Optical Investigation on Collective Dynamics of Charge-Orbital Density Wave in Layered Manganites

We have investigated the broad band optical spectra on the layered manganites $R_{1-x}$Sr$_{1+x}$MnO$_4$ (R=Nd and La) to reveal the collective charge/orbital density wave dynamics by means of the terahertz time domain spectroscopy [1]. The collective charge/orbital density wave excitation is observed around 9 meV in the charge/orbital stripe phase, when the nominal $e_g$-electron filling (1-$x$) of Mn-ion is less than around 1/3. By contrast, such a collective mode almost vanishes at $x$=1/2, which is explained in terms of the enhanced Jahn-Teller interaction cooperative with the electron correlation effect. \\[4pt] [1] J. Fujioka {\it et al.}, Phys. Rev. B. {\bf 82}, 140409(R) (2010).