Charge Ordering Fluctuation and Optical Pseudogap in La${}_{1-x}$Ca${}_x$MnO${}_3$

Abstract

Optical spectroscopy was used to investigate the optical gap (2$\Delta$) due to charge ordering (CO) and related pseudogap developments with x and temperature (T) in La${}_{1-x}$Ca${}_x$MnO${}_3$ (0.48 <= x <= 0.67). Surprisingly, we found 2$\Delta$/k_{B}T_{CO} 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 T_{CO} becomes drastically enhanced near x=0.5, producing non-BCS T-dependence of 2$\Delta$ with the large magnitude far above T_{CO}, and systematic increase of T^* for x~0.5. These results unequivocally indicate systematically-enhanced CO correlation when x approaches 0.5 even though T_{CO} decreases.

5 pages, 4 figures embedded, submitted to Phys. Rev. Lett

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