Muonium diffusion in “dirty” solid krypton

Muonium spin dynamics in solid krypton in the presence of deliberately introduced impurities has been studied in the temperature range from 2 K to 115 K. At temperatures up to 40 K, muonium transverse relaxation rate turns out to be virtually unaffected by impurities. This implies that the Mu energy level shifts in the neighboring wells induced by impurities are smaller than the characteristic Mu activation energy in the one‐phonon regime of quantum diffusion. This revised version was published online in August 2006 with corrections to the Cover Date.     

Entropy bounds for endomorphisms commuting withK actions

Shereshevsky has shown that a shift-commuting homeomorphism from the two-dimensional full shift to itself cannot be expansive, and asked if such a homeomorphism can have finite positive entropy. We formulate an algebraic analogue of this problem, and answer it in a special case by proving the following: ifT : X → X is a mixing endomorphism of a compact metrizable abelian groupX, andT commutes with a completely positive entropyZ ²-actionS onX by continuous automorphisms, thenT has infinite entropy. Dedicated to the memory of Dr. Elizabeth Mary Hartley (1923–1998) The authors gratefully acknowledge support from EPSRC award no. 9570016X, N.S.F. grant No. DMS-94-01093, and the hospitality of the Warwick Mathematics Research Institute.     

Reduction kinetics of ceria surface by hydrogen

Powdered cerium dioxide (ceria, CeO₂) as compressed, sintered pellets, of porosity 16.4% and density 5.99 g cm^?3, were treated in hydrogen flow at 1 atm and various temperatures to effect reduction. The electrical conductivity was measured in situ during the reduction process. The conductivity increased continuously during the hydrogen treatment because of the creation of anion vacancies and accompanying small polaron electrons. The conductivity–time relationship exhibits three distinct regions indicated as I, II, and III. For each of steps I and II, the conductivity increases exponentially with the reduction. It is suggested from the kinetic analysis of the data that region I is due to desorption of adsorbed oxygen states. Region II appears to be the reduction of surface lattice oxygen. The kinetics of the reactions in both regions I and II obey first‐order rate laws with similar activation energies of 86 and 115 kJ mol^?1, respectively. Thermogravimetric experiments were used to determine the time needed to remove one monolayer of adsorbed oxygen from the surface. This could be used to estimate the activation energy of the desorption process at 95 kJ mol^?1—close to the value measured by conductivity measurements. After completing the surface reduction the electrical conductivity subsequently increased slowly during region III. This step is assigned to a diffusion‐controlled process during which the bulk of the pellets are reduced. ¹H MASNMR and in situ PXRD experiments confirmed the chemical nature of each of the three steps. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 293–301, 2004     

Affine transformations of finite vector spaces with large orders or few cycles

Let V be a d-dimensional vector space over a field of prime order p. We classify the affine transformations of V   of order at least p^d/4$p^d/4$, and apply this classification to determine the finite primitive permutation groups of affine type, and of degree n  , that contain a permutation of order at least n/4$n/4$. Using this result we obtain a classification of finite primitive permutation groups of affine type containing a permutation with at most four cycles.     

On the use of crack-tip-opening displacement to predict the fracture strength of notched graphite/epoxy laminates

The fracture strength and crack-opening displacement of notched graphite/epoxy laminates were measured experimentally using the center-cracked tension-specimen geometry. Four replicate tests were conducted for a variety of laminate stacking sequences, thicknesses, and notch lengths. Most laminates exhibited extensive notch-tip damage prior to fracture. Values of crack-tip-opening displacement (CTOD) at fracture were estimated from values of crack-opening displacement measured at the crack center line. CTOD was independent of specimen crack length for the [0/±45/90] _s , [0/±45/90]_15s , [0/±45] _s , [0/±45/]_15s , and [0/90]_24s laminates. In addition, notched laminate strength was accurately predicted using a Dugdale-type model along with the estimated CTOD.Paper was presented at V International Congress on Experimental Mechanics held in Montreal, Quebec, Canada on June 10–15, 1984.     

On infinitesimal stability and instability of pendulum type oscillations

For the pendulum type of oscillations governed by the equation ? + φ(x) = 0, with φ(x) an odd function, it is shown that according to the linearized disturbance equation, stability is predicted if and only if dTd_x = 0. where T is the period and α is the amplitude of the non-linear steady-state oscillations. From this it follows that for a given non-linear function φ(x). infinitesimal stability can at most be predicted only for certain discrete values of α. It is shown analytically that for a simple pendulum, a power-law spring and a cubic hard or soft spring, the oscillations are infinitesimally unstable for all α. It is further shown, however, that particular cases of non-linear restoring forces do exist for which infinitesimal stability is predicted for certain α's, in contrast to the actual Liapunov instability in these cases.     

