Superconductivity induced by current injection into non-superconducting Bi$_2$Sr$_2$CaCu$_2$O$_8$

The carrier-doping induced transition from the antiferromagnetic state to the superconducting phase is still one of the most fascinating properties of high-Tc materials. Usually hole doping is achieved by non-stoichiometry. However, we already have shown that we can change the carrier concentration of Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ single crystals by current injection along the c-axis. This effect is persistent up to annealing temperatures of approximately 270 K. Now, the interesting question is if ``chemical'' doping by oxygen excess is necessary at all. For this purpose we performed current-injection experiments on fully oxygen depleted Bi$_2$Sr$_2$CaCu$_2$O$_8$ which was not superconducting above 4.2 K. In order to eliminate the contact resistance of the highly resistive depleted material, we realized a ``true'' 4-point geometry by fabricating double cross-bar crystal stacks. C-axis resistivity, critical current, and critical temperature were measured by c-axis transport. We have observed that by carrier injection the conductivity can be increased until superconductivity above 4.2 K is reached. Continuing the doping by current injection, optimum-doped and even overdoped states were obtained. Using current injection at higher bias, we were able to reduce the hole concentration again.