A negative photoconductivity is observed due to the twohyphen;dimensional (2D) holes at GaAs/Al0.5Ga0.5As heterojunctions under illumination by a red lighthyphen;emitting diode below sim;6 K. The photoconductivity follows a nonexponential decay after the illumination is turned off. Quantum transport measurements are made to demonstrate explicitly that the photoconductivity is due to a decrease in the density and mobility of the 2D holes, and a model is proposed to explain the phenomenon.
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