Epitaxial CrO 2 (100) island films have been grown on TiO 2 (100) substrates by a chemical-vapor deposition technique. Well-controlled surface and interface properties of the CrO 2 (100) films were confirmed by scanning tunneling microscopy and transmission electron microscopy, respectively. Spin-and angle-resolved photoemission spectroscopy at room temperature revealed an energy gap of about 2 eV below Fermi level E F for spin-down electrons and a spin polarization of about ϩ95% at E F . After extended sputtering, the spin polarization can be recovered from about ϩ10% up to ϩ85% upon annealing. 1-3,7,10-13The HMF nature of CrO 2 has been predicted on the basis of the local-spin-density approximation ͑LSDA͒ to the density-functional theory.1,3 Only recently, spin polarization values of over 90% near E F were found for CrO 2 at 1.8 K using superconducting point contact spectroscopy, 7,11-13 although values of 95% had been obtained earlier by spinpolarized photoemission at 293 K for binding energies of about 2 eV below E F .14 In this letter, we present a study of structural properties and of the spin-dependent electronic structure of epitaxial CrO 2 (100) island films prepared on isostructural TiO 2 (100) substrates by a chemical-vapor deposition ͑CVD͒ technique. 15 The CrO 2 (100) surface shows in scanning tunneling microscopy ͑STM͒ the growth of large elongated rectangular-like epitaxial islands with a preferential growth direction along the in-plane ͓001͔ axis. A sharp CrO 2 (100)/TiO 2 (100) interface without intermixing and a good crystalline quality of CrO 2 (100) were found by transmission electron microscopy ͑TEM͒. The spin-dependent electronic structure of the CrO 2 (100) films has been investigated at 293 K by means of spin-and angle-resolved photoemission spectroscopy ͑PES͒. Below the Fermi level E F an energy gap of about 2 eV was observed for spin-down electrons, and thus a spin polarization of up to ϩ95% at E F was found at 293 K. Both the magnitude of the spin-down gap and the value of the spin polarization are in good agreement with theoretical band-structure calculations, which predict only majority spin electrons at E F .1,3 Sputtering of the asprepared surface up to 210 s increases the spin polarization from ϩ80% to ϩ95%. Continued sputtering up to 750 s results in about ϩ10% spin polarization, which can be increased again up to ϩ85% by annealing for 12 h at 150°C in ultrahigh vacuum ͑UHV͒.The CrO 2 (100) films were prepared by a CVD technique proposed by Ishibashi.15 As substrates isostructural TiO 2 (100) ͑rutile; aϭ4.592 Å; cϭ2.959 Å; space group: P4 2 /mnm͒ has been used. CrO 3 is decomposed at a temperature of 260°C within a two-zone tube furnace. A wellcontrolled oxygen flow ͑0.5 l/min͒ transports the decomposed precursor material (CrO 3 ) and its related oxide phases (Cr 2 O 5 , Cr 3 O 8 ) into the deposition zone where the substrate is placed. The substrate temperature is adjusted to 390°C enabling the epitaxial growth of CrO 2 . Immediately after the growth the CrO 2 (100) films were i...

We report on the decomposition of platinum acetylacetonate (Pt(acac)(2)) in hydrogen induced by flash heating. The changes in the local Pt structure were followed by high energy resolution off-resonant spectroscopy uniquely performed with sub-second time resolution. The decomposition consists of a two-step reduction process of the Pt(II) species.

The sensitivity of X-ray absorption spectroscopy (XAS) can be increased by using the modulated excitation approach, where the sample is excited with a periodic external stimulation (e.g., changes in temperature, gas/liquid concentration, etc.) and the corresponding spectra are filtered with the frequency of the excitation. The resulting demodulated spectra contain similar spectral features as difference spectra but to much higher k values because of the isolation of the structural change that occurs with the same frequency as that of the excitation. Furthermore, multiple intermediate species that occur with a phase delay can uniquely be differentiated. We present a robust approach for the quantitative analysis of demodulated extended X-ray absorption fine structure (EXAFS) spectra. Simulations show that the proposed fitting approach resolves small changes in the sample's structure with greatly enhanced precision. Experimentally, this is demonstrated by the formation of ruthenium oxide species upon partial oxidation of a Ru metal particle, which cannot be detected in a standard EXAFS experiment.

Highly a-axis-textured CrO 2 films have been deposited on Al 2 O 3 (0001) and on isostructural TiO 2 (100) substrates by a chemical vapour deposition technique. For Al 2 O 3 substrates a columnar growth of CrO 2 (010) on an initial Cr 2 O 3 (0001) layer has been found in transmission electron microscopy as well as in x-ray diffraction investigations. The sixfold in-plane symmetry of a (0001)-oriented Cr 2 O 3 initial layer leads to three equivalent in-plane orientations of the CrO 2 unit cell as confirmed by electron diffraction and scanning electron microscopy. The growth can be understood by a simple model of the in-plane symmetries of the Al 2 O 3 (0001), Cr 2 O 3 (0001), and CrO 2 (010) lattices. The growth on TiO 2 (100) substrates leads to (100)-oriented CrO 2 films of higher crystalline quality than the ones grown on Al 2 O 3 (0001). Transmission electron microscope images show growth of CrO 2 (100) directly on the TiO 2 (100) substrates and no significant Cr 2 O 3 inclusions within the CrO 2 (100) layer. All contributions to the magnetoresistance (MR) due to anisotropic MR, Lorentz MR, spin disorder, and intergrain tunnelling MR have been determined and partly correlated with the crystalline properties of the samples investigated. For films of both types the intrinsic linear contribution to the high-field MR does not depend on the crystalline quality of the films and supports the suggested intrinsic doubleexchange mechanism for CrO 2 .

The electronic structure of high-quality CrO 2 ͑100͒ films was investigated by means of x-ray absorption and resonant photoemission spectroscopy at the 2p-3d excitation threshold. The obtained binding energy of the occupied Cr 3d states is in agreement with the results predicted within the local spin-density approximation using the dynamical mean-field theory ͓L. Craco et al., Phys. Rev. Lett. 90, 237203 ͑2003͔͒. The reported data support a model of CrO 2 as a half-metallic ferromagnet with strong electron-correlation effects.

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We report a combined experimental-computational investigation of the electronic structure of CrO 2 . We have measured the magneto-optical Kerr spectra of CrO 2 at 10 K and 300 K. At 10 K the Kerr signal is significantly enhanced over that obtained at 300 K. We compare the measured Kerr spectra to first-principles theoretical spectra, which we computed using three different approximations to the exchange-correlation functional, i.e., the local spin-density approximation ͑LSDA͒, generalized gradient approximation ͑GGA͒, and LSDAϩU. The experimental low-temperature magneto-optical Kerr spectra are best explained by calculations employing the GGA functional. The addition of an on-site Coulomb correlation U does not lead to reasonable Kerr spectra.