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 ${\mathrm{La}}_{1-x}{\mathrm{Ca}}_x{\mathrm{MnO}}_3$ ( $0.48\le x\le 0.67$). Surprisingly, we found $2\Delta {/k}_{\mathrm{B}}{T}_{\mathrm{CO}}$ is as large as 30 for $x\approx 0.5$, and decreases rapidly with increasing $x$. Simultaneously, the optical pseudogap, possibly starting from $T^{*}$ far above $T_{\mathrm{CO}}$ becomes drastically enhanced near $x=0.5\mathrm{}$, producing non-BCS $T$-dependence of $2\Delta$ with the large magnitude far above $T_{\mathrm{CO}}$, and systematic increase of $T^{*}$ for $x\approx 0.5$. These results unequivocally indicate systematically enhanced CO correlation when $x$ approaches 0.5 even though $T_{\mathrm{CO}}$ decreases.

• Received 20 April 2001

DOI:https://doi.org/10.1103/PhysRevLett.88.167204