Common microscopic origin of the phase transitions in Ta 2 NiS 5 and the excitonic insulator candidate Ta 2 NiSe 5

(submitted), (2021)

Common microscopic origin of the phase transitions in Ta 2 NiS 5 and the excitonic insulator candidate Ta 2 NiSe 5

Lukas Windgätter, Malte Rösner, Giacomo Mazza, Hannes Hübener, Antoine Georges, Andrew J. Millis, Simone Latini, Angel Rubio

The envisioned existence of an excitonic-insulator phase in Ta 2 NiSe 5 has attracted a remarkable interest in this material. The origin of the phase transition in Ta 2 NiSe 5 has been rationalized in terms of crystal symmetries breaking driven by both electronic correlation and lattice distortion. However, the role of structural and electronic effects has yet to be disentangled. Meanwhile its complementary material Ta 2 NiS 5 , which has the chalcogen species exchanged with Sulfur, does not show any experimental evidence of an excitonic insulating phase. Here we present a microscopic investigation of the electronic and phononic effects involved in the structural phase transition in Ta 2 NiSe 5 and Ta 2 NiS 5 by means of extensive first-principles calculations for both the high temperature orthorhombic and low-temperature monoclinic crystal phases. We show that, despite the difference in electronic behaviour, the structural origin of the phase transition is the same in the two crystals. In particular our first-principles results suggest, that the high temperature phase of Ta 2 NiSe 5 is metallic and the structural transition to the low-temperature phase leads to the opening of an electronic gap. By analysing the phononic modes of the two phases we single out the mode responsible for the structural transition and demonstrate how this phonon mode strongly couples to the electronic structure. We demonstrate that, despite the very similar phononic behaviour, in Ta 2 NiS 5 the electronic transition from metal to semiconductor is lacking and the crystal remains a semiconductor in both phases. To disentangle the effect of electronic correlation, we calculate electronic bandstructures with increasing accuracy in the electron-electron interaction and find that the structural transition alone allows for the metal to semiconductor phase transition, ...

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We are grateful to E.Baldini, I.Mazin and Yann Gallais for enlightening discussions throughout the course of this work. This work is supported by the European Research Council (ERC-2015-AdG-694097), Grupos Consolidados (IT1249-19) and the Flatiron Institute, a division of the Simons Foundation. We acknowledge funding by the Deutsche Forschungsgemeinschaft (DFG) under Germany's Excellence Strategy - Cluster of Excellence Advanced Imaging of Matter (AIM) EXC 2056 - 390715994 and by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) { SFB-925 { project 170620586. Support by the Max Planck Institute - New York City Center for Non-Equilibrium Quantum Phenomena is acknowledged.. S. L. acknowledges support from the Alexander von Humboldt foundation. G. M. acknowledge support of support of the Swiss National Science Foundation FNS/SNF through an Ambizione grant.

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