Observation of Charge-Density-Wave Excitations in Manganites

In the optical conductivity of four different manganites with commensurate charge order (CO), strong peaks appear in the meV range below the ordering temperature $T_{\mathrm{CO}}$. They are similar to those reported for one-dimensional charge density waves (CDW) and are assigned to pinned phasons. The peaks and their overtones allow one to obtain, for ${\mathrm{La}}_{1-n/8}{\mathrm{Ca}}_{n/8}{\mathrm{MnO}}_3$ with $n=5$, 6, the electron-phonon coupling, the effective mass of the CO system, and its contribution to the dielectric constant. These results support a description of the CO in La-Ca manganites in terms of moderately weak coupling and of the CDW theory.