Suppressed superconductivity of the surface conduction layer in ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+x}$ 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 $T_c\simeq 90$ 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 I-$V$ data of the weakened Josephson junction fits well to the tunneling characteristics of a d-wave superconductor${(}^{\prime })$–insulator–$d$-wave superconductor junction. Also, the tunneling resistance in the range $T_c^{\prime }<T<\mathrm{}{T}_c$ 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.