  We theoretically investigate the unexpected occurrence of an extra emission
peak that has been experimentally observed in off-resonant studies of cavity
QED systems. Our results within the Markovian master equation approach
successfully explain why the central peak arises, and how it reveals that the
system is suffering a dynamical phase transition induced by the phonon-mediated
coupling. Our findings are in perfect agreement with previous reported
experimental results and for the first time the fundamental physics behind this
quantum phenomenon is understood.
