopic framework for computing the configurational entropy and relaxational spectrum of specific substances.Comment: 11 pages, 5 figures, Final version published in J Phys Chem

Profile scaling in decay of nanostructures

The flattening of a crystal cone below its roughening transition is studied by means of a step flow model. Numerical and analytical analyses show that the height profile, h(r,t), obeys the scaling scenario dh/dr = F(r t^{-1/4}). The scaling function is flat at radii r<R(t) \sim t^{1/4}. We find a one parameter family of solutions for the scaling function, and propose a selection criterion for the unique solution the system reaches.Comment: 4 pages, RevTex, 3 eps figure

A erradicação do trabalho escravo até 2030 e os desafios da vigilância em saúde do trabalhador

A erradicação das formas contemporâneas de escravidão é uma relevante questão científica, social e institucional. De fato, esforços globais têm sido feitos para compreender, mapear e eliminar a escravidão contemporânea, como um dos objetivos do Desenvolvimento Sustentável das Nações Unidas até 2030. Entretanto, pouca atenção tem sido dada para o lugar da vigilância em saúde do trabalhador (VISAT) nas estratégias e lutas pela erradicação dessas relações de exploração. Para suprir essa lacuna, este artigo discute o trabalho escravo contemporâneo (TEC) e suas especificidades no Brasil, na perspectiva da VISAT. Inicialmente, destacamos as relações entre trabalho escravo, saúde do/a trabalhador/a e a vigilância em saúde e, em seguida, apresentamos três desafios da vigilância para o enfretamento do TEC: (1) o desafio de caracterizar setores econômicos, regiões e populações afetadas; (2) de identificar determinantes, riscos e efeitos à saúde; (3) e de fortalecer práticas e serviços de saúde do trabalhador para desencadear ações de formação, informação e intervenção em regiões de maior presença de TEC. Conclui-se que a vigilância em saúde do trabalhador pode trazer contribuições significativas para emancipação de trabalhadores em contextos de trabalho escravo. Eradicating modern slavery is a relevant scientific, social, and institutional challenge issue. Indeed, efforts are being made at a global scale to understand and eradicate contemporary slavery as a target of the United Nations Sustainable Development Goals by 2030. However, little attention has been given to the Worker Health Surveillance (WHS) in the struggle against contemporary forms of slavery. To fill this gap, the paper discuss contemporary slave labour (CSL) from a workers health surveillance perspective, calling attention to challenges evident in the case of Brazil. Further, we explain the connection of CSL to workers health and to workers health surveillance (WHS). We then identify and discuss three challenges of CSL to WHS: 1. help to characterise and identify economic sectors and populations most affected by slave labour; 2. identify determinants, risks, and health effects related to CSL; and 3. strengthen workers health services to trigger specific actions in terms of formation, information, and intervention in regions of high CSL prevalence. We conclude that Workers Health Surveillance can play an important role towards workers emancipation slavery relations

A contiuum model for low temperature relaxation of crystal steps

High and low temperature relaxation of crystal steps are described in a unified picture, using a continuum model based on a modified expression of the step free energy. Results are in agreement with experiments and Monte Carlo simulations of step fluctuations and monolayer cluster diffusion and relaxation. In an extended model where mass exchange with neighboring terraces is allowed, step transparency and a low temperature regime for unstable step meandering are found.Comment: Submitted to Phys.Rev.Let