Abstract
Transient photoconductivity (σ
ph) experiments have been carried out in single crystals of insulating La
2CuO
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
−
α
, in the nanosecond time regime, and the exponent, α, decreases significantly with increasing
I
L. As
I
L increases to 5 × 10
15 photons/cm
2, 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.