Optical spectroscopy of the charge-ordering transition

Optical spectra of a single crystal of (Formula presented)(Formula presented)(Formula presented), which undergoes a charge-ordering transition at (Formula presented)∼240 K, have been investigated for wide ranges of photon energy (0.008-3 eV) and temperature (10-480 K). The opening of a charge gap as well as change of the gap magnitude (2Δ) with temperature is clearly observed below (Formula presented), with an anomalously large ratio of 2Δ(T=0)/(Formula presented)(Formula presented) (∼13). Conspicuous spectral change with temperature is also observed above (Formula presented) over the energy region up to 2 eV, which suggests persistent fluctuation of the charge ordering or formation of small polarons above (Formula presented).