Novel electronic and magnetic properties of ultrathin chromium oxide films grown on Pt(111)

The growth of epitaxial metal–oxide films on lattice-mismatched metal substrates often results in the formation of unique overlayer structures. In particular, epitaxial chromium oxide films grown on Pt(111) exhibit a p(2×2) symmetry through the first two monolayers of growth which is followed by a (√3×√3)R30° phase that is attributed to the growth of a Cr2O3(0001) overlayer. Ultraviolet photoelectron spectroscopy measurements have been performed on the CrOx/Pt(111) system. The electronic structures of CrO2, Cr2O3, and Cr3O4 were calculated using the linear muffin-tin orbital method in the atomic sphere approximation. Comparison of the photoemission valence band spectra with the calculated density of states indicates that the CrOx initially grows in a cubic spinel Cr3O4 structure. Beyond ∼0.2 monolayers, the metallic behavior of the CrOx overlayer begins a transformation to an insulating state. The measured valence emission for the p(2×2) phase beyond ∼0.2 monolayers is more consistent with either a γ-Cr2O3(111) overlayer or possibly a reconstructed Cr2O3(0001) overlayer. © 1998 American Vacuum Society

H2S adsorption on chromium, chromia, and gold/chromia surfaces: Photoemission studies

The reaction of H2S with chromium, chromia, and Au/chromia films grown on a Pt(111) crystal has been investigated using synchrotron-based high-resolution photoemission spectroscopy. At 300 K, H2S completely decomposes on polycrystalline chromium producing a chemisorbed layer of S that attenuates the Cr 3d valence features. No evidence was found for the formation of CrSx species. The dissociation of H2S on Cr3O4 and Cr2O3 films at room temperature produces a decrease of 0.3–0.8 eV in the work function of the surface and significant binding-energy shifts (0.2–0.6 eV) in the Cr 3p core levels and Cr 3d features in the valence region. The rate of dissociation of H2S increases following the sequence: Cr2O33O4. For chromium, the density of states near the Fermi level is large, and these states offer a better match in energy for electron acceptor or donor interactions with the frontier orbitals of H2S than the valence and conduction bands of the chromium oxides. This leads to a large dissociation probability for H2S on the metal, and a low dissociation probability for the molecule on the oxides. In the case of Cr3O4 and Cr2O3, there is a correlation between the size of the band gap in the oxide and its reactivity toward H2S. The uptake of sulfur by the oxides significantly increases when they are “promoted” with gold. The Au/Cr2O3 surfaces exhibit a unique electronic structure in the valence region and a larger ability to dissociate H2S than polycrystalline Au or pure Cr2O3. The results of ab initio SCF calculations for the adsorption of H2S on AuCr4O6 and AuCr10O15 clusters show a shift of electrons from the gold toward the oxide unit that enhances the strength of the Au(6s)↔H2S(5a1,2b1) bonding interactions and facilitates the decomposition of the molecule. © 1997 American Institute of Physics

Spin-resolved photoelectron spectroscopy of Fe3O4 - Revisited

Recently Tobin et al (2007 J. Phys.: Condens. Matter 19 315218) reported on the spin-resolved photoemission study of Fe3O4(001) films, claiming magnetite being a case against half-metallicity. In the present communication we re-examine recent spin-resolved photoemission experiments on Fe3O4 and explain why their criticism is unfounded

Tropical Cyclone Prediction on Subseasonal Time-Scales

Here we discuss recent progress in understanding tropical cyclone (TC) subseasonal variability and its prediction. There has been a concerted effort to understand the sources of predictability at subseasonal time-scales, and this effort has continued to make progress in recent years. Besides the Madden-Julian Oscillation (MJO), other modes of variability affect TCs at these time-scales, in particular various equatorial waves. Additionally, TC activity is also modulated by extratropical processes via Rossby wave breaking. There has also been progress in the ability of models to simulate the MJO and its modulation of TC activity. Community efforts have created multi-model ensemble datasets, which have made it possible to evaluate the forecast skill of the MJO and TCs on subseasonal time-scales in multiple forecasting systems. While there is positive skill in some cases, there is strong dependence on the ensemble system considered, the basin examined, and whether the storms have extratropical influences or not. Furthermore, the definition of skill differs among studies. Forecasting centers are currently issuing subseasonal TC forecasts using various techniques (statistical, statistical-dynamical and dynamical). There is also a strong interest in the private sector for forecasts with 3-4 weeks lead time

Growth morphology and electronic structure of ultra-thin TaOx films on Ag(100)

