Structural phase transformations and physical properties of intercalation compounds in the Cr0.5Ti(Se1−x Tex)2 system

The atomic structure and magnetic and electric properties of the Cr_0.5TiSe₂-Cr_0.5TiTe₂ system of intercalated phases were studied in detail by gradually replacing selenium by tellurium. It was revealed that this replacement changes the crystalline structure from monoclinic in the initial compounds to hexagonal in the compounds containing various types of chalcogen atoms; this is accompanied by disordering of chromium atoms in the van der Waals gaps. The electrical resistance and magnetic characteristics vary nonmonotonically on replacement of selenium by tellurium, which is associated with a change in the degree of atomic disordering during the transition from Cr_0.5TiSe₂ to Cr_0.5TiTe₂.     

Low-loss multilayered metamaterial exhibiting a negative index of refraction at visible wavelengths

We experimentally demonstrate a low-loss multilayered metamaterial exhibiting a double-negative refractive index in the visible spectral range. To this end, we exploit a second-order magnetic resonance of the so-called fishnet structure. The low-loss nature of the employed magnetic resonance, together with the effect of the interacting adjacent layers, results in a figure of merit as high as 3.34. A wide spectral range of negative index is achieved, covering the wavelength region between 620 and 806 nm with only two different designs.     

Observation of resonant behavior in the energy velocity of diffused light

In this Letter we demonstrate Mie resonances mediated transport of light in randomly arranged, monodisperse dielectric spheres packed at high filling fractions. By means of both static and dynamic optical experiments we show resonant behavior in the key transport parameters and, in particular, we find that the energy transport velocity, which is lower than the group velocity, also displays a resonant behavior.     

Neutron diffraction study of the magnetic ordering in the series R 2 BaNiO 5 (R = Rare Earth)

A neutron diffraction study, as a function of temperature, of the title compounds is presented. The whole family (space group Immm, a ≈ 3.8?, b ≈ 5.8?, c ≈ 11.3?) is structurally characterised by the presence of flattened NiO₆ octahedra that form chains along the a-axis, giving rise to a strong Ni-O-Ni antiferromagnetic interaction. Whereas for Y-compound only strong 1D correlations exist above 1.5 K, presenting the Haldane gap characteristic of 1D AF chain with integer spin, 3D AF ordering is established simultaneously for both R and Ni sublattices at temperatures depending on the rare earth size and magnetic moment. The magnetic structures of R₂BaNiO₅ ( R = Nd, Tb, Dy, Ho, Er and Tm) have been determined and refined as a function of temperature. The whole family orders with a magnetic structure characterised by the temperature-independent propagation vector = (1/2, 0, 1/2). At 1.5 K the directions of the magnetic moments differ because of the different anisotropy of the rare earth ions. Except for Tm and Yb (which does not order above 1.5 K), the magnetic moment of the R³⁺ cations are close to the free-ion value. The magnetic moment of Ni²⁺ is around 1.4 , the strong reduction with respect to the free-ion value is probably due to a combination of low-dimensional quantum effects and covalency. The thermal evolution of the magnetic structures from T _N down to 1.5 K is studied in detail. A smooth re-orientation, governed by the magnetic anisotropy of R³⁺, seems to occur below and very close to T _N in some of these compounds: the Ni moment rotates from nearly parallel to the a-axis toward the c-axis following the R moments. We demonstrate that for setting up the 3D magnetic ordering the R-R exchange interactions cannot be neglected. Received 19 July 2001     

Comparative Study by MS and XRD of Fe50Al50 Alloys Produced by Mechanical Alloying, Using Different Ball Mills

In this work we report a comparative study of the magnetic and structural properties of Fe₅₀Al₅₀ alloys produced by mechanical alloying using two different planetary ball mills with the same ball mass to powder mass relation. The Fe₅₀Al₅₀ sample milled during 48 h using the Fritsch planetary ball mill pulverisette 5 and balls of 20 mm, presents only a bcc alloy phase with a majority of paramagnetic sites, whereas that sample milled during the same time using the Fritsch planetary ball mill pulverisette 7 with balls of 15 mm, presents a bcc alloy phase with paramagnetic site (doublet) and a majority of ferromagnetic sites which include pure Fe. However for 72 h of milling this sample presents a bcc paramagnetic phase, very similar to that prepared with the first system during 48 h. These results show that the conditions used in the first ball mill equipment make more efficient the milling process.     

