enabled by mirror-symmetry breaking*

We present a fresh look at the experimental electronic structure of the excitonic insulator candidate Ta₂NiSe₅. Our angle-resolved photoemission spectroscopy measurements unambiguously establish the normal state as a semimetal with a significant band overlap of > 100 meV. Our temperature-dependent measurements indicate how these low-energy states hybridize when cooling through the well-known 327 K phase transition in this system. From our calculations and polarization-dependent photoemission measurements, we demonstrate the importance of a loss of mirror symmetry in enabling the band hybridization, driven by a shear-like structural distortion which reduces the crystal symmetry from orthorhombic to monoclinic. Our supporting ab-initio calculations in the monoclinic show the opening of an energy gap comparable with the experimental results, pointing to the key role of the lattice distortion in enabling the phase transition of Ta₂NiSe₅. We will show the links between the experimental electronic structure and other recent experimental results on this material. We will also briefly present a comparison with the latest experimental results in the related material TiSe₂.

Matthew D. Watson, Igor Marković, Edgar Abarca Morales, Patrick Le Fèvre, Michael Merz, Amir A. Haghighirad, and Philip D. C. King, Phys. Rev. Research 2, 013236 (2020)