Anisotropic ultrafast dynamics of quasiparticles on CuO₂ planes of Y_0.7 Ca_0.3 Ba₂ Cu₃ O_7-δ

The orientation-resolved femtosecond spectroscopy, combined with the well-textured (110)- and (001)-Y _0.7 Ca _0.3 Ba ₂ Cu ₃ O _7-δ thin films, serves as an effective probe to quasiparticle relaxation dynamics on the ab planes and along the diagonal orientations. The significant divergences in the temperature-dependent relaxation time (τ) associated with the opening of superconducting gap were observed along the nodal directions and on the CuO ₂ planes which are dominated by the a axis and b axis in the over-doped region. Moreover, the divergence in the temperature-dependent τ along the nodal direction disappears around optimal doped region. This implies that the superconducting gap evolves from the dominant s-wave symmetry in over-doped region into the dominant d-wave symmetry in optimal doped region.