Ultrafast electron dynamics in a metallic quantum well nanofilm with spin splitting

Using time- and angle-resolved two-photon photoemission spectroscopy, we investigate the energy-and momentum-dependent ultrafast electron dynamics in the Rashba spin-split quantum-well nanofilm Bi/Cu(111). We find an expected increase of electron lifetimes towards the band bottom due to a competition of intra- and interband scattering processes. In addition, we find an unexpected peculiar decrease of the lifetimes around the intersection of the split bands. We compare the experimental results with calculated lifetimes due to electron-electron scattering in a model system of a 2D electron gas including a Rashba interaction term and an effective statically screened Coulomb interaction. Although the Rashba model reproduces the increase of lifetimes towards the band bottom well, there is no indication of the experimentally observed decrease around the intersection point in this simple model system. To investigate spin-orbit coupling effects, beyond those contained in a pure Rashba model, we introduce a phenomenological k-dependent spin mixing that leads to a "spin hot spot." It is shown that such a mixing would strongly increase the electron-electron scattering rate around the band intersection and thus improves the agreement with experiment.