Observation of Collective Excitation Mode in Pr$_{\text{0.7}}$Ca$_{\text{0.3}}$MnO$_{\text{3}}$

The observation of the broken symmetry state manifesting itself as the collective excitation mode in the complex optical spectrum, is an important and difficult task in condensed matter physics. We used THz time-domain spectroscopy to directly elaborate low-energy (0.5--5 meV) complex optical conductivity spectra of the charge ordered manganite Pr$_{0.7}$Ca$_{0.3}$MnO$_3$ exhibiting the colossal magnetoresistance and revealed the existence of a finite peak structure around 2--3 meV well below the charge gap $2\Delta\sim300$ meV. This observed structure is attributed to the collective excitation mode arising from the charge-density-wave condensate.