Signatures of Ultrafast Reversal of Excitonic Order in Ta 2 NiSe 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 ${\mathrm{Ta}}_{2}{\mathrm{NiSe}}_{5}$ 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 ${\mathrm{Ta}}_{2}{\mathrm{NiSe}}_{5}$ based on the anomalous behavior of its coherently excited order-parameter-coupled phonons. Our Letter expands the field of ultrafast order parameter control beyond spin and charge ordered materials.