We demonstrate that the excitonic insulator ground state of ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$ can be electrically controlled by electropositive surface adsorbates. Our studies utilizing angle-resolved photoemission spectroscopy reveal intriguing wave-vector-dependent deformations of the characteristic flattop valence band of this material upon potassium adsorption. The observed band deformation indicates a reduction of the single-particle band gap due to the Stark effect near the surface. The present study provides the foundation for the electrical tuning of the many-body quantum states in excitonic insulators.

• Received 2 February 2019

DOI:https://doi.org/10.1103/PhysRevLett.123.206401