Optical spectroscopy of the charge-ordering transition in La1.67Sr0.33NiO4

Optical spectra of a single crystal of La1.67Sr0.33NiO4, which undergoes a charge-ordering transition at T(CO)similar to 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 (2 Delta) with temperature is clearly observed below T-CO, with an anomalously large ratio of 2 Delta(T=0)/k(B)T(CO) (similar to 13). Conspicuous spectral change with temperature is also observed above T-CO over the energy region up to 2 eV, which suggests persistent fluctuation of the charge ordering or formation of small polarons above T-CO.