Sharp resonant multiplet in femto-second optical pair-breaking spectroscopy of YBa2Cu3O7-delta

Femto-second optical pair breaking spectroscopy is performed on optimally doped YBa2Cu3O7-delta near 1.5eV. Dramatic photon energy dependence with a sharp triplet fine structure is seen for the first time. The peak separations are attributable to the reported phonon and magnetic excitations in the system. The power dependence is non-linear with differing exponents for the three contributions. These results imply the presence of insulating antiferromagnetic domains in the superconducting state of YBCO over sub-picosecond time scale, and thereby favor an electronic phase separation picture.