Anomalous Melting Transition of the Charge-Ordered State in Manganites

Melting of the charge-ordered state in $(\mathrm{La},\mathrm{Ca}){\mathrm{MnO}}_3$ and $(\mathrm{Pr},\mathrm{Ca}){\mathrm{MnO}}_3$ is characterized by an anomalously large decrease of the wave vector—as much as $25\%$ through the transition. High resolution lattice images obtained from electron microscopy reveal that the large change occurs as a result of formation of new clusters with charge-ordering modulation wavelength increased by integral lattice parameter. Our findings can be attributed to the discrete nature of the charge ordering on the manganese sites.