Analysis of the escape in systems with four exit channels

In this paper, we have performed a numerical investigation of the escape of a particle from two different dynamical systems with the same number of exit channels. We have chosen specific values of the parameters of the systems so that the openings of the potential well in both systems are approximately of the same size. We have found that, in the galactic system, the distribution of the times of escape follows a sequential pattern that has never been detected before. Moreover, we have proved that this pattern is directly related to the geometry of the stable manifolds to the Lyapunov orbits located at the openings of the potential. Finally, we have shown that the different nature of the two systems affects the way the escape occurs, due to the difference in the geometry of the manifolds to the Lyapunov orbits in both systems.     

Formation and Evolution of Carbonate Species in Na-Doped Al2O3 and V2O5-Al2O3

A study is carried out by FT-IR spectroscopy of the carbonate species formed upon interaction of CO₂ with alumina and vanadia-alumina catalysts doped with sodium. It is found that the presence of sodium enhances the ability of the catalyst surface to adsorb CO₂, yielding to carbonate formation. The species formed changes in the presence of vanadium, shifting the ν_COO stretching bands towards higher wavenumbers than those recorded in Na-Al₂O₃ systems.     

Effect of temperature on the synthesis of silver nanoparticles with polyethylene glycol: new insights into the reduction mechanism

Polyethylene glycol (PEG) molecules act as a reducing and stabilizing agent in the formation of silver nanoparticles. PEG undergoes thermal oxidative degradation at temperatures over 70 °C in the presence of oxygen. Here, we studied how the temperature and an oxidizing atmosphere could affect the synthesis of silver nanoparticles with PEG. We tested different AgNO₃ concentrations for nanoparticles syntheses using PEG of low molecular weight, at 60 and 100 °C. At the higher temperature, the reducing action of PEG increased and the effect of PEG/Ag⁺ ratio on nanoparticles aggregation changed. These results suggest that different synthesis mechanisms operate at 60 and 100 °C. Thus, at 60 °C the reduction of silver ions can occur through the oxidation of the hydroxyl groups of PEG, as has been previously reported. We propose that the thermal oxidative degradation of PEG at 100 °C increases the number of both, functional groups and molecules that can reduce silver ions and stabilize silver nanoparticles. This degradation process could explain the enhancement of PEG reducing action observed by other authors when they increase the reaction temperature or use a PEG of higher molecular weight     

<Emphasis Type="Italic">Z</Emphasis><Superscript>′</Superscript> boson signal at Fermilab-Tevatron and CERN-LHC in a 331 model

We analyse the possibilities to detect a new Z ^′ boson in di-electron events at Tevatron and LHC in the framework of the 331 model with right-handed neutrinos. We also study other fermions and Higgs events as final state at LHC. Using collision data collected by the CDF II detector at Fermilab Tevatron, we find that the 331 Z ^′ boson is excluded with masses below 920 GeV. For an integrated luminosity of 100 fb⁻¹ at LHC, and considering a central value GeV, we obtain the invariant-mass distribution in the process pp→Z ^′→e ⁺ e ⁻, where a huge peak, corresponding to 800 signal events/(20 GeV), is found above the SM background. The number of di-electron events vary from 50000 to 2400 in the mass range of –2000 GeV. We also obtain branching ratios and cross sections in other fermion and Higgs channels at LHC, where a heavy top quark T exhibits the biggest ratio with m _T =300 GeV.