UC Irvine

Transient photoconductivity (σph) experiments have been carried out in single crystals of insulating La2CuO4+δ near the metal-insulator transition (δ≈ 1-2%). The time evolution of the σph changes dramatically with light intensity (IL). At low IL, σph is characterized by power law time decay, t-α, in the nanosecond time regime, and the exponent, α, decreases significantly with increasing IL. As IL increases to 5 × 1015 photons/cm2, σph reaches ∼ 15 S/cm, and the lifetime of the conducting state is enhanced by more than two orders of magnitude: σph exhibits a delayed peak centered at approximately 30 ns. followed by exponential decay with time constant of ∼ 360 ns. The data reveal a continuous distribution of localized electronic states above the (hole) mobility edge; the M-I transition is achieved when the Fermi level is shifted away from the localized states and across the mobility edge either by chemical dopiing or by photo-excitation. The time of σph at high IL implies the formation of metastable metallic droplets after photo-excitation. © 1992.