Transport measurements on individual CuO2 double layers

Oxygen loss at the surface of Bi2Sr2CaCu2O8+x single crystals provides CuO2 layers with different critical temperatures. Particularly for overdoped crystals this allows to achieve a T-c maximum near the surface. We performed transport measurements of this oxygen depleted layer. Samples were prepared by evaporating four Pb or Ag electrodes on top of freshly cleaved Bi2Sr2CaCu2O8+x single crystals. Using overdoped crystals with a bulk T-c of 55 to 60 K, the outermost layer was found to be underdoped with T-c down to 30 K while the adjacent one was near optimal doping with T-c between 80 and 90 K. The T-c of the third layer was close to the bulk value. For temperatures above the bulk T-c often only one CuO2 double layer was found to be superconducting. We discuss current transport measurements with current flow along the CuO2 layers.