Bi₂Sr₂CaCu₂O_8+δ Bicrystalc -axis twist josephson junctions: A new phase-sensitive test of order parameter symmetry

We prepared atomically clean Bi₂Sr₂CaCu₂O_8+δJosephson junctions between identical single crystal cleaves stacked and twisted an angle φ₀ about the c axis. For each bicrystal, the ratio J^J_c/J^S_c of the c axis twist junction critical current density to that across either single crystal part is unity, independent of φ₀ and the ratio A^J/A^S of junction areas. Our results provide strong evidence for incoherent c-axis tunneling and that the dominant superconducting order parameter contains an isotropic s-wave component for T ≤T_c, but not the purported d_x²−y²-wave component.