Ultrafast Electron Relaxation in SuperconductingBi2Sr2CaCu2O8+δby Time-Resolved Photoelectron Spectroscopy
Time-resolved photoelectron spectroscopy is employed to study the dynamics of photoexcited electrons in optimally doped ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ (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 $\ensuremath{\lambda}{\ensuremath{\Omega}}_{0}^{2}=360\ifmmode\pm\else\textpm\fi{}30\text{ }\text{ }{\mathrm{meV}}^{2}$. For the typical phonon energy of copper-oxygen bonds (${\ensuremath{\Omega}}_{0}\ensuremath{\simeq}40--70\text{ }\text{ }\mathrm{meV}$), this results in an average electron-phonon coupling $\ensuremath{\lambda}<0.25$.