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Synchrotron-radiation glancing-incidence and angle-resolved photoelectron spectroscopy (PES) is used to study the oxides grown in air on a single-crystal Nb(100) surface. Both core-level and valance-band PES are measured for various heat treatments. Glancing-incidence excitation was used to characterize the outer 2-nm Nb2O5 layer without influence from the substrate and to profile through various oxides by varying the incidence angle. Immediately adjacent to this amorphous layer is a layer of NbO2, which contributes to the density of states at 1.2eV below the Fermi level. Thereafter, there exist other oxides that interface with the metal substrate. Annealing at 430K induces a “mild” chemical reaction, which is contained essentially in the oxide layer. Annealing at 550K causes oxygen diffusion into the metal and leaves an oxide layer consisting primarily of Nb2O and a roughened surface, as evidenced by angle-resolved photoemission measurements. Oxide composition alteration caused by annealing at these moderated temperatures is believed to be interface mediated.

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The local structure of an amorphous a-Gd x Si 1Ϫx (xϭ0.04,0.07,0.12,0.18) magnetic semiconductor is determined from combined x-ray absorption fine structure ͑XAFS͒ measurements at the Gd L 3 and Si K absorption edges. XAFS data indicate that the structure is amorphous, with Gd atoms surrounded predominantly by Si atoms, consistent with a random substitution of Gd in the Si network. Gd substitution induces a large local distortion resulting in a Gd-Si distance of 2.98͑3͒ Å compared to a Si-Si distance of 2.39͑1͒ Å. The lack of x dependence in the Gd local environment indicates that the distortions around the Gd do not interact with one another, even at xϭ0.18. In addition, a small but systematic increase in the mean squared disorder of the Si-Si interatomic distance is observed with x. These results suggest that the amorphous Si network is very effective in relaxing the local strain through random displacements of Si atoms within a few Å of the perturbing Gd atoms.