Spectroscopic evidence for a charge-density-wave condensate in a charge-ordered manganite: Observation of a collective excitation mode in ${\mathrm{Pr}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3$ by using THz time-domain spectroscopy

THz time-domain spectroscopy was used to directly probe the low-energy (0.5–5 meV) electrodynamics of the charge-ordered manganite ${\mathrm{Pr}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3.$ We revealed the existence of a finite peak structure around 2–3 meV well below the charge gap $\sim 300\hspace{0.5em}\mathrm{}\mathrm{meV}.$ In analogy to the low-energy optical properties of the well-studied low-dimensional materials, we attributed this observed structure to the collective excitation mode arising from the charge-density-wave condensate. This finding provides the importance role of the quasi-one-dimensional nature of the charge and orbital ordering in ${\mathrm{Pr}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3.$