T. J. Wagener

Single‐crystal YBa2Cu3O7−x and Bi2Ca1+xSr2−xCu2O8+y surfaces and Ag adatom‐induced modification

Comparison of x-ray photoemission results for single crystal and polycrystalline YBa/sub 2/Cu/sub 3/O/sub 7-//sub x/ allows us to identify oxygen vacancies in the Cu-O chains and determine their effects on the Cu, Ba, and Y core level line shapes. The deposition of adatoms of Ag onto single crystals of YBa/sub 2/Cu/sub 3/O/sub 7-//sub x/ and Bi/sub 2/Ca/sub 1+//sub x/Sr/sub 2-//sub x/Cu/sub 2/O/sub 8+//sub y/ leads to changes in the Cu 2p/sub 3//sub ///sub 2/ emission indicative of slight Cu/sup 2 +/ to Cu/sup 1 +/ surface reduction, with greater modification for the 1-2-3s than the 2-1-2-2s. O 1s core level results show Ag-induced broadening because of these surface effects. The Ba 3d and Y 3d structures showed minimal changes for the 1-2-3 surface. Ag deposition onto the 2-1-2-2 samples also leads to Bi dissociation and subsequent surface segregation.Comparison of x‐ray photoemission results for single crystal and polycrystalline YBa2Cu3O7−x allows us to identify oxygen vacancies in the Cu‐O chains and determine their effects on the Cu, Ba, and Y core level line shapes. The deposition of adatoms of Ag onto single crystals of YBa2Cu3O7−x and Bi2Ca1+xSr2−xCu2O8+y leads to changes in the Cu 2p3/2 emission indicative of slight Cu2+ to Cu1+ surface reduction, with greater modification for the 1‐2‐3’s than the 2‐1‐2‐2’s. O 1s core level results show Ag‐induced broadening because of these surface effects. The Ba 3d and Y 3d structures showed minimal changes for the 1‐2‐3 surface. Ag deposition onto the 2‐1‐2‐2 samples also leads to Bi dissociation and subsequent surface segregation.

Spectroscopic evidence for passivation of the La1.85Sr0.15CuO4 surface with gold

High‐resolution x‐ray photoemission and inverse photoemission have been used to study the microscopic formation of Au overlayers on the high Tc superconductor La1.85Sr0.15CuO4. Our results indicate that Au does not induce disruption or surface reaction, unlike the reactive transition metals. The electronic states of the superconductor are attenuated as the Au overlayer covers the surface. We conclude that Au is a promising material for metallization and for passivation.

Reaction and intermixing at metal-superconductor interfaces: Fe/YBa2Cu3O6•9

Inverse photoemission studies of the interface formed when Fe is deposited onto the high‐temperature superconductor YBa2Cu3O6.9 show that Fe leaches oxygen from the near surface region to nucleate an insulating form of Fe‐O. Ultimately, a metallic Fe layer grows but it has surface segregated Ba, even after 250 A of coverage. These results demonstrate that reactive metal overlayers drastically alter the chemical and physical properties of the superconductor.

High-energy auger and medium-energy backscattered electron diffraction as a probe of ultra-thin epitaxial overlayers, sandwiches and superlattices

Abstract High-energy Auger and medium-energy backscattered electron diffraction are shown to be useful structural probes for ultra-thin epitaxial films. Coherent scattering of such electrons in the energy regime of 500 to 1000 eV leads to intensity modulations of the order of 50% in an angle-resolved measurement. These modulations are well accounted for by a straightforward kinematical scattering formalism in which trial geometries are employed and varied. Optimal agreement with experiment consistently results from a single geometry, leading to a unique solution to the structural problem. Atomic coordinates and elastic strain associated with lattice mismatch at the interface can be routinely determined with sub-Angstrom precision. Moreover, first layer morphology and structural rearrangement associated with intermixing can be monitored. We present results from the Cu Ni (001) , Co Ni (001) , Fe Cu (001) , Cu/Fe/Cu(001) and Fe GaAs (001) systems which illustrate these conclusions.

Photoemission, inverse photoemission, and ab initio SCF investigations of the electronic structure of polyimide

Abstract Inverse photoemission was combined with direct photoemission for a unique investigation of clean films of poly(N,N′-bis(phenoxyphenyl)pyromellitimide). Experimental results for the valence and conduction states extending from 22 eV below the Fermi level to 10 eV above the Fermi level are presented. The occupied and empty states of PMDA-ODA have also been investigated with ab initio self-consistent field molecular orbital calculations, and comparison with experimental results to identify the origin of spectral features shows excellent agreement.

