Suppressed superconductivity of the surface conduction layer in Bi 2 Sr 2 CaCu 2 O 8 + x single crystals probed by c -axis tunneling measurements

We fabricated small-size stacks on the surface of ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}Z{\mathrm{O}}_{8+x}$ single crystals with the bulk transition temperature ${T}_{c}\ensuremath{\simeq}90$ K, each containing a few intrinsic Josephson junctions. Below a critical temperature ${T}_{c}^{\ensuremath{'}}(\ensuremath{\ll}{T}_{c}),$ 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 I-$V$ data of the weakened Josephson junction fits well to the tunneling characteristics of a d-wave superconductor${(}^{\ensuremath{'}})$--insulator--$d$-wave superconductor junction. Also, the tunneling resistance in the range ${T}_{c}^{\ensuremath{'}}lTl{T}_{c}$ agrees well with the tunneling in a normal-metal--insulator--$d$-wave superconductor junction. In spite of the suppressed superconductivity at the surface layer the symmetry of the order parameter appears to remain unaffected.