[en] Li et al. found that the critical current density J_c^J across atomically clean c-axis twist junctions of Bi₂Sr₂CaCu₂O_8+δ is the same as that of the constituent single crystal, J_c^S, independent of the twist angle φ₀, even at and below Tc. They investigated theoretically if a d_x²-y²-wave order parameter might twist by mixing in d_xy--wave components, but found that such mixing cannot possibly explain the data near to Tc. Combined with group theoretical arguments, they then conclude that the order parameter contains at least a substantial s-wave component, but does not contain any purported d_x²_-y²-wave component, except possibly below a second, unobserved phase transition. By studying tunneling models, they further conclude that the intrinsic c-axis Josephson tunneling in Bi₂Sr₂CaCu₂O_8+δ is likely to be mostly incoherent. They also propose a c-axis junction version of a tricrystal experiment, which does not rely upon expensive substrates