Photoemission studies of the valence bands in YBa 2Cu 3O 7-δ

Abstract

Studies of the valence band electronic structure of the high T c superconductor YBa 2Cu 3O 7-δ (YBCO), obtained from photoelectron spectroscopy measurements, are reviewed. We include a brief discussion of angle integrated measurements acquired with X-ray photoemission and with ultraviolet photoemission. However, the major effort is devoted to a discussion of high resolution angle resolved photoemission spectroscopy (ARPES) studies on samples cleaved in high vacuum at cryogenic temperatures. These samples show a clear (metallic) Fermi edge cutoff, and band dispersion that is generally consistent with predictions of band theory. Mostly, electron states lying close to E F (within about 1 eV) are examined. The highly two-dimensional nature of the YBCO structure allows for reasonably straightforward measurement of the Fermi surface; consequently the Fermi surface has been measured in some detail. Evidence, from ARPES measurements, for the existence of a van Hove singularity near the Fermi level, that might play a role in high T c superconductivity, is considered. Systematic studies are presented that show changes in the band structure and Fermi surface as the oxygen stoichiometry is reduced; with oxygen depletion, YBCO changes from a high T c superconductor to a magnetically ordered insulator. ARPES studies of YBCO crystals with Co and Zn atoms substituted on Cu sites are also included. When ARPES measurements are performed at room temperature, the samples show rapid degradation; consequently, room temperature measurements are not extensively reviewed here. Despite extensive studies, some issues still remain largely unresolved or controversial. The ARPES technique is surface sensitive and the cleavage plane has not been clearly identified; consequently concerns associated with measurement of the cleaved surface are addressed. Efforts, generally unsuccessful, to observe a superconducting gap, are discussed.