Signatures of ultrafast reversal of excitonic order in Ta₂NiSe₅

In the presence of electron-phonon coupling, an excitonic insulator can harbor two degenerate ground states described by an Ising-type order parameter. Starting from a microscopic Hamiltonian, we derive the equations of motion for the electronic and structural order parameters in the excitonic insulator Ta₂NiSe₅. We show that the excitonic order can be controllably reversed with appropriate laser excitation sequences. Using a combination of theory and coherent phonon spectroscopy measurements, we report evidence of such reversal in Ta₂NiSe₅. Our work expands the field of ultrafast order parameter control beyond conventional spin and charge ordered materials.