Item

Abstract

Symmetry-broken states are characterized by an order parameter, which usually appears if a material is cooled below a critical temperature. These fragile states are typically destroyed by strong optical pulses on an ultrafast timescale. A nontrivial challenge is therefore the enhancement of an order parameter by optical excitations, as this implies a strengthening of long-range order in a perturbed system. Here, we investigate the non-equilibrium dynamics of the electronic structure of the layered semiconductor Ta₂NiSe₅ using time- and angle-resolved photoelectron spectroscopy. We show that below the critical excitation density of F_C=0.2 mJ cmF⁻², the direct band gap is transiently reduced, while it is enhanced above F_C. An analysis based on Hartree-Fock calculations reveals that this intriguing effect can be explained by the exotic low-temperature ordered state of Ta₂NiSe₅, which hosts an exciton condensate whose order parameter is connected to the gap size. These results demonstrate the ability to manipulate condensates of bound electron-hole pairs with light, and due to the similarity to BCS theory, this approach might also be applicable to the case of superconductors.