Hyperfine interactions in copper sites of dielectric and superconducting copper metal oxides

It is shown using Mössbauer spectroscopy on $^{61}$Cu($^{61}$Ni) isotopes that dielectric metal oxides of divalent copper exhibit combined quadrupole and Zeeman interaction of $^{61}$Ni nuclei with local fields in copper sites, whereas superconducting metal oxides exhibit only quadrupole interaction of $^{61}$Ni nuclei. The valence contributions to the electric field gradient tensor on $^{61}$Ni$^{2+}$ and $^{63}$Cu$^{2+}$ nuclei are found to be identical for all metal oxides.