A study of the growth morphology and electronic structure of TaOx films on the Ag(100) substrate has been performed to determine the properties of ultra-thin TaOx films without the influence of a mixed interfacial oxide (i.e., a disordered SiO2/TaOx interface for growth on Si). The TaOx films were grown by thermal evaporation of Ta in an oxygen atmosphere of 1 × 10-6 Torr. Growth on a Ag(100) surface held at room temperature results in an amorphous TaOx overlayer, as determined by low energy electron diffraction. The onset of ordering of these films occurs for a post-anneal at ∼500 °C. A diffraction pattern that corresponds to a multi-domain overlayer structure is observed for anneals at ∼550 °C. Deposition of Ta without oxygen results in the formation of Ta islands. These results indicate that there is a very weak adsorbate-substrate interaction. Photoemission measurements of the TaOx films show the formation of a band gap with the valence band maximum residing at 3.5 eV below the Fermi level. Core level shifts of ∼3.5 eV are observed for the Ta with no indication of metallic Ta at the surface

Metallic-to-nonmetallic transition of Na coadsorbed with CO2 and H2O on the Cr2O3(111)/Cr(110) surface

Coadsorption of Na with CO2 and H2O on a thin Cr2O3(111) single crystal epitaxially grown on a single-crystal Cr(110) substrate has been performed to study the effects of the coadsorbates on the electronic properties of the Na species. High-resolution electron-energy-loss spectroscopy spectra as a function of Na overlayer coverage reveal a continuous attenuation of the Fuchs-Kliewer surface-phonon loss modes with near total attenuation by ∼2 monolayers (ML). Coadsorption with either H2O or CO2 returns the substrate Fuchs-Kliewer modes to a relative intensity approximately equal to that observed on the clean Cr2O3(111) surface. Angle-resolved photoemission spectroscopy (ARPES) measurements from the Cr2O3(111) surface demonstrate almost total attenuation of the Cr2O3(111) valence-band emission by a Na coverage of ∼2 ML with no return of Cr2O3(111) valence-band emission features upon coadsorption. These observations provide strong evidence that the development of a metallic Na overlayer results in a damping of the coupling interaction between the electron and the Fuchs-Kliewer phonons and that upon reaction with the coadsorbed species, a uniform dielectric overlayer is formed which permits the return of the Fuchs-Kliewer losses. The metallic-to-nonmetallic transition of the overlayer is also supported by the ARPES data which reveal suppression of emission from the Fermi level, the disappearance of the LVV Auger transition, and a shifting to higher binding energy of the Na 2p emission upon compound formation during coadsorption. © 1992 The American Physical Society

Bulk-defect dependent adsorption on a metal oxide surface: S/TiO2(1 1 0)

The adsorption of molecular sulfur on TiO2(1 1 0)(1 × 1) has been studied with scanning tunneling microscopy and photoelectron spectroscopy. At room temperature S binds dissociatively to 5-fold coordinated Ti atoms and oxygen vacancies. At elevated temperatures (120-440°C) sulfur replaces surface oxygen atoms. Evidence was found that the reduction state of TiO2 crystals strongly affects the surface coverage of S at elevated temperatures. The rate of the O-S site exchange is kinetically limited by the arrival of diffusing bulk defects at the surface. © 2001 Elsevier Science B.V. All rights reserved

An Open Randomized Controlled Study on the Efficacy of Low-Sodium Water Intake Evaluated by Non-Invasive Methods in Patients with Cellulite

Cellulite is the result of complex physiological changes of the subcutaneous fat layer and of microcirculation, clinically manifesting as orange peel skin especially in women and involving thighs, buttocks, and abdomen. An adequate water intake has been suggested to be helpful in controlling the development and worsening of the disease. An open randomized controlled study has been performed to evaluate modification induced by an adequate diet associated to low-sodium or high-sodium water intake on some clinical features of patients affected with mild to moderate cellulite. Non-invasive instrumental investigations (Bioelectrical Impedance Analysis (BIA), thermography and skin echography) were used before and after dietary treatment. A significant improvement of the disease, in terms of weight loss, cutaneous microcirculation and reduction of subcutaneous fat layer, was observed in the group of patients who underwent low-sodium water intake. An adequate dietary treatment associated to a low-sodium water intake is able to efficiently controll some biological and clinical parameters of mild to moderate cellulite

The adsorption of chlorine on TiO2(1 1 0) studied with scanning tunneling microscopy and photoemission spectroscopy

The adsorption of chlorine, dosed from an electrochemical cell, on the (1 1 0)-surface of a bulk-reduced TiO2 crystal was studied with scanning tunneling microscopy, X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy (UPS). At room temperature chlorine adsorbs on the fivefold coordinated Ti atoms and on the vacancies in the bridging oxygen rows. Chlorine exposure at 200 and 300 °C leads to adsorption at the position of bridging oxygen atoms. In addition, ∼15 Å wide protrusions are formed that contain several chlorine atoms. No long-range ordering was found in the adsorbed layer. UPS shows that adsorption of Cl on the hot surface leads to an increase in the Ti3d-related defect state. This indicates a replacement of bridging oxygen atoms by chlorine. Interestingly, the rate of the site exchange depends on the bulk reduction state of the TiO2 crystal. After dosing Cl at 200 °C, negligible adsorption was found on a slightly reduced, light blue crystal. The data are compared and contrasted with similar experimental results on the S/TiO2(1 1 0) system. © 2002 Elsevier Science B.V. All rights reserved