Determination of the energy gap for charged excitations in insulatingLa2CuO4

Abstract
Measurements are reported of the photoconductivity (PO) in reduced, undoped, single-crystal La2 CuO4. The spectral dependence of the PC shows a threshold at 2.0±0.1 eV, corresponding to the minimum energy for the creation of charged excitations. Transient measurements indicate that the PC is surprisingly persistent: the average recombination time is longer than 10 s at room teperature.