Hyperfine field and electric field gradient at various cation sites in RBa2Cu2NbO8 and RSr2Cu2GaO7

We have investigated antiferromagnetic nuclear resonance (AFNMR) for Cu-site as well as nuclear magnetic (NMR) and quadrupolar (NQR) resonances for La, Nb, Ga-sites in antiferromagnetic insulators RBa2Cu2NbO8 (R = La, Pr, Nd) and RSr2Cu2GaO7 (R = La, Pr, Nd, Y). Hyperfine magnetic fields at the Cu-site are very close to values published for RBa2Cu3O6 analogs. Quadrupolar frequencies for Cu in RSr2Cu2GaO7 are also close to those for RBa2Cu3O6 but in RBa2Cu2NbO8 these values are about 50% higher. Axially symmetric electric field gradient at the La and the Nb-sites in RBa2Cu2NbO8 and asymmetric one at the La and Ga-sites in RSr2Cu2GaO7 are in accord with their structures. Split NQR spectrum for Ga in NdSr2Cu2GaO7 evidences the different Cu-AF spin stacking sequence in this material. Possible cation disorder in RSr2Cu2GaO7 is discussed. (author)