The spontaneous symmetry breaking in Ta2NiSe5 is structural in nature

Nature Materials (submitted), (2020)

The spontaneous symmetry breaking in Ta2NiSe5 is structural in nature

Edoardo Baldini, Alfred Zong, Dongsung Choi, Changmin Lee, Marios H. Michael, Lukas Windgaetter, Igor I. Mazin, Simone Latini, Doron Azoury, Baiqing Lv, Anshul Kogar, Yao Wang, Yangfan Lu, Tomohiro Takayama, Hidenori Takagi, Andrew J. Millis, Angel Rubio, Eugene Demler, Nuh Gedik

The excitonic insulator is an electronically-driven phase of matter that emerges upon the spontaneous formation and Bose condensation of excitons. Detecting this exotic order in candidate materials is a subject of paramount importance, as the size of the excitonic gap in the band structure establishes the potential of this collective state for superfluid energy transport. However, the identification of this phase in real solids is hindered by the coexistence of a structural order parameter with the same symmetry as the excitonic order. Only a few materials are currently believed to host a dominant excitonic phase, Ta2NiSe5 being the most promising. Here, we test this scenario by using an ultrashort laser pulse to quench the broken-symmetry phase of this transition metal chalcogenide. Tracking the dynamics of the material's electronic and crystal structure after light excitation reveals surprising spectroscopic fingerprints that are only compatible with a primary order parameter of phononic nature. We rationalize our findings through state-of-the-art calculations, confirming that the structural order accounts for most of the electronic gap opening. Not only do our results uncover the long-sought mechanism driving the phase transition of Ta2NiSe5, but they also conclusively rule out any substantial excitonic character in this instability.

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We thank F. Boschini, S. Kaiser, D. Chowdhury, A. Georges, G. Mazza, and A. Subedi for insightful discussions. We are grateful to F. Mahmood, E.J. Sie, T. Rohwer, B. Freelon for early instrumentation work of the trARPES and UED setups at MIT. The work at MIT was supported by DARPA DSO under DRINQS program grant number D18AC00014 (trARPES data taking and analysis), Army Research Office Grant No. W911NF-15- 1-0128 (instrumentation for the trARPES setup), and the US Department of Energy BES DMSE (UED measurements). The work at Harvard was supported by HarvardMIT CUA, AFOSR-MURI: Photonic Quantum Matter (award FA95501610323), and DARPA DRINQS program (award D18AC00014). E.B acknowledges additional support from the Swiss National Science Foundation under fellowships P2ELP2-172290 and P400P2-183842. Y.W. acknowledges the Postdoctoral Fellowship in Quantum Science of the Harvard-MPQ Center for Quantum Optics. The theory work was supported by the European Research Council (ERC-2015-AdG694097), the Cluster of Excellence “Advanced Imaging of Matter” (AIM), Grupos Consolidados (IT1249-19) and SFB925. The Flatiron Institute is a division of the Simons Foundation. 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. I.I.M. acknowledges support from the Office of Naval Research (ONR) through the grant #N00014-20-1-2345. A.J.M. acknowledges e support from DOE BES Pro-QM EFRC (de-sc0019443). Part of the calculations used resources of the National Energy Research Scientific Computing Center (NERSC), a US Department of Energy Office of Science User Facility operated under Contract No. DE-AC02-05CH11231. A.Z. and A.K. thank Y. Zhang for assisting us at the MRSEC Shared Experimental Facilities at MIT, supported by the NSF under award number DMR-14-19807. A.Z. and A.K. also thank C. Marks for the assistance in preparing UED samples at Center for Nanoscale Systems, a member of the National Nanotechnology Coordinated Infrastructure Network (NNCI), which is supported by the National Science Foundation under NSF award no. 1541959. CNS is part of Harvard University.

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