Photoemission and inverse photoemission studies of La adatom interactions with YBa2Cu3O6.9

The surface reactions that occur when adatoms of La are deposited onto the high‐temperature superconductor YBa2Cu3O6.9 have been studied using high‐resolution synchrotron radiation photoelectron spectroscopy and inverse photoelectron spectroscopy. These adatoms lead to the depletion of oxygen from the substrate as La‐O forms at low coverage. The process of oxygen withdrawal quickly becomes kinetically limited as the thickening overlayer impedes mass transfer. At the same time, Cu atoms released from the disrupted, no‐longer‐ superconducting substrate outdiffuse into, and segregate to the top of, the growing La overlayer. The complex reacted interface, therefore, exhibits phases which are likely to be superconducting, insulating, and metallic.

Surface and interface properties of high temperature superconductors

The energy distribution of the occupied and unoccupied electronic states of the high temperature superconductors La1.85Sr0.15CuO4, YBa2Cu3O7−x, Bi2Ca1+xSr2−xCu2O8+y and related compounds have been investigated using x‐ray and inverse photoemission spectroscopy. Results from polycrystalline and single crystal samples show very similar valence band characteristics, and comparison with theory shows the importance of correlation effects. Core level results reveal a nominally 2+ valence state for copper and inequivalent chemical environments for oxygen. The empty state spectra show structure that can be identified with La 5d and 4f levels for La1.85Sr0.15CuO4, Ba 5d and 4f and Y 4d levels for YBa2CuO3O7−x, and Bi 6p, Sr 4d and Ca 3d levels for Bi2Ca1+xSr2−xCu2O8+y. Inverse photoemission studies also show that the excitation of O 2s core levels near threshold results in resonant behavior of the unoccupied O levels just above EF. Interface studies which include overlayers of metals (Au, Ag, Ti, Fe, Cu, Pd, La, A...

Interface formation: High‐temperature superconductors with noble metals, reactive transition metals, and semiconductors

X‐ray photoemission and inverse photoemission have been used to study interface formation between La1.85Sr0.15CuO4 or YBa2Cu3O6.9 and overlayers that include noble metals (Au, Cu), transition metals (Ti, Fe), and semiconductors (Ge). While Au passivates the surface, the reactive transition metals and Cu deplete oxygen from the substrate and form heterogeneous metal‐oxide overlayers over a scale of a few tens of Angstroms. Reaction is observed for Ge, but to a lesser degree than for the reactive metals, Changes in the substrate core level and valence band spectra indicate regions rich in adatom‐induced reactions and the consequences in terms of suppression of superconductivity are discussed.

High‐temperature superconductors: Occupied and unoccupied electronic states

The energy distributions of the occupied and unoccupied electronic states of the high temperature superconductors La1.85Sr0.15CuO4 and YBa2Cu3O6.9 have been identified using x‐ray photoemission, high resolution synchrotron radiation photoemission, inverse photoemission, high resolution synchrotron radiation photoemission, inverse photemission and bremsstrahlung isochromat spectroscopy. These results show low state density from ∼0.5 eV below to ∼3.5 eV above the Fermi energy, EF, well‐defined Cu‐O occupied levels extending to ∼7 eV, the occupied core levels, and the empty Ba, Y, and La states above EF. Copper is shown to be in a nominally 2+ valence state. Comparisons are made with band structure calculations.

Spectroscopic examinations of surface stability of high‐temperature superconductors

The surface stabilities of La1.85Sr0.15CuO4 and YBa2Cu3O6.9 have been studied for samples exposed to ultrahigh vacuum, oxygen, ∼1500 eV electron bombardment, and zero order synchrotron radiation. Oxygen chemisorption saturates by 10 L and induces energy states ∼1.6 eV above EF. High energy electron bombardment eliminates this structure for La1.85Sr0.15CuO4 but not for YBa2Cu3O6.9. The effects of monochromatic photon irradiation for uv or x‐ray beams are negligible, but surface modifications are observed when the sample is exposed to the zero‐order white beam generated by the Aladdin synchrotron radiation source. The surface modifications are related to BaCO3 residuals, probably in grain boundaries of the polycrystalline samples.