J. C. Fuggle

Experimental electronic structure studies of La2−xSrxCuO4

X-ray induced photoelectron spectroscopy, bremsstrahlung-isochromat spectroscopy, and electron energy-loss spectroscopy were used to investigate the electronic structure of the high-temperature superconductor La2−xSrxCuO4. In general, good agreement was obtained with band structure calculations of the tetragonal phase of La2CuO4. Near the Fermi energyEF the systems shows forx=0 a smeared out gap or a region of very low density of states ∼2eV wide. Upon replacement of La by Sr, the gap is filled with defect states, the intensity of which is proportional tox. The experimental data are not consistent with the closing of a Peierls gap alone.

2p X-ray absorption of titanium in minerals

Ti 2p X-ray absorption spectra for a series of minerals have been measured. Crystal field multiplet calculations can explain the spectral shape. The asymmetry of the eg, peak is shown to be related to distortions of the TiIV octahedron. It is found, theoretically as well as experimentally, that the absorption spectra are more sensitive to tetragonal distortions than to trigonal distortions. A number of silicate minerals and metamict minerals containing titanium are measured and TiIIIhas not been observed in any of these minerals. A comparison is made to the 1s X-ray absorption, and the potential of both for the study of minerals is discussed.

Soft X-ray absorption spectroscopy of vanadium oxides

Abstract We present the V2p and O1s X-ray absorption spectra of V 2 O 3 , VO 2 and V 2 O 5 . The V2p spectra show strong multiplet effects caused by large 2p-3d and 3d-3d Coulomb and exchange interactions. The spectra exhibit noticeable chemical shifts and completely different multiplets. The shapes of the multiplet are related to the symmetry and spin of the ground state. The V2p spectrum of V 2 O 3 can be simulated by an atomic-multiplet calculation projected in the appropriate crystal field. The O1s spectra are related to unoccupied Op character mixed in the conduction band. The spectra reflect V3d bands near the edge and V4sp bands at higher energies. The V3d bands are split by crystal field effects. Changes in the relative intensities are related to differences in the metal-ligand hybridization. The extra dispersion of the V3d bands in V 2 O 3 is attributed to larger metal-metal interactions. The O1s spectrum of VO 2 compares favorably to a symmetry-projected band structure calculation. Finally, the O1s spectra of VO 2 taken at room temperature (insulator phase) and at T ≈ 120°C (metallic phase) show clearly the splitting of the V3d | band across the metal-insulator transition.

A bibliography for high energy spectroscopy studies of high-Tc superconductors (1987–1989)

Abstract A bibliography of papers on high energy spectroscopic studies of the new superconductors is given, together with some selected work on related oxides. Tables are given to allow quick tracing of works on particular topics. The results of some of the more important areas of investigation are discussed.

The interaction of N with Ti and the oxidation of TiN studied by soft X-ray absorption spectroscopy

Abstract The N1s XAS spectrum of a reference TiN sample is compared to an N p-projected DOS. Good overall agreement is found except for a small discrepancy at the edge which could be due to core-hole effects. Teh N1s XAS spectra of N 2 chemisorbed on Ti (3000 L) and N + implanted in Ti (250 eV) compare well with TiN; this indicate a close chemical similarity between them. The differences in the spectra are attributed to disorder in the implanted layer and to the two-dimensional character of the chemisorbed layer. The N1s XAS spectra of TiN oxidized at different temperatures indicate that O progressively displaces N to form TiO 2 . The results are consistent with a complete TiN/TiO 2 phase separation and rule out the possible formation of oxynitride. The appearance of a narrow peak in the spectrum of TiN oxidized at 300°C suggests that some of the N atoms displaced during the oxidation process remain relatively unbound within the TiO 2 matrix. At 500°C this peak disappears; this is attributed to N migration towards the surface followed by thermal desorption.

Protection of stainless-steels against corrosion in sulphidizing environments by Ce oxide coatings: X-ray absorption and thermogravimetric studies

In this paper a study is reported concerning ceramic coatings containing cerium oxide, prepared by the sol-gel method, used to protect Incoloy 800H against sulphidation. When the coating is sintered in air at 850°C good protection is obtained. In an X-ray absorption spectroscopic study of the coatings it was observed that the best protective coating contains all cerium as CeIV after pretreatment. After sulphidizing cerium was reduced to CeIII. Possible mechanisms to explain the protective properties are discussed.

The Ba/Si(100)-2 × 1 interface: II. XPS, BIS and synchrotron PS studies of Schottky-barrier formation

Abstract In the preceding paper [Surf. Sci. 260 (1992) 97] we have reported the results of XPS and XAES studies of silicide formation at the Ba/Si(100) interface at elevated temperatures. In this paper we present the results of XPS, BIS, and synchroton PS studies on the same system but at room temperature. We find that Ba grows layer by layer on the Si substrate and that electron energy losses and final state effects are important for the correct interpretation of the data. Observed shifts in the Si 2p core levels are ascribed to a lowering of the Schottky barrier and are explained with the model presented by Monch [Phys. Rev. Lett. 58 (1987) 1260], while measured shifts in the Ba 4d core levels are related to the reduction of the silicon work function. This model requires the presence of metal induced gap states (MIGS) which are indeed observed with BIS. Here we find increasing spectral intensity near the Fermi level upon Ba deposition. This intensity is assigned to Ba 5d states which probably fulfill the role of the MIGS in Monchs model.

Electronic structure of La2−xSrxCuO4 and YBa2Cu3O7−y

Since the discovery of the new superconducting cuprates, numerous models for the mechanism for the high superconducting transition temperatures have been developed.1–3 At present there is no clear evidence for one or another model. Therefore, it is extremely important to elaborate the electronic structure of this new class of materials and to determine the relevant parameters by experiments in order to guide theory in the choice of a model. In particular, there is a strong discussion whether band-structure calculations using the local density approximation (LDA) yield reasonable information on the electronic structure or whether the electron-electron interaction of the 3d electrons on the Cu sites is important for the high-Tc superconductors similar to the oxides, chalcogenides and halides of the late 3d transition metals. Having accepted the strong e-e correlation, a further question is, what is the influence of the correlation effects on the electronic structure and what is the nature of the charge carriers in the new superconductors. This contribution reviews recent high-energy spectroscopic investigations on the electronic structure of the superconducting cuprates.4–6 Recent results on single crystals have been included. X-ray induced photoemission (XPS), Auger-electron spectroscopy (AES), bremsstrahlung-isochromat spectroscopy (BIS) and electron energy-loss spectroscopy (EELS) were used.

Magnetic-Properties of Fe and Tb in Tbxfe1-X Amorphous Films Studied with Soft-X-Ray Circular and Linear Dichroism

Abstract We have used linearly and circularly polarized X-rays to determine the magnetic properties of several Tb x Fe 1− x amorphous films. Absorption measurements on the M 4.5 edges of Tb and the L 2.3 edges of Fe allowed us to obtain information about the size and direction of local magnetic moments. Our results confirm that linear dichorism in rare earth M 4.5 edges can give useful information about both crystal field and magnetic effects.

Electronic structure and bonding properties in TiSi2

Both occupied and unoccupied electronic states of TiSi2 have been studied by a variety of electron and X-ray spectroscopies as well by electronic structure calculations. The data allows us to build up a detailed picture of the site and symmetry selected, partial densities of states. We show that the metald states contribute to silicide chemical bonding, by distortion of the Sip state distribution. We also point out evidence that the Sis states give a significant contribution to the chemical bonding.