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

  • Author(s): Yu, G
  • Lee, CH
  • Heeger, AJ
  • Cheong, SW
  • Fisk, Z
  • et al.
Abstract

Transient photoconductivity (σph) experiments have been carried out in single crystals of insulating La2CuO4+δnear the metal-insulator transition (δ≈ 1-2%). The time evolution of the σphchanges dramatically with light intensity (IL). At low IL, σphis characterized by power law time decay, t-α, in the nanosecond time regime, and the exponent, α, decreases significantly with increasing IL. As ILincreases to 5 × 1015photons/cm2, σphreaches ∼ 15 S/cm, and the lifetime of the conducting state is enhanced by more than two orders of magnitude: σphexhibits 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 σphat high ILimplies the formation of metastable metallic droplets after photo-excitation. © 1992.

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