A 3D-TEM study of the shape of mesopores in SBA-15 and modified SBA-15 materials

The mesopores in SBA-15 are curved on a mesoscopic length scale and even more so in the case of modified SBA-15, which implies that using the particle size of these types of materials in diffusion studies may strongly underestimate the path length relevant for intraparticle diffusion.     

Shear at twin domain boundaries in YBa2Cu3O7-x

The microstructure and strain state of twin domains in YBa2Cu3O7-x are discussed based upon synchrotron white-beam x-ray microdiffraction measurements. Intensity variations of the fourfold twin splitting of Laue diffraction peaks are used to determine the twin domain structure. Strain analysis shows that interfaces between neighboring twin domains are strained in shear, whereas the interior of these domains are regions of low strain. These measurements are consistent with the orientation relationships of twin boundaries within and across domains and show that basal plane shear stresses can exceed 100 MPa where twin domains meet. Our results support stress field pinning of magnetic flux vortices by twin domain boundaries.     

Force barriers for membrane tube formation

We used optical tweezers to measure the force-extension curve for the formation of tubes from giant vesicles. We show that a significant force barrier exists for the formation of tubes, which increases linearly with the radius of the area on which the pulling force is exerted. The tubes form through a first-order transition with accompanying hysteresis. We confirm these results with Monte Carlo simulations and theoretical calculations. Whether membrane tubes can be formed in, for example, biological cells, thus depends on the details of how forces are applied.     

The molecular structure of a complex of tetramethylcyclobutadiene and aluminium chloride,containing a σ-AlC bond

The molecular structure of the primary reaction product of 2-butyne and aluminium cloride has been deduced from single crystal X-ray diffraction data (1148 reflections) (a = 15.664(8); b = 10.886(4), c = 7.222(2) Å, Z = 4, space group Pnma). The aluminium chloride is coordinated via a σ-aluminium carbon bond (1.979 Å) to a non-planar tetramethylcyclobutadiene moiety. The bond length pattern indicates a charge dislocation within the four-membered ring.     

Test of time reversal invariance in 2-MeV neutron scattering from aligned165Ho

We describe a search for the time reversal non-invariant five-fold correlation (s·Ixk)(I·k) in the scattering of 2 MeV polarized neutrons from an aligned¹⁶⁵Ho target. Here s is the neutron spin,I is the target spin, andk is the neutron momentum vector. Differences in transmission were sought when s was reversed. The target was a single crystal of holmium, cryogenically aligned with a specially built dilution refrigerator. Rotating the target axis of alignment while cold permits the parity-even, five-fold correlation to be identified among other spin-dependent correlations in the forward elastic scattering amplitude. An analyzing power was found to be (1±6)×10⁻⁴, consistent with time reversal invariance.We obtain a bound of 5×10⁻³ for α_T, the ratio of T-odd to T-even couplings in the effective nucleon-nucleon interaction.     

Polyhedral results for the edge capacity polytope

Network loading problems occur in the design of telecommunication networks, in many different settings. For instance, bifurcated or non-bifurcated routing (also called splittable and unsplittable) can be considered. In most settings, the same polyhedral structures return. A better understanding of these structures therefore can have a major impact on the tractability of polyhedral-guided solution methods. In this paper, we investigate the polytopes of the problem restricted to one arc/edge of the network (the undirected/directed edge capacity problem) for the non-bifurcated routing case.?As an example, one of the basic variants of network loading is described, including an integer linear programming formulation. As the edge capacity problems are relaxations of this network loading problem, their polytopes are intimately related. We give conditions under which the inequalities of the edge capacity polytopes define facets of the network loading polytope. We describe classes of strong valid inequalities for the edge capacity polytopes, and we derive conditions under which these constraints define facets. For the diverse classes the complexity of lifting projected variables is stated.?The derived inequalities are tested on (i) the edge capacity problem itself and (ii) the described variant of the network loading problem. The results show that the inequalities substantially reduce the number of nodes needed in a branch-and-cut approach. Moreover, they show the importance of the edge subproblem for solving network loading problems. Received: September 2000 / Accepted: October 2001?Published online March 27, 2002     

Pressure effects in the kinetics of cw laser induced reactions

Rate constantsk and translational temperaturesT have been determined for the decompositions of ethyl acetate and perfluorocyclobutane at various pressures induced by a continuous (cw) CO₂ laser. Arrhenius plots give activation energies in good agreement with those reported for thermal rate constantsk ₁. The ratiosk/k ₁ become progressively larger than unity with decreasing reactant pressure, indicative of laser induced non-thermalization, but the values ofk/k _t appear to reach limiting values at low pressures, similar to effects reported elsewhere for CH₃CF₂Cl. Analysis of the data suggests important general features of vibrational-translational energy transfer in polyatomic compounds.