Small and Large Polarons in Nickelates, Manganites, and Cuprates

By comparing the optical conductivities of La1.67Sr0.33NiO4 (LSNO), Sr1.5La0.5MnO4 (SLMO), Nd2CuO4-y (NCO), and Nd1.96Ce0.04CuO4 (NCCO), we have identified a peculiar behavior of polarons in this cuprate family. Whereas in LSNO and SLMO small polarons localize into ordered structures below a transition temperature, in these cuprates the polarons appear to be large, and at low T their binding energy decreases. This reflects an increase of the polaron radius, which may trigger coherent transport.