Continuously Doping Bi_2Sr_2CaCu_2O_8+\delta into Electron-Doped Superconductor by CaH_2 Annealing Method

As a typical hole-doped cuprate superconductor, Bi_2Sr_2CaCu_2O_8+\delta(Bi2212) carrier doping is mostly determined by its oxygen content. Traditional doping methods can regulate its doping level within the range of hole doping. Here we report the first application of CaH_2 annealing method in regulating the doping level of Bi2212. By continuously controlling the anneal time, a series of differently doped samples can be obtained. The combined experimental results of x-ray diffraction, scanning transmission electron microscopy, resistance and Hall measurements demonstrate that the CaH_2 induced topochemical reaction can effectively change the oxygen content of Bi2212 within a very wide range, even switching from hole doping to electron doping. We also found evidence of a low-T_\rm c superconducting phase in the electron doping side.