10,000 publications from this institution
We present a low-temperature structural model for lithium imide $({\text{Li}}_{2}\text{NH})$ that is consistent with experimental studies. Using the cluster expansion formalism and density-functional theory, we have identified a low-energy crystal structure for lithium imide with 96 atoms per unit cell. This low-energy structure is consistent with experimental diffraction patterns, and we propose that the symmetry of the structure may be increased at finite temperature due to thermal fluctuations. In addition, our results suggest that lithium motion is relatively facile between octahedral and tetrahedral sites, which may help explain how lithium diffuses through this material.
Debugging large-scale, data-intensive, distributed applications running in a datacenter (datacenter applications) is complex and time-consuming. The key obstacle is non-deterministic failures—hard-to-reproduce program misbehaviors that are immune to traditional cyclic debugging techniques. Datacenter applications are rife with such failures because they operate in highly non-deterministic environments: a typical setup employs thousands of nodes, spread across multiple datacenters, to process terabytes of data per day. In these environments, existing methods for debugging non-deterministic failures are of limited use. They either incur excessive production overheads or don't scale to multi-node, terabyte-scale processing. To help remedy the situation, we have built a new deterministic replay tool. Our tool, called DCR, enables the reproduction and debugging of non-deterministic failures in production datacenter runs. The key observation behind DCR is that debugging does not always require a precise replica of the original datacenter run. Instead, it often suffices to produce some run that exhibits the original behavior of the control-plane —the most error-prone component of datacenter applications. DCR leverages this observation to relax the determinism guarantees offered by the system, and consequently, to address key requirements of production datacenter applications: lightweight recording of long-running programs, causally consistent replay of large-scale clusters, and out-of-the box operation with existing, real-world applications running on commodity multiprocessors.
Multivariate resultants generalize the Sylvester resultant of two polynomials and characterize the solvability of a polynomial system. They also reduce the computation of all common roots to a problem in linear algebra. We propose a determinantal formula for the sparse resultant of an arbitrary system of n + 1 polynomials in n variables. This resultant generalizes the classical one and has significantly lower degree for polynomials that are sparse in the sense that their mixed volume is lower than their Bézout number. Our algorithm uses a mixed polyhedral subdivision of the Minkowski sum of the Newton polytopes in order to construct a Newton matrix. Its determinant is a nonzero multiple of the sparse resultant and the latter equals the GCD of at most n + 1 such determinants. This construction implies a restricted version of an effective sparse Nullstellensatz. For an arbitrary specialization of the coefficients, there are two methods that use one extra variable and yield the sparse resultant. This is the first algorithm to handle the general case with complexity polynomial in the resultant degree and simply exponential in n . We conjecture its extension to producing an exact rational expression for the sparse resultant.
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An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.
Summary form only given, as follows. Photonic crystals, the electromagnetic analog of semiconductor crystals, have stirred the imagination toward photonic integrated circuits. At the same time, the build-out of the telecommunications infrastructure is creating a demand for large volumes of optical communications components and subsystems. Integration at the tiniest scale of photonic crystals allows the largest number of components to be produced from a single wafer, reducing cost, and allowing considerable optical complexity. There have been a series of practical difficulties standing in the way of building practical microphotonic circuits, that are gradually being solved; including, the input/output coupling efficiency problem, the nanofabrication accuracy problem, the active device issues, electrical modulation schemes, device design software and simulation. Some of these problems are already solved, and we can project solutions to the others over the next few years.
OBJECTIVES: Endoplasmic reticulum (ER) stress is involved in the progression of several diseases, including diet and obesity-related conditions such as nonalcoholic fatty liver disease. Our goal was to understand the role of diet on the unfolded protein response (UPR), an important pathway in ER stress response, in efforts to elucidate the role of the UPR in the progression to non-alcoholic fatty liver disease. METHODS: We used stable isotope labeling with tandem mass spectrometric analysis to characterize proteome-wide synthesis rates and de novo lipogenesis rates in vivo in mouse liver to generate metabolic flux signatures of the unfolded protein response. We initiated the unfolded protein response through treatment with tunicamycin. Diets rich in either unsaturated, oleic, acids, or saturated, palmitic, acids were given to mice for five weeks to determine the effect of dietary fatty acids on this induced ER stress response. RESULTS: With induction of the unfolded protein response, we observed reduced protein synthesis across most ontologies, but increased synthesis of ER proteins and chaperones. We also found reduced de novo lipogenesis after 48 and 72 hours of induced ER stress. Reduction in food intake and significant weight loss also occurred after 48 and 72 hours. Electron microscopy revealed striking morphological differences in the ER and accumulation of lipid droplets with ER stress. Diets high in unsaturated fatty acids had a lesser impact on the progression of the unfolded protein response. CONCLUSIONS: These data begin to characterize how the unfolded protein response progresses over time, and the metabolic changes that occur with ER stress. Diets rich in saturated or unsaturated fatty acids had different effects on the metabolic signatures of the UPR, suggesting the type dietary fatty acid is important in properly handling ER stress. FUNDING SOURCES: NIH.
A number of experimental studies have shown recently that ppm-level additions of nitric oxide (NO) enhance the rate of nitrous oxide (N(2)O) decomposition catalyzed by Fe-ZSM-5 at low temperatures. In the present work, the NO-assisted N(2)O decomposition over mononuclear iron sites in Fe-ZSM-5 was studied on a molecular level using density functional theory (DFT) and transition-state theory. A reaction network consisting of over 100 elementary reactions was considered. The structure and energies of potential-energy minima were determined for all stable species, as were the structures and energies of all transition states. Reactions involving changes in spin potential-energy surfaces were also taken into account. In the absence of NO and at temperatures below 690 K, most active single iron sites (Z(-)[FeO](+)) are poisoned by small concentrations of water in the gas phase; however, in the presence of NO, these poisoned sites are converted into a novel active iron center (Z(-)[FeOH](+)). These latter sites are capable of promoting the dissociation of N(2)O into a surface oxygen atom and gas-phase N(2). The surface oxygen atom is removed by reaction with NO or nitrogen dioxide (NO(2)). N(2)O dissociation is the rate-limiting step in the reaction mechanism. At higher temperatures, water desorbs from inactive iron sites and the reaction mechanism for N(2)O decomposition becomes independent of NO, reverting to the reaction mechanism previously reported by Heyden et al. [J. Phys. Chem. B 2005, 109, 1857].
