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 lowT their binding energy decreases. This reflects an increase of the polaron radius, which may trigger coherent transport.