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.
Highlights
In 1he Cu-oxide materials ~a~ch exhibh high temperature superconductivity (HTSC), the antiferromagnetic (AF) transition, the metal-insulator (MI} transition and the sulccrconduc~ing .(SC) transition are closely correlated with the densily of charge carriers on CuO2 layers {l ]
As It. increases, the initial o~h increases toward the minimum metal!ie conductivity. ~.... and the exponem of the power law decay decreases signifieamly
The magnitude o f a ~ w~s estimated by assuming that the photo-induced c~TM rent is carried in a layer with thickness equal to d'~c optical absorption deoth, which ~s about 400 .& | 13 ], The data show that o~v~follows a power law decay, t - ° with the exponent ~ decree.sing in magnitude as IL increases
Summary
In 1he Cu-oxide materials ~a~ch exhibh high temperature superconductivity (HTSC), the antiferromagnetic (AF) transition, the metal-insulator (MI} transition and the sulccrconduc~ing .(SC) transition are closely correlated with the densily of charge carriers on CuO2 layers {l ]. The magnitude o f a ~ w~s estimated by assuming that the photo-induced c~TM rent is carried in a layer with thickness equal to d'~c optical absorption deoth, which ~s about 400 .& (as inferred from measurements of the optic~J constunts) | 13 ], The data show that o~v~follows a power law decay, t - ° with the exponent ~ decree.sing in magnitude as IL increases.
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