C. T. Simmons

On the Suppression of Superconductivity in Y1-xPrxBa2Cu3O7-δ

Y1-xPrxBa2Cu3O7-δ compounds exhibit a progressive decrease in superconducting transition temperature with increasing x. Using soft-X-ray absorption spectroscopy at the Pr-MIV,V thresholds, Pr is shown to be trivalent for all x in these compounds contrary to expectations, excluding an obvious mechanism for the suppression of Tc based on the tetravalency of Pr. From a discussion of other possible mechanisms it is suggested that Tc decreases through changes in electronic-band structure upon Pr substitution leading ultimately to a semiconducting behaviour for x = 1.0.

Surface and bulk electronic structure of α-Ce metal and CeIr2

Abstract Resonant 4f photoemission spectra of α-Ce metal and α-like CeIr 2 , taken in the region of the Ce4d and 3d thresholds, reveal strong surface-induced changes of the correlated electronic structure. By decomposing the spectra into surface and bulk components, it is shown that in both α-like systems the surface layers are almost γ-like, while in the bulk the 4f states are much stronger hybridized than previously assumed. The results support a description of the bulk electronic structure of CeIr 2 in terms of narrow 4f bands. Core-hole induced final-state changes in 4f localization are briefly discussed.

Investigations on Superconducting RbTl1.5-Doped Fullerene

Superconductivity with an onset temperature of 27.5 K has been found in samples of a nominal composition (RbTl1.5)3C60. Investigations of the d.c. magnetization, the a.c. susceptibility as well as preliminary results of 13C NMR and STM studies are reported.

On the 3d-configuration of copper in high-Tc superconductors

Abstract The Cu-3d configuration is studied in high-T c superconductors by x-ray absorption (XA) spectroscopy at the Cu L 2,3 thresholds. Signals from 3d 9 and 3d 9 L ground-state configurations are identified, while those from Cu-3d 8 are below detectability.

Tc suppression and rare-earth valency in Y1−xMxBa2Cu3O7−δ (M = Ce, Pr, Tb)

The question of rare-earth (RE) valency in Y 1-x M x Ba 2 Cu 3 O 7−δ compounds (with M = Ce, Pr, Tb) is addressed by x-ray absorption (XA) near-edge structure (XANES) studies at the RE-M 4.5 and -L 2,3 thresholds. While Pr is found to be essentially trivalent for all x, the related Ce systems are characterized by tetravalent Ce; Tb is found to be trivalent for small x, but tetravalent for x = 1. Possible mechanisms for T c suppression are discussed.

Electronic structure of high-Tc superconductors: Effects of oxygen stoichiometry and surface reactions with alkali metals

Abstract The effects of oxygen stoichiometry on valence-band photoemission and 0 ·· ls soft x-ray absorption spectra were studied on MBa 2 Cu 3 O 7−δ (with M = Y, Sm) using synchrotron radiation. With decreasing oxygen content, varied by sample preparation or interface formation, a decrease of 0–2p derived states close to E F is observed. For Rb coverages ⩾ 5A, a rigid shift of all spectral features indicates a metal/semiconductor transition.