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 σ and in-plane π 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 π 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.