Charge-inhomogeneity doping relations in YBa2Cu3Oy detected by angle-dependent nuclear quadrupole resonance

The origin of charge inhomogeneity in YBa2Cu3Oy is investigated using an experimental method designed to determine the nuclear quadrupole resonance (NQR) asymmetry parameter eta for very wide NQR lines at different positions on the line. The method is based on the measurement of the echo intensity as a function of the angle between the radio frequency field H-1 and the principal axis of the electric field gradient. Static charge inhomogeneity deduced from eta > 0 is found in this compound, but only in conjunction with oxygen deficiency. This limits considerably the possible forms of charge inhomogeneity in bulk YBa2Cu3Oy.