Work Hardening Effects in Gellan Gum Gels1

Abstract

Gellan gum is a bacterial polysaccharide that is marketed as a broad spectrum gelling agent. The shear and Young's moduli, and the fracture strength of gellan gels increase with increasing ionic strength, pass through maxima and then decrease to zero at higher ionic strength. The position of the maxima depend on the valency of added counterions and are virtually independent of polymer concentration. At low ionic strength the measured rheological parameters show small variability but these values become increasingly variable with increasing ionic strength. Stress relaxation experiments were carried out in order to examine the mechanical behaviour of gellan gels and to explain these effects. At low ionic strength the gels approximate to elastic solids whereas at high ionic strength the increased stress relaxation is colloid-like in behaviour. However, unlike colloidal dispersions, the magnitude of the stress relaxation decreases on successive applications of stress and the relaxation modulus increases. The gels work harden. Such behaviour is attributed to a heterogeneous structure within the gel.     

Solvent Vapor Annealing of Block Copolymers in Confined Topographies: Commensurability Considerations for Nanolithography

The directed self‐assembly of block copolymer (BCP) materials in topographically patterned substrates (i.e., graphoepitaxy) is a potential methodology for the continued scaling of nanoelectronic device technologies. In this Communication, an unusual feature size variation in BCP nanodomains under confinement with graphoepitaxially aligned cylinder‐forming poly(styrene)‐block‐poly(4‐vinylpyridine) (PS‐b‐P4VP) BCP is reported. Graphoepitaxy of PS‐b‐P4VP BCP line patterns (C_II) is accomplished via topo­graphy in hydrogen silsequioxane (HSQ) modified substrates and solvent vapor annealing (SVA). Interestingly, reduced domain sizes in features close to the HSQ guiding features are observed. The feature size reduction is evident after inclusion of alumina into the P4VP domains followed by pattern transfer to the silicon substrate. It is suggested that this nano­domain size perturbation is due to solvent swelling effects during SVA. It is proposed that using a commensurability value close to the solvent vapor annealed periodicity will alleviate this issue leading to uniform nanofins.

     

Label‐Free Nanoimaging of Neuromelanin in the Brain by Soft X‐ray Spectromicroscopy

A hallmark of Parkinson's disease is the death of neuromelanin‐pigmented neurons, but the role of neuromelanin is unclear. The in situ characterization of neuromelanin remains dependent on detectable pigmentation, rather than direct quantification of neuromelanin. We show that direct, label‐free nanoscale visualization of neuromelanin and associated metal ions in human brain tissue can be achieved using synchrotron scanning transmission x‐ray microscopy (STXM), through a characteristic feature in the neuromelanin x‐ray absorption spectrum at 287.4 eV that is also present in iron‐free and iron‐laden synthetic neuromelanin. This is confirmed in consecutive brain sections by correlating STXM neuromelanin imaging with silver nitrate‐stained neuromelanin. Analysis suggests that the 1s–σ* (C?S) transition in benzothiazine groups accounts for this feature. This method illustrates the wider potential of STXM as a label‐free spectromicroscopy technique applicable to both organic and inorganic materials.     

Label-Free Nanoimaging of Neuromelanin in the Brain by Soft X-ray Spectromicroscopy

A hallmark of Parkinson's disease is the death of neuromelanin-pigmented neurons, but the role of neuromelanin is unclear. The in situ characterization of neuromelanin remains dependent on detectable pigmentation, rather than direct quantification of neuromelanin. We show that direct, label-free nanoscale visualization of neuromelanin and associated metal ions in human brain tissue can be achieved using synchrotron scanning transmission x-ray microscopy (STXM), through a characteristic feature in the neuromelanin x-ray absorption spectrum at 287.4 eV that is also present in iron-free and iron-laden synthetic neuromelanin. This is confirmed in consecutive brain sections by correlating STXM neuromelanin imaging with silver nitrate-stained neuromelanin. Analysis suggests that the 1s–σ* (C−S) transition in benzothiazine groups accounts for this feature. This method illustrates the wider potential of STXM as a label-free spectromicroscopy technique applicable to both organic and inorganic materials.