Low-temperature softening of the polaron band in electron-doped

The aÐb plane optical conductivity of three, semiconducting, Nd 2~xCexCuO 4By single crystals with di¤erent values of x and y has been measured between 70 and 18 000 cm~1 and between 300 and 20 K. In all samples, a polaron peak is observed at room temperature at&1000 cm~1. As¹decreases, the peak remains at the same energy in two low-doping samples with di¤erent chemical compositions, while in the most strongly electron-doped crystal it moves to the far-infrared due to a transfer of spectral weight within the polaron band. This unexpected behavior indicates a delocalization of the polaron in this cuprate, which could provide coherent transport at low temperature.