Observation of an Impurity Band in Cuprates with Low Carrier Density

The electrodynamics of small numbers of carriers added to the cuprate oxides have been studied intensively to understand the origins of superconductivity at high temperatures. We have observed an absorption band in insulating Ba sub 2 YCu sub 3 O sub (6+x) and La sub 2 CuO sub 4:Pb that is similar to that studied previously in Si:P near and below the metal-insulator transition. Calculations by Bhatt and Rabe indicate an acceptor binding energy in the cuprates that is consistent with the characteristic energy of our observed band. The Bohr radii and the Mott criterion allow an analysis of the critical density of the metal-insulator transition.