Implications of the Nuclear Quadrupole Frequency in High-TcCuprates

The 63 Cu 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 CuO 2 plane. It is shown that a primary contribution from the Cu 3 d ( x 2 - y 2 ) hole is cancelled largely by the contribution from the Cu 4 p ( 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 CuO 2 plane.