One- and two-particle phenomena in the electronic lifetimes in metals: Quasiparticles and transient excitons

The experimental and theoretical investigation of the lifetime of excited electrons is of great importance for a variety of different fields in condensed matter physics. In this review two distinct classes of processes are discussed, which determine the lifetime of excited electrons in crystalline systems. One class is single-particle processes, which in many cases is able to describe the decay of excited electrons. For systems with weakly correlated electrons the state-of-the-art method is the solution of the Dyson equation using the GW approximation for the electronic self-energy. If applicable this approach leads to very good results. However, many of the experimental studies about the lifetime of excited electrons have been done using time-resolved two-photon photoemission spectroscopy utilizing ultrashort laser pulses. This technique, applied to materials with localized d electrons, can lead to the creation of bound, excitonic-like states in metals on a very short time scale, which are beyond the physics described in the single-particle approach. In this review the experimental evidence for both mechanisms is given and the theoretical tools to describe them are discussed. Furthermore, theoretical results are presented and compared to experimentally available data