Using temperature to tune film roughness: nonintuitive behavior in a simple system

Ag(100) homoepitaxy constitutes one of the simplest systems in which to study thin-film growth. Yet we find that the roughness variation with temperature is extraordinarily complex. Specifically, as the deposition temperature is reduced from 300 to 50 K, the roughness of 25 monolayer films first increases, then decreases, then increases again. A transition from mound formation to self-affine (semifractal) growth occurs at approximately 135 K. The underlying mechanisms are postulated. An atomistic model incorporating these mechanisms reproduces the experimental data quantitatively.     

Step structure on the fivefold Al-Pd-Mn quasicrystal surface, and on related surfaces

We compare step morphologies on surfaces of Al-rich metallic alloys, both quasicrystalline and crystalline. We present evidence that the large-scale step structure observed on Al-rich quasicrystals after quenching to room temperature reflects equilibrium structure at an elevated temperature. These steps are relatively rough, i.e., have high diffusivity, compared to those on crystalline surfaces. For the fivefold quasicrystal surface, step diffusivity increases as step height decreases, but this trend is not obeyed in a broader comparison between quasicrystals and crystals. On a shorter scale, the steps on Al-rich alloys tend to exhibit local facets (short linear segments), with different facet lengths, a feature which could develop during quenching to room temperature. Facets are shortest and most difficult to identify for the fivefold quasicrystal surface.     

Site-specific oxidation of the [Fe4S4] cubanes in high-potential iron sulfur proteins as probed by EPR and orientation-selective proton ENDOR spectroscopy:Ectothiorhodospira halophila I versusRhodocyclus tenuis

Electron paramagnetic resonance (EPR) and proton ENDOR (electron-nuclear double resonance) spectroscopies were used to analyze the structural and electronic parameters of the oxidized [Fe₄S₄] cubane clusters in high-potential iron sulfur proteins (HiPIPs) fromEctothiorhodospira halophila (HiPIP I) andRhodocyclus tenuis. TheE. halophila HiPIP I EPR spectra at X- and Q-band revealed a dominant species (simulated withg _max=2.1425,g _int=2.0315,g _min=2.0296) and a minor species (ca. 5–10% contribution) which was not analyzed further. ForR. tenuis HiPIP the EPR spectrum contained a single species only (g _max=2.1140,g _int=2.0392,g _min=2.0215), i.e., withg _max significantly smaller than that of theE. halophila protein. Orientation-selected proton ENDOR spectra of HiPIP I ofE. halophila were reconstructed by simulation with slight modifications of the crystal structure data. ENDOR from two mutants, F36S and F36G, ofE. halophila HiPIP I gave evidence for a common assignment of a HB2 proton of a phenylalanine residue (36 and 44, respectively, in isoenzymes I and II as reported earlier) interacting with the mixed-valence pair iron ions Fe2 and Fe3. ForR. tenuis HiPIP, the ENDOR spectra were assigned to arise from Fe3 and Fe4 as mixedvalence pair under the assumption of an unchanged intrinsicg-tensor symmetry. The resulting site specificity of the cubane oxidation was discussed in relation to structural requirements and redox potentials of the two HiPIPs.     

Importance of shear in the bcc-to-hcp transformation in iron

Iron shows a pressure-induced martensitic phase transformation from the ground state ferromagnetic bcc phase to a nonmagnetic hcp phase at approximately 13 GPa. The exact transformation pressure (TP) and pathway are not known. Here we present a multiscale model containing a quantum-mechanics-based multiwell energy function accounting for the bcc and hcp phases of Fe and a construction of kinematically compatible and equilibrated mixed phases. This model suggests that shear stresses have a significant influence on the bcc<-->hcp transformation. In particular, the presence of modest shear accounts for the scatter in measured TPs. The formation of mixed phases also provides an explanation for the observed hysteresis in TP.     

Energetics and kinetics of vacancy diffusion and aggregation in shocked aluminium via orbital-free density functional theory

A possible mechanism for shock-induced failure in aluminium involves atomic vacancies diffusing through the crystal lattice and agglomerating to form voids, which continue to grow, ultimately resulting in ductile fracture. We employ orbital-free density functional theory, a linear-scaling first-principles quantum mechanics method, to study vacancy formation, diffusion, and aggregation in aluminium under shock loading conditions of compression and tension. We calculate vacancy formation and migration energies, and find that while nearest-neighbor vacancy pairs are unstable, next-nearest-neighbor vacancy pairs are stable. As the number of nearby vacancies increases, we predict that vacancy clusters preferentially grow through next-nearest-neighbor vacancies. The energetics are found to be greatly affected by expansion and compression, leading to insight as to how vacancies behave under shock conditions.     

Structure of ethnic violence in Sudan: a semi-automated network analysis of online news (2003–2010)

Mining textual sources of data can be used to design studies and test theories at temporal and spatial scales unheard of in the past. This opens up new opportunities for conflict studies and ethnographic research. We conducted a semi-automated network analysis of the 2003–2010 Sudan Tribune online news articles and modeled ethnic-group conflict in Sudan. We tested whether an ethnic group’s connections to the environment (livestock, biomes, and other resources) and other ethnic groups was associated with severe conflict and peace terms and whether ethnic-group richness at a given geospatial location was associated with severe conflict. Ethnic groups with more connections to livestock, environmental resources, and those ethnic groups with denser environmental–resource networks in their sphere of influence had more co-occurrences with severe conflict terms. Ethnic groups that had more connections to biomes in their sphere of influence were co-associated with peace terms. Locations that had high ethnic-group richness were associated with a higher frequency of severe conflict terms.