Restoring the surface layer of Bi2Sr2CaCu2O8+? single crystals by current injection

In 2010, we reported a new method to electronically tailor the hole concentration of the high-Tc superconductor Bi2Sr2CaCu2O8+delta (Bi-2212). We discovered that current injection along the c-axis of Bi-2212 provides a convenient tool to reversibly increase the carrier concentration. In this study, we have investigated the current injection effect on the surface junction of Bi-2212 by a series of current injection experiments carried out in samples of slightly overdoped and of extremely underdoped Bi-2212 crystals. The c-axis transport characteristics of bulk and surface intrinsic Josephson junctions are examined in a wide range of doping by current injection. Before injection treatment, typically, our samples demonstrate non-superconducting characteristics across their surface junctions, because standard preparation procedures cause the topmost CuO2 layers to be heavily degraded by oxygen loss. The electronic doping enables to substantially restore the hole concentration of the topmost CuO2 layers. The contact resistance is significantly reduced, and the intrinsic junction at the surface gets back its superconducting tunneling characteristic. We examine systematically the doping results of surface and bulk junction stacks and discuss the enhancement of their superconducting properties.