Nature of the Conduction-Band States in YBa2Cu3O7 as Revealed by Its Yttrium Knight Shift

Abstract

The small positive ${}_{}{}^{89}{}_{}{}^{}\mathrm{Y}$ Knight shift in Y${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ ($K_Y\simeq 2\times {10}^{-4\mathrm{}}$) reported by two groups is shown to require that the in-plane conduction band be formed from the $\pi$-bonding $\mathrm{Cu} {3d}_{\mathrm{xy}}$ and $\mathrm{O} 2p{\pi }_x,\left(y\right)\mathrm{}$ orbitals with $\mathrm{O}-2p\pi$-like antibonding states at the Fermi level. The negative $K_Y\simeq {10}^{-4\mathrm{}}$ reported by another group is consistent with either this $\pi$ conduction band or one formed from the $\sigma$-bonding $\mathrm{Cu} {3d}_{x^2-y^2}$ and $\mathrm{O} 2p{\sigma }_x,\left(y\right)\mathrm{}$ orbitals provided again that the Fermi-level states are largely oxygenlike.