Evidence for Two-Component High-Temperature Superconductivity in the Femtosecond Optical Response of ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\mathit{\delta }}$

Femtosecond time-resolved spectrocopy has been used to investigate electronic excitations contributing to the superconducting gap function $\Delta (\omega ,T)$ in ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$. The optical response is strongly peaked at 1.5 eV, and contains two distinct components: one with a characteristic relaxation time of $\sim 5\mathrm{ps}$, whose amplitude displays a two-fluid-like temperature dependence, and a long-lived component $(>10\mathrm{}\mathrm{ns})$ which is consistent with localized quasiparticle states at the Fermi energy. The latter shows activated behavior below $T_c$ with an activation energy $2{\Delta }_0\approx {3.5kT}_c$.