Nature of Symmetry Breaking at the Excitonic Insulator Transition:Ta2 NiSe5

Physical Review Letters 124, 19, 197601 (2020)

Nature of Symmetry Breaking at the Excitonic Insulator Transition:Ta2 NiSe5

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

Ta 2 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 Ta 2 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 also leads to the experimentally observed lattice distortion into a monoclinic phase. Our results provide a theoretical framework to understand and analyze the excitonic transition in Ta 2 NiSe 5 and settle the fundamental questions about symmetry breaking governing the spontaneous formation of excitonic insulating phases in solid-state materials.

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Discussions with Jennifer Cano, Denis Goleˇz, Tatsuya Kaneko, and Jernej Mravlje are gratefully acknowledged. We thank Merzuk Kaltak for sharing his (c)RPA implementation [36] with us. This work was supported by (AG, GM) the European Research Council (ERC- 319286-QMAC) and (AM) the US Department of Energy under grant DE-SC 0019443. SL, LW, HH and AR were supported by the European Research Council (ERC-2015-AdG694097), the Cluster of Excellence AIM,Grupos Consolidados (IT1249-19) and SFB925. SL acknowledges support from the Alexander von Humboldt foundation. The Flatiron Institute is a division of the Simons Foundation.

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