Excitonic insulator through coherent pulse excitation?

The excitonic insulator was predicted numerous times by theory, but the exp erimental observation is conspicuous by absence. We demonstrate that, even under ideal circumstances, an excitonic insulator cannot be created by mean s of coherent optical excitation. If dephasing is neglected, a non-stationa ry excitonic insulator may arise. If a dephasing is taken into account, a h ypothetical excitonic insulator would disappear within the characteristic d ephasing time. Moreover, we find that dephasing prevents the buildup of the excitonic insulator from the beginning.