Observation of nuclear spin-spin structure in zero and low external static magnetic fields

Nuclear quadrupole resonance structure in zero external static magnetic field and coupled transitions in low magnetic fields have been observed for the $I_z=\pm \frac{1}{2}\to I_z=\pm \frac{3}{2}$ transition of praseodymium in LiY${\mathrm{F}}_4$ in an optically detected magnetic resonance experiment. Praseodymium nuclear magnetic moment is significantly enhanced by the second-order magnetic hyperfine interaction resulting in a strong coupling between praseodymium and fluorine spins. Nuclear-nuclear dipolar coupling between a praseodymium nucleus and one of the near-neighbor fluorine spins seem to adequately account for the zero and 100-G magnetic field results. Experimental and calculated values of the dipolar coupling constants are presented.