Bi2Sr2CaCu2O8+δ bicrystal c-axis twist Josephson junctions: a new phase-sensitive test of order parameter symmetry

Li et al. (Phys. Rev. Lett. 83 (1999) 4160) prepared atomically clean Bi2Sr2CaCu2O8+δ (BSCCO) Josephson junctions between identical single crystal cleaves stacked and twisted an angle phi0 about the c-axis. For each bicrystal, the ratio JcJ/JcS of the c-axis twist junction critical current density to that across either single crystal part is unity, independent of phi0 and the ratio AJ/AS 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≤Tc. Recently, Takano et al. (Phys. Rev. B 65 (2002) 140513(R)) 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 from small Fermi surface hot spots