Photoconductivity and Optical Conductivity in Lightly Doped Nd2CuO4-d

Abstract

The photoconductivity, σph(ω), and the optical conductivity, σ(ω), of lightly-doped single crystals of Nd2CuO4−δ are compared over the photon energy range . We find that σph(ω) and σ(ω) 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, σph(ω) follows σ(ω). At energies below the charge transfer transition, a broad spectral feature is observed in σ(ω), centered at 0.6 0.7 eV. This feature is suppressed in σph(ω), which shows a weak and relatively narrow peak at 0.7 eV at relevant temperatures. The discrepancies between σph(ω) and σ(ω) 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.