Nearly isotropic photonic bandgap structures in two dimensions

It is suggested that a material with a novel spatial dielectric distribution can exhibit a bandgap that is approximately independent of propagation angle. This independence is accomplished by development of the dielectric constant from reflection vectors of equal strength that are ideally equispaced in angle.     

Muon bonding sites in rare earth orthoferrites

A muon site search has been performed for the RFeO₃ series based on a calculation of dipole fields and assumptions that a μ-0 bond is formed at identical sites in each sample. The site previously identified by Holzschuh et al. /1/ has been verified and additional sites located which together explain all the observed μSR frequencies in the orthoferrites. Effects due to muon motion and covalent contributions to the internal fields are briefly discussed.     

Muon Knight shift and zero-field relaxation in (U,Th)Be13

We report μ⁺ zero-field relaxation and Knight shift studies of the heavy-fermion superconductors U_1−xTh_xBe₁₃, x=0 and 0.033. The Knight shift in UBe₁₃ shows a strong decrease as the temperature is reduced in the superconducting state, unlike U_0.967Th_0.033Be₁₃ in which the shift remains at about the normal state value. If the superconducting state in UBe₁₃ has odd-parity, the decrease of K_μ below T_c suggests that the order parameter is pinned to the lattice. Either spin-orbit scattering or the existence of two distinct superconducting states with different spin susceptibilities in the (U,Th)Be₁₃ system would explain the differences observed in the Th-doped and pure UBe₁₃ materials. The latter hypothesis would exclude conventional BCS superconductivity. No evidence for magnetic order is seen in the zero-field relaxation rate for either material down to 0.3 K.     

Protecting workers and the environment: An environmental NGO’s perspective on nanotechnology

Nanotechnology, the design and manipulation of materials at the atomic scale, may well revolutionize many of the ways our society manufactures products, produces energy, and treats diseases. New materials based on nanotechnology are already reaching the market in a wide variety of consumer products. Some of the observed properties of nanomaterials call into question the adequacy of current methods for determining hazard and exposure and for controlling resulting risks. Given the limitations of existing regulatory tools and policies, we believe two distinct kinds of initiatives are needed: first, a major increase in the federal investment in nanomaterial risk research; second, rapid development and implementation of voluntary standards of care pending development of adequate regulatory safeguards in the longer term. Several voluntary programs are currently at various stages of evolution, though the eventual outputs of each of these are still far from clear. Ultimately, effective regulatory safeguards are necessary to provide a level playing field for industry while adequately protecting human health and the environment. This paper reviews the existing toxicological literature on nanomaterials, outlines and analyzes the current regulatory framework, and provides our recommendations, as an environmental non-profit organization, for safe nanotechnology development.     

Considerations for potency equivalent calculations in the Ah receptor-based CALUX bioassay: normalization of superinduction results for improved sample potency estimation

The chemically activated luciferase expression (CALUX) system is a mechanistically based recombinant luciferase reporter gene cell bioassay used in combination with chemical extraction and clean-up methods for the detection and relative quantitation of 2,3,7,8-tetrachlorodibenzo-p-dioxin and related dioxin-like halogenated aromatic hydrocarbons in a wide variety of sample matrices. While sample extracts containing complex mixtures of chemicals can produce a variety of distinct concentration-dependent luciferase induction responses in CALUX cells, these effects are produced through a common mechanism of action (i.e. the Ah receptor (AhR)) allowing normalization of results and sample potency determination. Here we describe the diversity in CALUX response to PCDD/Fs from sediment and soil extracts and not only report the occurrence of superinduction of the CALUX bioassay, but we describe a mechanistically based approach for normalization of superinduction data that results in a more accurate estimation of the relative potency of such sample extracts.     

Data-directed scan sequence for the general assignment of C-glycosylflavone O-glycosides in plant extracts by liquid chromatography-ion trap mass spectrometry

An ion trap LC-MS/MS method is described for the analysis of C-glycosylflavone O-glycosides in crude methanolic extracts of plants. The method employs survey scans with and without the application of up-front collision induced dissociation (CID) to generate diagnostic ions for data-directed MS/MS. The spectra acquired allow assignment of the C-linked sugar to either the C-6 or C-8 position of the aglycone and provide data on the molecular mass of the compound, the number and type of O-linked sugars and the molecular mass of the flavone aglycone. These data for the majority of C-glycosylflavone O-glycosides in an extract are obtained automatically in one LC-MS/MS analysis without manual pre-programming. Key to the assignment of the C-6 or C-8 site of C-glycosylation is the generation, by up-front CID, of the (0,1)X+ product ion formed by internal cleavage of the C-linked sugar. MS/MS of this ion is found to have diagnostic value in addition to the (0,2)X+ product ion described by other authors. Ion trap MS/MS spectra of [M+H]+ of the 6,8-di-C-glycosylflavones schaftoside and isoschaftoside show an additional and previously unreported diagnostic product ion that is useful in determining the type of sugar at the C-6 position. The product ion spectra of protonated kaempferol 3-O-glucosylrhamnosides show similarities to the spectra of C-glycosylflavone O-glycosides; this is a potential source of confusion if the analysis of such glycosides is limited solely to MS/MS of [M+H]+.     

Muon spin relaxation and knight shift in the heavy-fermion superconductor UPt3

Positive muon spin relaxation experiments have been conducted on the heavy-fermion superconductor UPt₃ in both the normal and superconducting states for zero, transverse, and longitudinally applied magnetic fields. Below 6 K in zero applied field, the μ⁺ relaxation rate is approximately twice that expected from¹⁹⁵Pt nuclear dipolar relaxation alone. Transverse- and longitudinal-field measurements show that the observe relaxation rate depends on magnetic field and is quasistatic in origin. It is suggested that the onset of very weak (≈10⁻³ μ_B/U atom) magnetic ordering below approximately 6 K is responsible for the observed increase in the relaxation rate. μ⁺ Knight shift measurements in the normal state of UPt₃ show a temperature dependent shift K_μ which tracks the bulk susceptibility X. From the K_μ vs. X plot, a μ⁺ hyperfine field of approximately 100 Oe/μ_B is extracted.