Charge Differentiation of Inequivalent Cu Sites of YBa₂Cu₃O_y (6.0≤y≤6.91) Investigated by NQR and NMR

Abstract

Nuclear quadrupole resonance (NQR) and nuclear magnetic resonance (NMR) techniques have been utilized to characterize the local oxygen coordination of inequivalent Cu sites in YBa₂Cu₃O_y (6.0≤y≤6.91). Essentially, four distinct NQR lines which correspond to 2, 3, 4 oxygen coordinated Cu(l) sites in the Cu-O chains and 5 oxygen coordinated Cu(2) sites in the Cu-O planes have been observed. The zero-field NQR frequencies of those are centered at about 30.1, 24.0, 22.0 and 31.5 MHz for ⁶³Cu, respectively. The electric filed gradient EFG which is mainly due to intra-atomic in origin depends only on the oxygen coordination, hence locally differentiated charged state of Cu atoms, in the Cu(1) sites. The charge differentiation is also found in the Cu(2) sites and the degree of it is found to be increased progressively with decreasing y from 6.91.