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/sub c/ superconductors. In particular, for YBa/sub 2/Cu/sub 3/O/sub 7/ and Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub 8/, the O 1s and Cu 2p absorption edges have been measured for momentum transfer parallel and perpendicular to the a,b plane. Both sigma and in-plane ..pi.. holes on O, formed upon doping, are compatible with our experimental results. For YBa/sub 2/Cu/sub 3/O/sub 7/, in both cases, holes must be in the planes and in the ribbons. Out-of-plane ..pi.. holes can be excluded for YBa/sub 2/Cu/sub 3/O/sub 7/ and Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub 8/. In the undoped and doped compounds, the unoccupied states on the Cu sites have predominantly in-plane 3d/sub x//sub <2/-y/sup =/ symmetry. There is an admixture of about 10% of lower-lying states probably with 3d/sub 3//sub z//sub <2/-r/sup =/ symmetry.
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