Momentum-Resolved Ultrafast Electron Dynamics in Superconducting Bi2Sr2CaCu2O8+δ

The nonequilibrium state of the high-Tc superconductor Bi2Sr2CaCu2O8-delta and its ultrafast dynamics have been investigated by femtosecond time- and angle-resolved photoemission spectroscopy well below the critical temperature. We probe optically excited quasiparticles at different electron momenta along the Fermi surface and detect metastable quasiparticles near the antinode, since their decay toward the nodal region through e-e scattering is blocked by phase space restrictions. The observed lack of momentum dependence in the decay rates is in agreement with relaxation dynamics dominated by Cooper pair recombination in a boson bottleneck limit.