Symmetry of holes in high-T_c superconductors

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 YBa₂Cu₃O₇ and Bi₂Sr₂CaCu₂O₈, 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 YBa₂Cu₃O₇, in both cases, holes must be in the planes and in the ribbons. Out-of-plane π holes can be excluded for YBa₂Cu₃O₇ and Bi₂Sr₂CaCu₂O₈. 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.