IMPLICATIONS OF THE NUCLEAR-QUADRUPOLE FREQUENCY IN HIGH-TC CUPRATES

The Cu-63 nuclear quadrupole resonance (NQR) frequency of a variety of high-T(c) cuprates is analyzed by a cluster model to examine the origin of the electronic field gradient and its implications on the electronic ground state of the CuO2 plane. It is shown that a primary contribution from the Cu 3d(x2-y2) hole is cancelled largely by the contribution from the Cu 4p(x, y) electrons, and this cancellation explains both the large material dependence of the frequency in the insulating phase and an extremely small frequency observed in the electron-doped cuprates. Measured NQR frequencies thus provide rich information on the ground-state charge distribution in the CuO2 plane.