Discovery of a topological exciton insulator with tunable momentum order

• URL: http://arxiv.org/abs/2312.15862v2
• Date: Thu, 30 Jan 2025 16:57:56 GMT
• Title: Discovery of a topological exciton insulator with tunable momentum order
• Authors: Md Shafayat Hossain, Tyler A. Cochran, Yu-Xiao Jiang, Songbo Zhang, Huangyu Wu, Xiaoxiong Liu, Xiquan Zheng, Byunghoon Kim, Guangming Cheng, Qi Zhang, Maksim Litskevich, Junyi Zhang, Zi-Jia Cheng, Jinjin Liu, Jia-Xin Yin, Xian P. Yang, Jonathan Denlinger, Massimo Tallarida, Ji Dai, Elio Vescovo, Anil Rajapitamahuni, Hu Miao, Nan Yao, Anna Keselman, Yingying Peng, Yugui Yao, Zhiwei Wang, Luis Balicas, Titus Neupert, M. Zahid Hasan,
• Abstract summary: Correlated topological materials often maintain a delicate balance among physical symmetries. It is rare to find cases where symmetry breaking induces a non-trivial topological phase. We present the discovery of such a phase in Ta2Pd3Te5, where Coulomb interactions form excitons, which condense below 100 K.
• Score: 7.4675813849473665
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• Abstract: Correlated topological materials often maintain a delicate balance among physical symmetries: many topological orders are symmetry protected, while most correlated phenomena arise from spontaneous symmetry breaking. It is rare to find cases where symmetry breaking induces a non-trivial topological phase. Here, we present the discovery of such a phase in Ta2Pd3Te5, where Coulomb interactions form excitons, which condense below 100 K, opening a topological gap and creating a topological excitonic insulator. Our spectroscopy reveals the full spectral bulk gap stemming from exciton condensation. This excitonic insulator state spontaneously breaks mirror symmetries but involves a very weak structural coupling, as indicated by photoemission spectroscopy, thermodynamic measurements, and a detailed structural analysis. Notably, scanning tunneling microscopy uncovers gapless boundary modes in the bulk insulating phase. Their magnetic field response, together with theoretical modeling, suggests a topological origin. These observations establish Ta2Pd3Te5 as the first confirmed topological excitonic insulator in a three-dimensional crystal. This allows to access the associated physics through bulk-sensitive techniques. Furthermore, we uncover another surprising aspect of the topological excitonic insulator, a secondary excitonic instability near 5 K that breaks the translational symmetry. The wavevector of this state shows an unprecedented magnetic field tunability. Thus, we unveil a unique sequence of topological exciton condensations in a bulk crystal, offering new opportunities to study critical behavior and excitations.

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