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Condensed Matter; Strongly Correlated Electrons
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 FC=0.2 mJ cmF−2, the direct band gap is transiently reduced, while it is enhanced above FC. 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.
