Site Assignment for Cu NQR Lines in YBa2Cu3O7-δSuperconductor

Abstract

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.