Probing the metal-nonmetal transition in thin metal overlayers using resonant photoemission

We have studied one and two monolayers of barium on Ni(111) and of mercury on Cu(100). Using resonant photoemission, we have found core excited electrons become delocalized with increasing barium coverage. Similarly, upon formation of the mercury bilayer (as determined by low-energy electron diffraction and by atom-beam scattering), there is a substantial increase in the screening of the photohole. A transition of the electronic structure akin to a metal-nonmetal (metal-insulator) transition is apparent in these final-state effects. The band structure for Hg is similar to the band structure expected for a free-standing film with a free-electron sd band. The delocalization of the core excited electrons resembles the exciton unbinding that occurs at the metal-nonmetal Mott transition

Electronic band structure study of (1,2,3) high-temperature superconductors

A general discussion of the electronic band structure properties of YBa2Cu3O7-x and other (1,2,3) high-temperature superconductors is presented. Both the occupied and unoccupied density of states are discussed. The key experimental results and their relation to proposed superconducting mechanisms are examined. The discovery of high-temperature superconductors has evoked several proposed mechanisms. Among the most important properties that bear on the proposed mechanism is the material electronic structure. As a result, several experiments probing the occupied and unoccupied density of states have been performed on YBa2Cu3O7-x and other (1,2,3) high-temperature superconductors. The present article provides a critical overview of the results as they bear on the proposed mechanism. After outlining the pertinent theoretical issues, each experimental technique and the results obtained will be discusse

Substrate-induced magnetic ordering of rare-earth overlayers

We have studied the magnetic ordering of terbium overlayers on Cu(100) and Ni(111), using angle-resolved photoemission. The 5p3/2 to 5p1/2 shallow-core-level branching ratios in different photoemission geometries provide a measure of the magnetic ordering in rare-earth-metal overlayers as a result of final-state effects in photoemission. We find that ferromagnetic substrates order paramagnetic terbium overlayers. This induced magnetic ordering is not a crystal-field effect and can be modeled by Ginzburg-Landau theory. Application of Ginzburg-Landau theory to our results suggests that the correlation length of paramagnetic terbium κ-1 is between 2.5 and 3.5 Å. Reversible increases in the extent of magnetic ordering at temperatures below the Tb Curie temperature are observed for terbium overlayers on both Cu(100) and Ni(111)