General Principles for the Nonequilibrium Relaxation of Populations in Quantum Materials

We examine the problem of how excited populations of electrons relax after they have been excited by a pump. We include three of the most important relaxation processes: (i) impurity scattering, (ii) Coulomb scattering, and (iii) electron-phonon scattering. The relaxation of an excited population of electrons is one of the most fundamental processes measured in pump-probe experiments, but its interpretation remains under debate. We show how several common assumptions about nonequilibrium relaxation that are pervasive in the field may not hold under quite general conditions. The analysis shows that nonequilibrium relaxation is more complex than previously thought, but it yields to recently developed theoretical methods in nonequilibrium theory. In this work, we show how one can use many-body theory to properly interpret and analyze these complex systems. We focus much of the discussion on implications of these results for experiment