  We have performed nuclear quadrupole resonance (NQR) experiments on $^{47}$Ti
nuclei in Dy$_2$Ti$_2$O$_7$ in the temperature range 70 -- 300 K in order to
investigate the dynamics of $4f$ electrons with strong Ising anisotropy. A
significant change of the NQR frequency with temperature was attributed to the
variation of the quadrupole moment of Dy $4f$ electrons. A quantitative account
was given by the mean field analysis of the quadrupole-quadrupole (Q-Q)
interaction in the presence of the crystalline-electric-field splitting. The
magnitude and the temperature dependence of the nuclear spin-lattice relaxation
rate was analyzed, including both the spin-spin and the Q-Q interactions. The
results indicate that these two types of interaction contribute almost equally
to the fluctuation of Dy magnetic moments.

Strongly Correlated Electrons