Photo-excitation of single crystals of La2CuO4+δ near the metal-insulator transition

Transient photoconductivity (σ_ph) experiments have been carried out in single crystals of insulating La₂CuO_4+δ near the metal-insulator transition (δ≈ 1–2%). The time evolution of the σ_ph changes dramatically with light intensity (I_L). At low I_L, σ_ph is characterized by power law time decay, t^?, in the nanosecond time regime, and the exponent, , decreases significantly with increasing I_L. As I_L increases to 5 × 10¹⁵ photons/cm₂, σ_ph reaches 15 S/cm, and the lifetime of the conducting state is enhanced by more than two orders of magnitude: σ_ph 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 σ_ph at high I_L implies the formation of metastable metallic droplets after photo-excitation.