First-principles prediction of half-metallic ferromagnetism in Cd(TM)O2 (TM=Cr, Mn, Fe, Co, Ni) compounds

We report the electronic structure of Cd(TM)O₂ (TM=Cr, Mn, Fe, Co, Ni) in the chalcopyrite structures. From this study we find that Cd(TM)O₂ is a half-metallic ferromagnetic compound. From the energy consideration we find that Cd(TM)O₂ is more stable in chalcopyrite structure rather than in rock salt structure. A careful analysis of the spin density reveals the ferromagnetic coupling between the p-d states and the cation dangling-bond p states, which is believed to be responsible for the stabilization of the ferromagnetic phase. The calculated heat of formation, bulk modulus and cohesive energy are reported.     

Analytical results of the generalized mean-field theory for diluted magnetic semiconductors with RKKY interaction

Analytical results have been obtained in the framework of the generalized mean-field theory for diluted semiconductors with RKKY interaction. That theory accounts for the non-equivalency of different lattice sites by introducing the distribution function of local effective magnetic fields for non-regular (random) systems with magnetic interaction. The procedure is described that permits to deduce the analytical expression for that function. Corresponding improvement of the traditional mean-field theory could be observed by comparing results of such a generalized analytical model with exact results known for some simple cases, with numerical results of different authors considering the disorder of magnetic impurities’ arrangement, and with experimental data, as well.     

Photoluminescence-spectra study of yttria-stabilized zirconia

Spectral analyses of photoluminescence of yttriastabilized zirconia single crystals are carried out in the temperature range from 90 K to 330 K. Over the whole temperature range the emission spectrum can be decomposed into two broad bands. The main band is centred in the yellow-orange region of the spectrum and the secondary in the green region. The temperature dependence of the emission spectrum is tentatively analyzed in terms of the first three moments of the two band shapes. In addition, thermal quenching energy is determined to be 0.10 eV for both the yellow-orange and green bands. Results are discussed on basis of the major defects, oxygen vacancies and complexes formed by associating with the Y³⁺ ions.     

Cathodic needle growth from Mo(CO)6 vapors at higher electric fields

It is shown that metallic needle crystals grow on electron emitting areas of field emission cathodes working in Mo(CO)₆ vapor. The growth is driven by ions impinging from a field emission arc plasma, and the products are classified into two categories depending on the thermal energy supplied externally to the substrate: 1) Mo particles are agglomerated in dendritic shapes at 300-1300 K, and 2) whisker-like crystals with an unknown structure are obtained at 1300-1500 K. The high-temperature needles are capped with Taylor cones during the growth, and it is shown that they are a metastable phase of Mo.     

Is Auger-free luminescence present in CeF3?

It is well known that Auger-free luminescence (AFL) is observable when the condition E_g>E_VC is satisfied, where E_g is the band-gap energy between the lowest unoccupied band and the highest occupied band and E_VC the energy difference between the top of the highest occupied band and the top of the next lower occupied band. From measurements of reflection and X-ray photoelectron spectra, CeF₃ is demonstrated to really satisfy this condition. No evidence for AFL is found, nevertheless. The absence of AFL in CeF₃ is related to a characteristic nature of its highest and next lower occupied bands, which are quite different from those of previously studied AFL-materials.     

Photoluminescence characteristics of neodymium oxide nanocrystal/titania/ormosil composite sol-gel thin films

Neodymium (III) oxide nanocrystal/titania/organically-modified silane (ormosil) composite thin films have been prepared using a chemical approach consisting of a combination of inverse microemulsion and sol-gel techniques at low temperature. Transmission electron microscopy shows that the neodymium (III) oxide nanoparticles have a needle-like nanocrystal structure. A strong room temperature emission at 1064 nm, corresponding to the ⁴ F _3/2?⁴ I _11/2 transition, has been observed as a function of the heat treatment temperature used for the production of the composite thin films. In addition to this emission, two other main emissions at 890 nm and at 1336 nm have also been observed. In particular, there was a clear shoulder peak at 1145 nm, probably be due to the host matrix, which was observed in all the measured samples and this shoulder peak gave a maximum intensity after heat treatment at 300 °C. Received: 6 September 2000 / Accepted: 15 November 2000 / Published online: 20 June 2001     

Luminescence properties from erbium oxide nanocrystals dispersed in titania/organically modified silane composite sol–gel thin films

Luminescence properties from erbium (III) oxide nanocrystals dispersed in titania/organically modified silane composite thin films were studied. Erbium oxide nanocrystals were prepared by an inverse microemulsion technique. A strong room-temperature photoluminescence was observed at 1.531 μm, with the full width at half maximum (FWHM) of 22 nm due to intra-atomic transitions between ⁴ I _13/2 and ⁴ I _15/2 levels in the erbium (III) ion. The shape, peak position, and FWHM of the photoluminescence signals from the composite thin films were quite comparable to those prepared by other methods. The photoluminescence peak of the composite thin films showed a maximum intensity at the heat-treatment temperature of 300 °C. A room-temperature green up-conversion emission at 543 nm (⁴ S _3/2?⁴ I _15/2) was observed for the composite thin films with different heat-treatment temperatures upon excitation at 993 nm. The up-conversion emission mechanism was explained by means of an energy-level diagram and the lifetime of the visible up-conversion emission was measured. Received: 10 July 2000 / Accepted: 11 July 2000 / Published online: 5 October 2000     

Photoluminescence in amorphous TiO2-PbO systems

Photoluminescence (PL) at room temperature has been achieved in amorphous thin films and powders of the TiO₂–PbO system. They were prepared by the polymeric precursor method with [PbO]/[TiO₂] molar ratios ranging from 0.0 to 1.0. The energy position of maximum PL emission and the PL intensity showed dependence on Pb concentration. The Pb addition suggests an increase in the number of non-bridging oxygens (NBO) in the amorphous TiO₂ network. These results support the relationship between photoluminescence and structure in TiO₂-based amorphous materials. Received: 7 February 2001 / Accepted: 8 February 2001 / Published online: 27 June 2001     

Resonance-reggeon and parton-hadron duality in strong interactions

By using the concept of duality between direct channel resonances and Regge exchanges we relate the small- and large-x behavior of the structure functions. We show that even a single resonance exhibits Bjorken scaling at large Q². Received: 30 October 2001 / Accepted: 16 November 2001     

SR evidence of low frequency spin fluctuations in the AF phase of hole-doped NiO

μSR measurements in the antiferromagnetic (AF) phase of Ni_1-xLi_xO for are reported. While in pure NiO the muon longitudinal depolarization rate is found almost temperature independent, in the Li-doped compounds broad maxima around 130 K are observed. These maxima are associated with the progressive freezing of the spin fluctuations of S =1/2 defects induced by the localization of the extra-holes. From the temperature dependence of and the stretched exponential form of the depolarization, insights on the distribution of correlation times for the fluctuating field at the muon site are derived. Received: 27 April 1998 / Received in final form and Accepted: 21 August 1998