Laue Crystallography: It's show time

AbstractNew crystallographic techniques make it possible to observe directly all of the intermediates in an enzymatic reaction. Such a series of structures can be combined to create a detailed movie of enzymatic catalysis

Low Velocity Granular Drag in Reduced Gravity

We probe the dependence of the low velocity drag force in granular materials on the effective gravitational acceleration (geff) through studies of spherical granular materials saturated within fluids of varying density. We vary geff by a factor of 20, and we find that the granular drag is proportional to geff, i.e., that the granular drag follows the expected relation Fprobe = {\eta} {\rho}grain geff dprobe hprobe^2 for the drag force, Fprobe on a vertical cylinder with depth of insertion, hprobe, diameter dprobe, moving through grains of density {\rho}grain, and where {\eta} is a dimensionless constant. This dimensionless constant shows no systematic variation over four orders of magnitude in effective grain weight, demonstrating that the relation holds over that entire range to within the precision of our data

Influence of Cation Order on the Dielectric Properties of Pb(Mg\u3csub\u3e1/3\u3c/sub\u3eNb\u3csub\u3e2/3\u3c/sub\u3e)O\u3csub\u3e3\u3c/sub\u3e-Pb(Sc\u3csub\u3e1/2\u3c/sub\u3eNb\u3csub\u3e1/2\u3c/sub\u3e)O\u3csub\u3e3\u3c/sub\u3e (PMN-PSN) Relaxor Ferroelectrics

The effect of the B-site cation chemistry and ordering on the dielectric properties of solid solutions in the (1-x)Pb(Mg1/3Nb2/3)O3-Pb(Sc1/2Nb1/2)O3 (PMN-PSN) perovskite system was examined in samples with 0.1 ≤ x ≤ 0.9. Thermal annealing treatments were effective in inducing long-range B-site order in the samples within this compositional range. The well-ordered, large chemical domain ceramics exhibit relaxor behavior up to x = ~0.5; for higher values of x, normal ferroelectric behavior was observed. For x ≤ 0.5 reductions in the chemical domain, size had no significant effect on the weak-field dielectric properties, but induced a transition to relaxor behavior for x \u3e ~0.6. The disordered PSN-rich samples undergo a spontaneous zero-field relaxor to ferroelectric transition similar to that reported previously for PSN. The field-dependent properties of compositions lying closest to the relaxor to ferroelectric crossover exhibited the highest sensitivity to alterations in the chemical order. The properties of this system are consistent with a random site description of the 1:1 ordered Pb(β′1/2 β1/2)O3 structure with β′ = (Mg(2-2x)/3Nb(1-x)/3Scx) and β″ = Nb

Geometric and homological finiteness in free abelian covers

We describe some of the connections between the Bieri-Neumann-Strebel-Renz invariants, the Dwyer-Fried invariants, and the cohomology support loci of a space X. Under suitable hypotheses, the geometric and homological finiteness properties of regular, free abelian covers of X can be expressed in terms of the resonance varieties, extracted from the cohomology ring of X. In general, though, translated components in the characteristic varieties affect the answer. We illustrate this theory in the setting of toric complexes, as well as smooth, complex projective and quasi-projective varieties, with special emphasis on configuration spaces of Riemann surfaces and complements of hyperplane arrangements.Comment: 30 pages; to appear in Configuration Spaces: Geometry, Combinatorics and Topology (Centro De Giorgi, 2010), Edizioni della Normale, Pisa, 201

Cation Ordering in Pb(Mg\u3csub\u3e1/3\u3c/sub\u3eNb\u3csub\u3e2/3\u3c/sub\u3e)O\u3csub\u3e3\u3c/sub\u3e–Pb(Sc\u3csub\u3e1/2\u3c/sub\u3eNb\u3csub\u3e1/2\u3c/sub\u3e)O\u3csub\u3e3\u3c/sub\u3e (PMN–PSN) Solid Solutions

Extended thermal annealing treatments were used to modify the B-site cation order in the (1 - x)PMN–xPSN perovskite system (where PMN is lead magnesium niobate, Pb(Mg1/3Nb2/3)O3, and PSN is lead scandium niobate, Pb(Sc1/2Nb1/2)O3). Extensive 1:1 ordering could be induced in compositions with x ≧ 0.1. The substitution of PSN into PMN produced a large increase in the thermal stability of the 1:1 ordered phase, with the maximum disordering temperature of ~1360oC being observed for the x = 0.5 composition. By monitoring the change in the degree of order with temperature, the order–disorder boundary for the PMN–PSN system was established and the transition temperature for pure PMN was estimated to be ~950oC. The changes in stability across the system were consistent with the random-site description of the cation order. The change in enthalpy associated with the ordering was affected by the size difference of the two ordered cation sites and by the size and charge mismatch of the metal cations that occupy the random-site position