Charge Ordering Fluctuation and Optical Pseudogap in La₁₋ₓCaₓMnO₃

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Optical spectroscopy was used to investigate the optical gap (2Δ) due to charge ordering (CO) and related pseudogap developments with x and temperature (T) in La₁₋ₓCaₓMnO₃ (0.48 <= x <= 0.67). Surprisingly, we found 2Δ/kBTCO is as large as 30 for x 0.5, and decreases rapidly with increasing x. Simultaneously, the optical pseudogap, possibly starting from T* far above TCO becomes drastically enhanced near x=0.5, producing non-BCS T-dependence of 2Δ with the large magnitude far above TCO, and systematic increase of T* for x 0.5. These results unequivocally indicate systematically-enhanced CO correlation when x approaches 0.5 even though TCO decreases.