Time-Dependence of the Kondo Screening Cloud Following a Quench*

We use the time-dependent numerical renormalization group to study time evolution following quenches in variants of the Kondo model. Quantities of interest include the expectation value of the impurity spin in broken-symmetry states, the overlap between the wave functions before and after the quench, and the quantum entanglement between the region around the impurity and the rest of the system. For the conventional Kondo model of a spin-half impurity in a metallic host, we focus on the effects of sudden switching on or off of the impurity-host exchange coupling. In the pseudogap Kondo model, we investigate quenches that take the system through a quantum critical point between a Kondo phase and a local-moment phase in which the impurity moment is asymptotically decoupled from the band at low temperatures.