Symmetry of holes in high-Tcsuperconductors

Abstract

High-energy electron energy-loss spectroscopy in transmission has been used to obtain information on the symmetry of unoccupied electronic states in the new high-${T}_{c}$ superconductors. In particular, for ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ and ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8}$, the O 1s and Cu 2p absorption edges have been measured for momentum transfer parallel and perpendicular to the a,b plane. Both \ensuremath{\sigma} and in-plane \ensuremath{\pi} holes on O, formed upon doping, are compatible with our experimental results. For ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$, in both cases, holes must be in the planes and in the ribbons. Out-of-plane \ensuremath{\pi} holes can be excluded for ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ and ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8}$. In the undoped and doped compounds, the unoccupied states on the Cu sites have symmetry. There is an admixture of about 10% of lower-lying states probably with symmetry.