Polaron softening with doping and temperature in high-T-c cuprates

The infrared reflectivity of a Nd2-xCexCuO4 single crystal with x = 0.10 co nfirms that in the semiconducting phase of this compound the polaron band a t 1000 cm(-1) increasingly softens when x approaches the insulator-to-metal transition. The latter may occur as the polaron system collapses at a crit ical concentration. A fraction of small polarons probably bound to the impu rities survives in the metallic phase. (C) 1999 Elsevier Science B.V. All r ights reserved.