Composition and plasticizing effect of poly(carboxylic acid) complexes with amino compounds

Poly(carboxylic acid)-dimethylalkylamine complexes were prepared and examined by various methods including infrared spectrometry and chemical titrations. FTIR measurements provided some of the most complete and detailed insights into the type and the stoichiometry of the acid-base complexes. A (1:1) stoichiometric complexation of the poly(carboxylic acid) with tertiary amines was involved in protic solvents. A threshold degree of complexation of 42% was obtained in such a macromolecular system. The equilibrium constant of the proton transfer reaction K was estimated to be independent on the alkyl chain length of the base reagent. Besides the formation of the acid-base complexes, a plasticizing effect of the amine derivatives was demonstrated by differential calorimetry. A T_g depression could be both attributed to the amine molecules linked to the polymer backbone via ionic bonds and to the free molecules dissolved within the free volume of the polymer.     

Hyperfine coupling constants of β-phosphorylated nitroxides: Subtle interplay between steric strain,hyperconjugation, and dipole-dipole interactions

Solvent effects in β-phosphorylated nitroxides show that nitrogen and phosphorus hyperfine coupling constants a_N and a_P, increase and decrease with increasing polarity, polarizity, and Hydrogen Bond Donor effects of solvents, respectively. In a series of articles, it was shown that the driving interaction controlling the change in a_P is the maximization of the N^+?—O^– … ???⁺PO^– dipole – dipole interaction. In this work, we show that the steric strain in spiro β-phosphorylated nitroxides affords the opposite trend for a_P, that is, a_P increases with increasing solvent properties features.     

Quantum repeaters with photon pair sources and multimode memories

We propose a quantum repeater protocol which builds on the well-known Duan-Lukin-Cirac-Zoller (DLCZ) protocol [L. M. Duan, M. D. Lukin, J. I. Cirac, and P. Zoller, Nature (London) 414, 413 (2001)10.1038/35106500], but which uses photon pair sources in combination with memories that allow to store a large number of temporal modes. We suggest to realize such multimode memories based on the principle of photon echo, using solids doped with rare-earth-metal ions. The use of multimode memories promises a speedup in entanglement generation by several orders of magnitude and a significant reduction in stability requirements compared to the DLCZ protocol.     

Slippery or sticky boundary conditions: control of wrinkling in metal-capped thin polymer films by selective adhesion to substrates

Wrinkling patterns at the metallized surface of thin polymer films are shown to be sensitive to the sticky or slippery character of the polymer-substrate interface. Existing theoretical models were expanded to specific boundary conditions (adhesive versus slippery) in order to rationalize these observations. Based on this concept, we were able to propose a new and simple method to orient the wrinkles by chemically patterning the substrate with regions of high and low adhesion.     

Photonic crystal nanocavities in GaAs/AlGaAs with oxidised bottom cladding

We present a solution to the difficult task of removing an oxide-based hard mask from a photonic crystal fabricated in the GaAs/AlGaAs system. We use a polymer backfill technique to seal the AlGaAs layer, thereby making it inaccessible to the wet-etch solution. This allows us to use a GaAs active layer for the photonic crystal placed on an oxidised AlGaAs layer which provides mechanical and thermal support. Using this technique, we fabricated GaAs-based photonic crystal cavities and measured respectable quality factors (Q ≈ 2200) despite the intrinsic asymmetry of the system. The technique presents a viable method for producing electrically injected photonic crystal cavities for operation on a mechanically stable and thermally conducting buffer layer.     

Simple model for active nematics: quasi-long-range order and giant fluctuations

We propose a simple microscopic model for active nematic particles similar in spirit to the Vicsek model for self-propelled polar particles. In two dimensions, we show that this model exhibits a Kosterlitz-Thouless-like transition to quasi-long-range orientational order and that in this nonequilibrium context, the ordered phase is characterized by giant density fluctuations, in agreement with the predictions of Ramaswamy et al.     

Sur les immeubles hyperboliques (On hyperbolic buildings)

The aim of this paper is to give a geometric approach to Tits' amalgam method to construct buildings and to initiate a study of hyperbolic buildings, i.e. whose types are reflexion systems of the real hyperbolic space. We construct lots of examples and study their cohomology at infinity. We construct CAT(–1) polyhedral complexes having big discrete parabolic groups of isometries.     

Sur de nouveaux composés oxyfluorés des terres rares de structure pyrochlore

The action of NaF on rare earth oxides with the columbite or aeschynite structure leads to pyrochlore-type phases of formula NaTTiNbO₆F (T = Y, Ce, …, Yb). On the other hand, the substitution of cadmium by a rare earth and of oxygen by fluorine in T₂Ti₂O₇ gives isostructural phases of formula CdTTI₂O₆F (T = Y, Nd … Yb). The dielectric properties of the solid solutions between both phases and Cd₂Nb₂O₇ characterize ferroelectric-paraelectric transitions. The substitution of oxygen by fluorine brings about a decrease in the Curie temperature, but a very small substitution rate increases the dielectric constant and the spontaneous polarization.     

Profiles and absorption coefficients of positrons implanted in solids from radioactive sources

Positron mass absorption coefficients in 18 elemental and 6 compound solids are directly measured from implantation profiles. Their dependences on sample atomic number and positron mean energ are correlated to the forbiddenness of β⁺ transitions.