A direct probe of superconducting order parameter symmetry in Bi₂Sr₂CaCu₂O_8+δ using bicrystal c-axis twist Josephson junctions

Atomically clean Bi₂Sr₂CaCu₂O_8+δ (B12212) bicrystal grain boundary junctions were prepared using identical single crystal cleaves stacked and twisted an arbitrary angle φ_o about the c-axis. We show that for each bicrystal, regardless of φ_o, the ratio of the measured c-axis Josephson critical current density across the twist junction to that across either single crystal part (J_c^J/J_c^S) is close to unity for T ≤ T_c. This result provides evidence that an isotropic s-wave superconducting order parameter is present near T_c.