Infrared properties of heavy-fermions: evolution from weak to strong hybridizations

In this article, we review the charge excitations of heavy fermion compounds probed by infrared spectroscopy. The article is not meant to be a comprehensive survey of experimental investigations. Rather it focuses on the dependence of charge excitations on the hybridization strength. In this context, the infrared properties of the Ce$_m$M$_n$In$_{3m+2n}$ family are discussed in detail since the hybridization strengths differ dramatically in different members despite their similar lattice structures. Investigations on some mixed valent compounds are also presented aiming to elucidate the generic trend on the evolution. In particular, we address the scaling between hybridization energy gap $\Delta_{dir}$ and the hybridization strength $\tilde{V}$($\propto\sqrt{WT_K}$) in a wide range of heavy fermions compounds which demonstrates that the periodic Anderson model can generally and quantitatively describe the low-energy charge excitations.