LOW-TEMPERATURE SOFTENING OF THE POLARON BAND IN ELECTRON-DOPED CUPRATES

The a-b plane optical conductivity of three, semiconducting, Nd2-xCexC uO4+/-y single crystals with different values of x and y has been meas ured between 70 and 18 000 cm(-1) and between 300 and 20 K. In all sam ples, a polaron peak is observed at room temperature at similar to 100 0 cm(-1). As T decreases, the peak remains at the same energy in two l ow-doping samples with different chemical compositions, while in the m ost strongly electron-doped crystal it moves to the far-infrared due t o a transfer of spectral weight within the polaron band. This unexpect ed behavior indicates a delocalization of the polaron in this cuprate, which could provide coherent transport at low temperature. (C) 1998 E lsevier Science B.V. All rights reserved.