${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$ is one of the most promising materials for hosting an excitonic insulator ground state. While a number of experimental observations have been interpreted in this way, the precise nature of the symmetry breaking occurring in ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$, the electronic order parameter, and a realistic microscopic description of the transition mechanism are, however, missing. By a symmetry analysis based on first-principles calculations, we uncover the discrete lattice symmetries which are broken at the transition. We identify a purely electronic order parameter of excitonic nature that breaks these discrete crystal symmetries and contributes to the experimentally observed lattice distortion from an orthorombic to a monoclinic phase. Our results provide a theoretical framework to understand and analyze the excitonic transition in ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$ and settle the fundamental questions about symmetry breaking governing the spontaneous formation of excitonic insulating phases in solid-state materials.

• Received 26 November 2019
• Accepted 23 April 2020

DOI:https://doi.org/10.1103/PhysRevLett.124.197601