Site Assignment for Cu NQR Lines in YBa2Cu3O7-δ Superconductor

A clear evidence is presented on the site assignment for two distinct 63 Cu NQR lines observed at around 22 MHz and 31 MHz in YBa 2 Cu 3 O 7-δ superconductor ( T c =91 K). Using the highly oriented powder sample under the applied magnetic field in the superconducting state where the c -axis being perpendicular to the field, the 63 Cu NMR spectra have carefully been examined. From analysis of the spectra, the directions of the principal axes of the electric field gradient (EFG) tensors at the two 63 Cu sites have been determined. Comparing these experimental results with the calculated EFG tensors at the crystallographically inequivalent two Cu sites [the chain Cu(1) and the planar Cu(2) sites] in YBa 2 Cu 3 O 7 , the site associated with the lower and the higher NQR frequencies are uniquely assigned to the Cu(1) and the Cu(2) sites, respectively.