Charge ordering fluctuation and optical pseudogap in La1-xCaxMnO3

Optical spectroscopy was used to investigate the optical gap (2Delta) due to charge ordering (CO) and related pseudogap developments with x and temperature (T) in La1-xCaxMnO3 (0.48 less than or equal to x less than or equal to 0.67 ). Surprisingly, we found 2Delta/k(B)T(CO) is as large as 30 for x approximate to 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 2Delta with the large magnitude far above T-CO, and systematic increase of T-{*} for x approximate to 0.5. These results unequivocally indicate systematically enhanced CO correlation when x approaches 0.5 even though T-CO decreases.