^63,65Cu nuclear quadrupole resonance study of Ni-doped YBa₂Cu₃O₇

We have prepared pure YBCO (YB₂Cu₃O_7-δ) and Ni-doped YBCO (YB_2-xLa_xCu_3-xNi_xO_7-δ, x = 0.1) samples and performed ^63,65Cu nuclear quadrupole resonance (NQR) measurements. La substitution introduced for charge compensation, as well as Ni doping, severely distorts the electric field gradient at the copper sites inducing extra ^63,65Cu NQR peaks other than the main peaks, for both the plane and the chain sites. ^63,65Cu NQR linewidths for the plane site increased significantly whereas those for the chain site remained similar. Furthermore, both the spin-lattice and the spin-spin relaxation times for the plane copper decreased whereas those for the chain copper increased. The Ni substitutional effects on linewidths and relaxation times confirm that Ni carrying a local moment substitutes for the plane copper.