Electric-field gradient calculations for YBa2Cu3O7-x

The electric-field gradients (EFGs) at the two Cu positions of YBa 2 Cu 3 O 7 and YBa 2 Cu 3 O 6 are calculated on the basis of full-potential linearised augmented-plane-wave calculations by an ab initio method. Good agreement with experimental EFGs is found for the Cu(1) position in both compounds, while the Cu(2) position in the superconductor the symmetry of the EFG agrees with experiment: the magnitude of the theoretical EFG is, however, only half the experimental value. A small transfer of 0.07 electrons from d(x 2 -y 2 ) to d(z 2 ) symmetry would be sufficient to achieve agreement with experiment. Thus the underlying local-density approximation is not perfect, but remains a good starting point for describing ground-state properties.