We observed a significant persistent change in the transport properties [current–voltage characteristics I(V) and resistance versus temperature R(T) curves] of bicrystal Bi2Sr2CaCu2O8+y grain boundary tunnel junctions by illumination with visible light. After illumination, the I(V) curves show a decrease of the energy gap while the R(T) curves show an increase of the critical temperature of the junction. We have also observed a characteristic temperature T*≃260 K above which the photoinduced conductivity is suddenly removed. These experiments can be understood as an increase of the doping by light in the oxygen depleted Bi2Sr2CaCu2O8+y grain boundary which is thought to be underdoped.
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