Temporal relaxation of gapped many-body quantum systems

Typicality of the orthogonal dynamics (TOD) is established as a generic feature of the temporal relaxation processes in isolated many-body quantum systems. The basic idea in the simplest case is that the transient nonequilibrium behavior is mainly governed by the component of the time-evolved system state parallel to the initial state, while the orthogonal component appears as equilibrated right from the beginning. The main emphasis is laid on the largely unexplored and particularly challenging case that one energy level exhibits a much larger population than all the others. Important examples are gapped many-body systems at low energies, for instance, due to a quantum quench. A general analytical prediction is derived and is found to compare very well with various numerically exact results.