Relationship of crystal growth parameters to the stoichiometry of EuO as determined by I.R. and conductivity measurements

Single crystals of EuO have been grown from melts containing excess Eu-metal. i.r. absorption and conductivity measurements were made on these crystals and the results correlated with the crystal growth parameters. It is seen that as the concentration of europium in the initial melt is increased, the growth temperature is lowered and the crystals become richer in europium. The variations in the i.r. absorption and conductivity were studied and are shown to be strongly related to composition. According to the i.r. and conductivity behavior, the EuO crystals can be classified into five types, with compositions ranging from oxygen-rich, to stoichiometric, to europium-rich. The oxygen-rich EuO crystals are insulators and exhibit i.r. absorption which is attributed to Eu₃O₄ and to Eu³⁺. Only weak i.r. absorption is seen in the stoichiometric crystals. Those with a deficiency of oxygen show a new i.r. absorption and have either metallic conduction or show large insulator-metal transitions which are attributed to oxygen vacancies.     

Strain-induced polarization rotation in epitaxial (001) BiFeO3 thin films

Direct measurement of the remanent polarization of high quality (001)-oriented epitaxial BiFeO3 thin films shows a strong strain dependence, even larger than conventional (001)-oriented PbTiO3 films. Thermodynamic analysis reveals that a strain-induced polarization rotation mechanism is responsible for the large change in the out-of-plane polarization of (001) BiFeO3 with biaxial strain while the spontaneous polarization itself remains almost constant.     

Intense geodesic acousticlike modes driven by suprathermal ions in a tokamak plasma

Intense axisymmetric oscillations driven by suprathermal ions injected in the direction counter to the toroidal plasma current are observed in the DIII-D tokamak. The modes appear at nearly half the ideal geodesic acoustic mode frequency, in plasmas with comparable electron and ion temperatures and elevated magnetic safety factor (q_{min}>or=2). Strong bursting and frequency chirping are observed, concomitant with large (10%-15%) drops in the neutron emission. Large electron density fluctuations (n[over ]_{e}/n_{e} approximately 1.5%) are observed with no detectable electron temperature fluctuations, confirming a dominant compressional contribution to the pressure perturbation as predicted by kinetic theory. The observed mode frequency is consistent with a recent theoretical prediction for the energetic-particle-driven geodesic acoustic mode.     

Mössbauer study of europium in fluorozirconate glass

¹⁵¹Eu Mössbauer spectra of fluorozirconate glasses of nominal composition 0.61 ZrF₄ · (0.32 ? x) BaF₂ · x EuF₂ · 0.07 ThF₄ with x = 0.10 and 0.20 show that Eu²⁺ is present in sites with isomer shifts in the range ?15.5 to ?12.9 mm/s, corresponding to EuF bond lengths of 2.3–2.8 Å and a coordination number ranging from 8 to 12. The glasses contain a minor amount (～10%) of Eu³⁺ in well-defined sites. Effective Debye temperatures deduced from the recoilless fractions for the two ions are θ_D²⁺ = 145 K and θ_D³⁺ = 261 K. The differences in coordination and bonding indicate that trivalent eurpoium is a network former, whereas the divalent ions are network modifiers.     

Reverse mathematics,well-quasi-orders,and Noetherian spaces

A quasi-order Q induces two natural quasi-orders on \({\mathcal{P}(Q)}\), but if Q is a well-quasi-order, then these quasi-orders need not necessarily be well-quasi-orders. Nevertheless, Goubault-Larrecq (Proceedings of the 22nd Annual IEEE Symposium 4 on Logic in Computer Science (LICS’07), pp. 453–462, 2007) showed that moving from a well-quasi-order Q to the quasi-orders on \({\mathcal{P}(Q)}\) preserves well-quasi-orderedness in a topological sense. Specifically, Goubault-Larrecq proved that the upper topologies of the induced quasi-orders on \({\mathcal{P}(Q)}\) are Noetherian, which means that they contain no infinite strictly descending sequences of closed sets. We analyze various theorems of the form “if Q is a well-quasi-order then a certain topology on (a subset of) \({\mathcal{P}(Q)}\) is Noetherian” in the style of reverse mathematics, proving that these theorems are equivalent to ACA₀ over RCA₀. To state these theorems in RCA₀ we introduce a new framework for dealing with second-countable topological spaces.     

Engineering the quadruplex fold: nucleoside conformation determines both folding topology and molecularity in guanine quadruplexes

Nucleic acid quadruplexes, based on the guanine quartet, can arise from one or several strands, depending on the sequence. Those consisting of a single strand are usually folded in one of two principal topologies: antiparallel, in which all or half of the guanine stretches are antiparallel to each other, or parallel, in which all guanine stretches are parallel to each other. In the latter, all guanine nucleosides possess the anti conformation about the glycosidic bond, while in the former, half possess the anti conformation, and half possess the syn conformation. While antiparallel is the more common fold, examples of biologically important, parallel quadruplexes are becoming increasingly common. Thus, it is of interest to understand the forces that determine the quadruplex fold. Here, we examine the influence of individual nucleoside conformation on the overall folding topology by selective substitution of rG for dG. We can reverse the antiparallel fold of the thrombin binding aptamer (TBA) by this approach. Additionally, this substitution converts a unimolecular quadruplex into a bimolecular one. Similar reverse substitutions in the all-RNA analogue of TBA result in a parallel to antiparallel change in topology and alter the strand configuration from bimolecular to unimolecular. On the basis of the specific substitutions made, we conclude that the strong preference of guanine ribonucleosides for the anti conformation is the driving force for the change in topology. These results demonstrate how conformational properties of guanine nucleosides govern not only the quadruplex folding topology but also impact quadruplex molecularity and provide a means to control these properties.     

Raman Spectroscopy and Fluorescence Photon Migration for Breast Cancer Diagnosis and Imaging

We are developing optical methods based on near infra-red Raman spectroscopy and fluorescence photon migration for diagnosis and localization of breast cancer. We demonstrate the ability of Raman spectroscopy to classify accurately normal, benign and malignant breast tissues, an important step in developing Raman spectroscopic needle probes as a tool for improving the accuracy of needle biopsy. We also show that photon migration imaging can be used to localize accurately small fluorescent objects imbedded in a thick turbid medium with realistic optical properties, thus demonstrating the potential of this technique for optical imaging.