We present a combined study from angle-resolved photoemission and density-functional-theory calculations of the temperature-dependent electronic structure in the excitonic insulator candidate ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$. Our experimental 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 shearlike structural distortion which reduces the crystal symmetry from orthorhombic to monoclinic. Our results thus point to the key role of the lattice distortion in enabling the phase transition of ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$.

• Received 5 December 2019
• Accepted 31 January 2020
• Corrected 27 August 2020

DOI:https://doi.org/10.1103/PhysRevResearch.2.013236

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Published by the American Physical Society