Spatial symmetry of the superconducting gap of YBa₂Cu₃O_7−δ obtained from femtosecond spectroscopy

The polarized femtosecond spectroscopies obtained from well characterized (100) and (110) YBa₂Cu₃O_7−δ thin films are reported. This bulk-sensitive spectroscopy, combined with the well-textured samples, serves as an effective probe to quasiparticle relaxation dynamics in different crystalline orientations. The significant anisotropy in both the magnitude of the photoinduced transient reflectivity change and the characteristic relaxation time indicates that the nature of the relaxation channel is intrinsically different in various axes and planes. By an orientation-dependent analysis, d-wave symmetry of the bulk-superconducting gap in cuprate superconductors emerges naturally.