Suppressed Superconductivity of the Surface Conduction Layer in Bi${}_2$Sr${}_2$CaCu${}_2$O${}_{8+x}$ Single Crystals Probed by {\it c}-Axis Tunneling Measurements

Abstract

We fabricated small-size stacks on the surface of Bi${}_2$Sr${}_2$CaCu${}_2$O${}_{8+x}$ (BSCCO-2212) single crystals with the bulk transition temperature $T_c$$\simeq$90 K, each containing a few intrinsic Josephson junctions. Below a critical temperature $T_c^{\prime }$ ($\ll$ $T_c$), we have observed a weakened Josephson coupling between the CuO${}_2$ superconducting double layer at the crystal surface and the adjacent one located deeper inside a stack. The quasiparticle branch in the $IV$ data of the weakened Josephson junction (WJJ) fits well to the tunneling characteristics of a d-wave superconductor(${}^{\prime }$)/insulator/d-wave superconductor (D${}^{\prime }$ID) junction. Also, the tunneling resistance in the range $T_c^{\prime }$$<$$T$$<$$T_c$ 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.

13 pages, 6 figures