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Bi2Sr2CaCu2O8+x Single Crystals Probed by c-Axis Tunneling Measurements

We fabricated small-size stacks on the surface of Bi2Sr2CaCu2O8+x (BSCCO2212) single crystals with the bulk transition temperature Tc≃90 K, each containing a few intrinsic Josephson junctions. Below a critical temperature T ′ c (≪ Tc), we have observed a weakened Josephson coupling between the CuO2 superconducting double layer at the crystal surface and the adjacent one located deeper inside a stack. The quasiparticle branch in the IV data of the weakened Josephson junction (WJJ) fits well to the tunneling characteristics of a d-wave superconductor()/insulator/d-wave superconductor (DID) junction. Also, the tunneling resistance in the range T ′ c<T<Tc agrees well with the tunneling in a normal metal/insulator/d-wave superconductor (NID) junction. In spite of the suppressed superconductivity at the surface layer the symmetry of the order parameter appears to remain unaffected.