Indirect superhyperfine interaction in HoVO4(Gd)

Exceptionally large linewidths are observed in the EPR spectra of Gd impurities in the singlet ground-state system HoV${\mathrm{O}}_4$. A new indirect superhyperfine interaction between the ${\mathrm{Gd}}^{3+}$ electrons and the ${\mathrm{Ho}}^{3+}$ nuclei through a highly polarizable ${\mathrm{Ho}}^{3+}$ electronic state is proposed to account for this behavior. The calculated linewidths are found to be in good agreement with the observations.