PHOTOCONDUCTIVITY AND OPTICAL CONDUCTIVITY IN LIGHTLY DOPED ND2CUO4-DELTA

The photoconductivity, sigma(ph)(omega), and the optical conductivity, sigma(omega), of lightly-doped single crystals of Nd2CuO4-delta are compared over the photon energy range O.6 < homegaBAR < 3 eV. We find that sigma(omega) and sigma(omega) exhibit a common edge at the charge transfer energy gap, 1.2 eV, with no indication of a significant exciton binding energy. At higher photon energies, sigma(ph)(omega) follows sigma(omega). At energies below the charge transfer transition, a broad spectral feature is observed in sigma(omega), centered at 0.6 approximately 0.7 eV. This feature is suppressed in sigma(ph)(omega), which shows a weak and relatively narrow peak at 0.7 eV at relevant temperatures. The discrepancies between sigma(ph)(omega) and sigma(omega) in the infrared spectral region provide evidence that the states in the CT band tails and the broad, doping-induced gap states in the CuO2 layers are localized.