  We investigate the quasiparticle dynamics in the prototype heavy fermion
CeCoIn$_5$ using ultrafast optical pump-probe spectroscopy. Our results
indicate that this material system undergoes hybridization fluctuations before
full establishment of the heavy electron coherence, as the temperature
decreases from $\sim$120 K ($T^\dagger$) to $\sim$55 K ($T^*$ ). We reveal that
the observed anomalous phonon softening and damping reduction below $T^*$ are
directly associated with opening of an indirect hybridization gap. We also
discover a distinct collective mode with an energy of $\sim$8 meV, which may be
the experimental evidence of the predicted unconventional density wave. Our
observations provide critical informations for understanding the hybridization
dynamics in heavy fermion materials.

Strongly Correlated Electrons;Materials Science