Abstract
The photoconductivity, σ
ph(ω), and the optical conductivity, σ(ω), of lightly-doped single crystals of Nd
2CuO
4−δ are compared over the photon energy range
0.6<
h
̵
ω<3 eV
. We find that σ
ph(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, σ
ph(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 σ
ph(ω), which shows a weak and relatively narrow peak at 0.7 eV at relevant temperatures. The discrepancies between σ
ph(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 CuO
2 layers are localized.