Evidence for two types of low-energy charge transfer excitations in Sr2CuO3

A comparative analysis of electron energy-loss spectroscopy (EELS) spectra for the 1D insulating cuprate Sr2CuO3 with transferred momentum q--> axially and radially to the chain axis allows one to elucidate the structure of the charge transfer gap in in-chain response. It is determined by the superposition of two types of excitations with different magnitudes of dispersion. The low-energy response with q--> radially to the chain direction, but yet within the plane of CuO4 plaquettes, exhibits also a dispersionless peak near 2 eV. The theoretical simulation of the EELS data using exact diagonalizations of an appropriate extended Hubbard Hamiltonian for relevant clusters requires the explicit consideration of low-lying oxygen 2p pi states within the CuO4 plaquette plane beyond the standard pd sigma extended Hubbard model widely used for cuprates with corner-shared CuO4 plaquettes.     

Une détermination du coefficient d'auto-diffusion de surface avec des pointes à émission de champ (tungstène)

To measure the surface self-diffusion coefficient, a method is described which is based on the increase of the curvature radius of a tip (Nichols and Mullins). This radius increase depending on temperature and time is measured under ultra high vacuum condition. The cleanliness of the surface is controlled by field electron microscopy. The radius variation is measured in situ by Fowler-Nordheim characteristics, and the initial and final radius by scanning electron microscopy. Measured are, for tungsten between 2100 and 2850°K, the activation energy ($\text{3.1}\text{eV}\text{atome}$) and the diffusivity ($\text{0.9}\text{cm}{}^2\text{s}\text{)}$.     

Surface self-diffusion studied by microscopic measurements of crystallite profile evolutions

Dendritic gold crystallites on graphite are heated in ultra high vacuum up to less than 0.5 of the melting point (T_m). Electron microscopy shows that the gold crystallites change their shapes by surface self-diffusion. The dendritic contours round off while the crystallite remains very flat (20 to 40 Å). The increase with time of the radii of dendrite tips is measured statistically. Such an evolution can be described by analogy to the blunting of either metal tips (Nichols and Mullins) or monoatomic cleavage tips (Höche and Bethge). Using this result, a new technique to measure surface self-diffusion coefficients (D) is proposed. Test measurements have shown that this is an interesting, very sensitive method to measure D (down to 10^?13 cm^?2 s^?1) which enables measurements to be made in an unusual low temperature range (0.25 T_m < T < 0.5T_m). In special cases the dendrites are split by the surface self-diffusion which is qualitatively in agreement wih the theory.     

Preparation of Polystyrene‐Poly(N‐isopropylacrylamide) (PS‐PNIPA) Core‐Shell Particles by Photoemulsion Polymerization

Summary: Monodisperse thermosensitive PS‐NIPA core‐shell particles composed of a PS core and a cross‐linked PNIPA shell can be successfully synthesized by a novel method: photoemulsion polymerization. Cryo‐TEM images indicate clearly the core‐shell morphology of the PS‐NIPA particles: A homogeneous regular PNIPA shell has been affixed on the spherical PS core. DLS measurements indicate that the obtained PS‐NIPA latex particles are thermosensitive. The shell of PNIPA networks with different cross‐linking densities can shrink and re‐swell with temperature and the volume transition temperature is around 32 °C in all cases.

Cryo‐TEM image of PS‐NIPA core‐shell particles.     

Ag Nanocomposite Particles: Preparation,Characterization and Application

Summary Herein, we report that different core-shell particles could be successfully used as the carrier systems for the deposition of silver nanoparticles. Firstly, thermosensitive core-shell microgel particles have been used as the carrier system for the deposition of Ag nanoparticles, in which the core consists of poly (styrene) (PS) whereas the shell consists of poly (N-isopropylacrylamide) (PNIPA) network cross-linked by N, N′-methylenebisacrylamide (BIS). Immersed in water the shell of these particles is swollen. Heating the suspension above 32 °C leads to a volume transition within the shell, which is followed by a marked shrinking of the network of the shell. Secondly, “nano-tree” type polymer brush can be used as “nanoreactor” for the generation of silver nanoparticles also. This kind of carrier particles consists of a solid core of PS onto which bottlebrush chains synthesized by the macromonomer poly (ethylene glycol) methacrylate (PEGMA) are affixed by “grafting from” technique. Thirdly, silver nanoparticles can be in-situ immobilized onto polystyrene (PS) core-polyacrylic acid (PAA) polyelectrolyte brush particles by UV irradiation. Monodisperse Ag nanoparticles with diameter of 8.5 nm, 7.5 nm and 3 nm can be deposited into thermosensitive microgels, “nano-tree” type polymer brushes and polyelectrolyte brush particles, respectively. Moreover, obtained silver nano-composites show different catalytic activity for the catalytic reduction of p-nitrophenol depending on the carrier system used for preparation.     

Nonlinear wave equation for torsion

Specializing the geometry of a Riemann-Cartan space-time U₄ to the case of a completely antisymmetric torsion tensor (axial vector torsion, ¹K_μ) a set of nonlinear wave equations and constraints are established relating the torsion of the U₄ geometry to an axial vector source current. This current obeys an anomaly relation similar to the axial current in spinor electrodynamics.     

Shear stresses of colloidal dispersions at the glass transition in equilibrium and in flow

We consider a model dense colloidal dispersion at the glass transition, and investigate the connection between equilibrium stress fluctuations, seen in linear shear moduli, and the shear stresses under strong flow conditions far from equilibrium, viz., flow curves for finite shear rates. To this purpose, thermosensitive core-shell particles consisting of a polystyrene core and a cross-linked poly(N-isopropylacrylamide) shell were synthesized. Data over an extended range in shear rates and frequencies are compared to theoretical results from integrations through transients and mode coupling approaches. The connection between nonlinear rheology and glass transition is clarified. While the theoretical models semiquantitatively fit the data taken in fluid states and the predominant elastic response of glass, a yet unaccounted dissipative mechanism is identified in glassy states.