Polaron absorption and the insulator to metal transition in Na2-xCexCuO4-ycuprates

The optical conductivity of Nd2-xCexCuO4-y single crystals with x ranging f rom 0 to 0.122 has been measured at different temperatures. The polaron ban d observed at 1100 cm(-1) in Nd2CuO4-y shifts to lower frequencies when the Ce concentration x approaches 0.125, namely, the critical value for the in sulator to metal transition. This unexpected behavior indicates a progressi ve delocalization of polarons in the Cu-O plane for increasing x and may ex plain the anomalous Drude behavior reported for the carriers in the metalli c phase of superconducting cuprates.