Ni Impurity Effect on the Transport Properties and Carrier Concentration of Bi2212

In typical hole-doped high-Tc cuprates, the optimum doping is achieved at the doping level p ∼ 0.16 (= popt). However impurity dopings sometimes alters popt value. In this sense, popt ∼ 0.16 is thought not to be universal. Here, we studied the doping dependence of Tc and some transport characteristics for several Ni impurity doped Bi2Sr2CaCu2O8+δ single crystals. Doping levels were precisely determined by thermoelectric measurements and oxygen contents δ. We found that 1. Ni valence is almost same as Cu, 2. popt shifts toward higher doping, 3. pseudogap opening temperature T* is not affected by Ni impurity. The results are consistent with the model that the shift in popt is due to the doping dependent pair breaking effect. This means that the shift is superficial and the essential universality popt ∼ 0.16 is impurity doped regime.