Bi_2 Sr_2 CaCu_2 O_{8+delta} Bicrystal c-Axis Twist Josephson Junctions: A New Phase-Sensitive Test of Order Parameter Symmetry

TL;DR

This study investigates the phase-sensitive properties of Bi-2212 bicrystal c-axis twist Josephson junctions to determine the symmetry of the superconducting order parameter, providing evidence for an s-wave dominant pairing and incoherent tunneling.

Contribution

It presents experimental results on c-axis twist junctions that strongly support an s-wave order parameter and incoherent tunneling in Bi-2212 superconductors, challenging previous d-wave assumptions.

Findings

Jc^J/Jc^S ratio is unity and independent of twist angle

Results support incoherent c-axis tunneling

Evidence suggests s-wave pairing symmetry

Abstract

Li {\it et al.} [Phys. Rev. Lett. {\bf 83}, 4160 (1999)] prepared atomically clean Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ (BSCCO) Josephson junctions between identical single crystal cleaves stacked and twisted an angle $\phi_0$ about the c axis. For each bicrystal, the ratio $J_c^J/J_c^S$ of the c-axis twist junction critical current density to that across either single crystal part is unity, independent of $\phi_0$ and the ratio $A^J/A^S$ of junction areas. From extensive theoretical studies involving a variety of tunneling and superconducting order parameter (OP) forms, we conclude that the results provide strong evidence for incoherent c-axis tunneling and that the dominant OP is s-wave for $T\le T_c$. Recently, Takano {\it et al.} [Phys. Rev. B {\bf 65}, 140315(R) (2002)] obtained results from BSCCO whisker twist junctions which also rule out a pure d-wave OP, but which are surprisingly suggestive of coherent c-axis tunneling form small Fermi surface hot spots.