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

TL;DR

This study investigates the suppressed superconductivity at the surface layer of Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$ crystals using c-axis tunneling, revealing weakened Josephson coupling and unchanged order parameter symmetry.

Contribution

It provides experimental evidence of suppressed surface superconductivity and characterizes the tunneling behavior, highlighting the surface effects on intrinsic Josephson junctions.

Findings

Weakened Josephson coupling at the surface below a critical temperature.

Tunneling characteristics consistent with d-wave superconductor junctions.

Surface suppression does not alter the order parameter symmetry.

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$$\simeq$90 K, each containing a few intrinsic Josephson junctions. Below a critical temperature $T_c'$ ($\ll$ $T_c$), we have observed a weakened Josephson coupling between the CuO$_2$ superconducting double layer at the crystal surface and the adjacent one located deeper inside a stack. The quasiparticle branch in the $IV$ data of the weakened Josephson junction (WJJ) fits well to the tunneling characteristics of a d-wave superconductor($'$)/insulator/d-wave superconductor (D$'$ID) junction. Also, the tunneling resistance in the range $T_c'$$<$$T$$<$$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.