Self-doping caused by oxygen displacements in heavy-ion-irradiated Bi₂Sr₂CaCu₂O_8+x crystals

Detailed irradiation and annealing experiments on Bi₂Sr₂CaCu₂O_8+x single'crystals of widely different doping levels confirm the mechanism of self-doping of oxygen by heavy-ion irradiation in bulk samples. The self-doping occurs in two steps: the initial expulsion of up to ten percent of the oxygen ions from the amorphous region, which can be partially suppressed by performing the irradiation at low temperature, and the subsequent slow diffusion of oxygen along the ab planes into the matrix. The diffusion process well describes the ion-dose dependence of the critical temperature and the aging of irradiated samples.