Evidence for an Excitonic Insulator State in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi>Ta</mml:mi></mml:mrow><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mrow><mml:mi>Pd</mml:mi></mml:mrow><mml:mn>3</mml:mn></mml:msub><mml:msub><mml:mrow><mml:mi>Te</mml:mi></mml:mrow><mml:mn>5</mml:mn></mml:msub></mml:mrow></mml:math>
Jierui Huang, Bei Jiang, Jingyu Yao, Dayu Yan, Xincheng Lei, Jiacheng Gao, Zhaopeng Guo, Feng Jin, Yupeng Li, Zhenyu Yuan, Congcong Chai, Haohao Sheng, Mojun Pan, Famin Chen, Junde Liu, Shunye Gao, Gexing Qu, Бо Лю, Zhicheng Jiang, Zhengtai Liu, X. Y. Ma, Shiming Zhou, Yaobo Huang, Chenxia Yun, Qingming Zhang, Shiliang Li, Shifeng Jin, Hong Ding, Jie Shen, Dong Su, Youguo Shi, Zhijun Wang, Tian Qian
Abstract
The excitonic insulator (EI) is an exotic ground state of narrow-gap semiconductors and semimetals arising from spontaneous condensation of electron-hole pairs bound by attractive Coulomb interaction. Despite research on EIs dating back to half a century ago, their existence in real materials remains a subject of ongoing debate. In this study, through systematic experimental and theoretical investigations, we provide evidence for the existence of an EI ground state in a van der Waals compound <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mrow><a:msub><a:mrow><a:mi>Ta</a:mi></a:mrow><a:mn>2</a:mn></a:msub><a:msub><a:mrow><a:mi>Pd</a:mi></a:mrow><a:mn>3</a:mn></a:msub><a:msub><a:mrow><a:mi>Te</a:mi></a:mrow><a:mn>5</a:mn></a:msub></a:mrow></a:math>. Density-functional-theory calculations suggest that it is a semimetal with a small band overlap, whereas various experiments exhibit an insulating ground state with a clear band gap. Upon incorporating electron-hole Coulomb interaction into our calculations, we obtain an EI phase where the electronic symmetry breaking opens a many-body gap. Angle-resolved photoemission spectroscopy measurements exhibit that the band gap is closed with a significant change in the dispersions as the number of thermally excited charge carriers becomes sufficiently large in both equilibrium and nonequilibrium states. Structural measurements reveal a slight breaking of crystal symmetry with exceptionally small lattice distortion in the insulating state, which cannot account for the significant gap opening. Therefore, we attribute the insulating ground state with a gap opening in <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mrow><c:msub><c:mrow><c:mi>Ta</c:mi></c:mrow><c:mn>2</c:mn></c:msub><c:msub><c:mrow><c:mi>Pd</c:mi></c:mrow><c:mn>3</c:mn></c:msub><c:msub><c:mrow><c:mi>Te</c:mi></c:mrow><c:mn>5</c:mn></c:msub></c:mrow></c:math> to exciton condensation, where the coupling to the symmetry-breaking electronic state induces a subtle change in the crystal structure. Published by the American Physical Society 2024