Ultrafast electron relaxation in superconducting Bi2Sr2CaCu2O8+δ by time-resolved photoelectron spectroscopy

Time-resolved photoelectron spectroscopy is employed to study the dynamics of photoexcited electrons in optimally doped Bi2Sr2CaCu2O8+delta (Bi-2212). Hot electrons thermalize in less than 50 fs and dissipate their energy on two distinct time scales (110 fs and 2 ps). These are attributed to the generation and subsequent decay of nonequilibrium phonons, respectively. We conclude that 20% of the total lattice modes dominate the coupling strength and estimate the second momentum of the Eliashberg coupling function lambda Omega(2)(0)=360 +/- 30 meV(2). For the typical phonon energy of copper-oxygen bonds (Omega(0)similar or equal to 40-70 meV), this results in an average electron-phonon coupling lambda < 0.25.