The Topological Representation of Oriented Matroids

We present a new proof of the Topological Representation Theorem for oriented matroids in the general rank case. Our proof is based on an earlier rank 3 version. It uses hyperline sequences and the generalized Schonflies theorem. As an application, we show that one can read off oriented matroids from arrangements of embedded spheres of codimension one, even if wild spheres are involved.     

Perfect metamaterial absorber

We present the design for an absorbing metamaterial (MM) with near unity absorbance A(omega). Our structure consists of two MM resonators that couple separately to electric and magnetic fields so as to absorb all incident radiation within a single unit cell layer. We fabricate, characterize, and analyze a MM absorber with a slightly lower predicted A(omega) of 96%. Unlike conventional absorbers, our MM consists solely of metallic elements. The substrate can therefore be optimized for other parameters of interest. We experimentally demonstrate a peak A(omega) greater than 88% at 11.5 GHz.     

Experimental demonstration of electromagnetic tunneling through an epsilon-near-zero metamaterial at microwave frequencies

Silveirinha and Engheta have recently proposed that electromagnetic waves can tunnel through a material with an electric permittivity (epsilon) near zero (ENZ). An ENZ material of arbitrary geometry can thus serve as a perfect coupler between incoming and outgoing waveguides with identical cross-sectional area, so long as one dimension of the ENZ is electrically small. In this Letter we present an experimental demonstration of microwave tunneling between two planar waveguides separated by a thin ENZ channel. The ENZ channel consists of a planar waveguide in which complementary split ring resonators are patterned on the lower surface. A tunneling passband is found in transmission measurements, while a two-dimensional spatial map of the electric field distribution reveals a uniform phase variation across the channel--both measurements in agreement with theory and numerical simulations.     

Purification of carbon-11 PET radiotracers from unlabeled precursors by preparative HPLC and SPE

A general methodology for the rapid purification of carbon-11 positron emission tomography (PET) radiotracers from radiolabeling reaction mixtures has been developed. Preparative HPLC and solid-phase extraction (SPE) techniques are described which can separate some commonly used radiopharmaceuticals such as [(11)C]raclopride, [(11)C]beta-CFT and [(11)C]choline from their unlabeled precursors.     

Application of on-line HPLC-FAB mass spectrometry to the analysis of enzymatic digests of ribonuclease B

A tryptic digest of bovine ribonuclease B was analysed by on-line HPLC-FAB mass spectrometry to verify the primary sequence. Peptides obtained by digestion with trypsin before and after treatment with N-glycanase were separated on a slurry-packed fused silica LC column. The entire eluent was introduced into the mass spectrometer source via a continuous flow interface. A single injection of 100 pinoles of material provided data covering over 70% of the sequence.     

Systems of conservation laws of mixed type

A local theory of weak solutions of first-order nonlinear systems of conservation laws is presented. In the systems considered, two of the characteristic speeds become complex for some achieved values of the dependent variable. The transonic “small disturbance” equation is an example of this class of systems. Some familiar concepts from the purely hyperbolic case are extended to such systems of mixed type, including genuine nonlinearity, classification of shocks into distinct fields and entropy inequalities. However, the associated entropy functions are not everywhere locally convex, shock and characteristic speeds are not bounded in the usual sense, and closed loops and disjoint segments are possible in the set of states which can be connected to a given state by a shock. With various assumptions, we show (1) that the state on one side of a shock plus the shock speed determine the state on the other side uniquely, as in the hyperbolic case; (2) that the “small disturbance” equation is a local model for a class of such systems; and (3) that entropy inequalities and/or the existence of viscous profiles can still be used to select the “physically relevant” weak solution of such a system.     

Strain-narrowing of excitons in germanium

We have observed a narrowing of the luminescence line from excitons in germanium with the application of a uniaxial stress along the [111] crystallographic axis. In zero stress the line is broadened by the two split excitons and the mass-reversal effect, while at our stress of 15 $\text{kg}\text{mm}{}^2$ only one band is occupied. We find that the high stress density of states varies nearly as the square root of energy.     

Wire initiation critical for radiation symmetry in z-pinch-driven dynamic hohlraums

Axial symmetry in x-ray radiation of wire-array z pinches is important for the creation of dynamic hohlraums used to compress inertial-confinement-fusion capsules. We present the first evidence that this symmetry is directly correlated with the magnitude of the negative radial electric field along the wire surface. This field (in turn) is inferred to control the initial energy deposition into the wire cores, as well as any current shorting to the return conductor.     

Determination of static microstructure of dilute and concentrated suspensions of anisotropic particles by ultra-small-angle X-ray scattering

Ultra-small-angle X-ray scattering was performed on suspensions of anisotropic polystyrene particles of varying degrees of anisotropy. The wave vector dependence of particle form factors is well described by a model developed by Debye for the scattering from fused spheres. As volume fraction is raised, all suspensions undergo a disorder/order phase transition. The scattering from disordered and ordered suspensions of anisotropic particles is the same as that of spheres up to volume fractions of 0.45, suggesting that, in the dilute crystalline phase, the anisotropic particles order into a rotator or plastic crystal phase, where the particle centers of mass are ordered, but the particle directors are randomly distributed. Further increase in particle volume fraction leads to differences in scattering between homonuclear dicolloids and spheres, implying that the homonuclear dicolloids form a body-centered tetragonal phase with both positional and directional order. This conclusion is supported by real-space imaging of dried films of the particles.