Direct Observation of Small-Polaron Ordering in Manganites

Transmission-electron-microscopy measurements characterizing the structural features of charge-ordering (CO) phases in $\mathrm{La}\left(\mathrm{Ca}\right){\mathrm{MnO}}_3$ and $\mathrm{Pr}\left(\mathrm{Ca}\right){\mathrm{MnO}}_3$ are presented. A rich variety of structural phenomena resulting from CO transition have been observed at low temperatures. The most interesting of all is the presence of a new structural modulation on the ${\mathbf{b}}^{*}-{\mathbf{c}}^{*}$ plane. This modulation shows some structural properties in sharp contrast with those of the known $d_{z^2}\left({\mathrm{Mn}}^{3+}\right)$-orbital-ordering state. We herein interpret this new structural modulation in terms of the small-polaron ordering.