Signatures of Ultrafast Reversal of Excitonic Order in Ta2NiSe 5
In the presence of electron-phonon coupling, an excitonic insulator harbors two degenerate ground states described by an Ising-type order parameter. Starting from a microscopic Hamiltonian, we derive the equations of motion for the Ising order parameter in the phonon coupled excitonic insulator Ta 2 NiSe 5 and show that it can be controllably reversed on ultrashort timescales using appropriate laser pulse sequences. Using a combination of theory and time-resolved optical reflectivity measurements, we report evidence of such order parameter reversal in Ta 2 NiSe 5 based on the anomalous behavior of its coherently excited order-parameter-coupled phonons. Finally, our Letter expands the field of ultrafast order parameter control beyond spin and charge ordered materials.