Experimental Limitation In Extending The Exciton Series In Cu2o Towards Higher Principal Quantum Numbers

So far, the observation of the yellow P exciton series in cuprous oxide has been limited to principal quantum numbers up to n(max) = 25 at a crystal temperature of T = 1.35 K. Here, we address the origin of this limitation and whether an extension to higher quantum numbers is possible. To that end, absorption experiments are performed to study the variation of n(max) with the spot position on a particular sample and from sample to sample. In addition the temperature is varied. By reducing T to below 1 K, we can extend n(max) to 28, not limited by the thermal energy exceeding the exciton binding energy. The data rather suggest that the ultimate limit is provided by residual charged impurities, despite their low density in the order of less than 10(9) cm(-2), which exert an electric field on the highly excited excitons causing their ionization.