Nature of the conduction-band states in YBa2Cu3O7 as revealed by its yttrium Knight shift.

The small positive /sup 89/Y Knight shift in YBa/sub 2/Cu/sub 3/O/sub 7/ (K/sub Y/approx. =2 x 10/sup -4/) reported by two groups is shown to require that the in-plane conduction band be formed from the ..pi..-bonding, Cu 3d/sub x//sub y/ and O 2p..pi../sub x//sub ,(//sub y//sub )/ orbitals with O 2p..pi..-like antibonding states at the Fermi level. The negative K/sub Y/approx. =-10/sup -4/ reported by another group is consistent with either this ..pi.. conduction band or one formed from the sigma-bonding Cu 3d/sub x//sub <2/-y/sup =/ and O 2psigma/sub x//sub ,(//sub y//sub )/ orbitals provided again that the Fermi-level states are largely oxygenlike.