Optical spectroscopy of charge-ordering transition

The change of the electronic structure and lattice dynamics upon the charge-ordering (CO) transition has been investigated for a (Formula presented) crystal by measurements of optical spectra. The CO transition, as characterized by sequential 2:1 ordering of nominal (Formula presented) and (Formula presented) sheets at (Formula presented) activates several additional optical phonon modes due to the charge modulation and opens an optical gap (up to 2Δ∼0.13 eV). The spectral intensity of the activated phonon mode shows a discontinuous increase at (Formula presented) reflecting the first-order nature of the CO transition. By contrast, the optical gap value increases rather continuously with decreasing temperature below (Formula presented) implying the effect of the concomitant antiferromagnetic spin ordering on the gap magnitude and structure.