Optical Excitations of Flat Bands Induced by Exciton Condensation in Ta$_2$Pd$_3$Te$_{5}$

Shaohui Yi; Zhiyu Liao; Chenhao Liang; Sheng Zhang; Xiutong Deng; Yongjie Xie; Lincong Zheng; Yujie Wang; Yubiao Wu; Zhijun Wang; Youguo Shi; Xianggang Qiu; Bing Xu

arXiv · 2506.18350

Optical Excitations of Flat Bands Induced by Exciton Condensation in Ta$_2$Pd$_3$Te$_{5}$

Shaohui Yi·Zhiyu Liao·Chenhao Liang·Sheng Zhang·Xiutong Deng·Yongjie Xie·Lincong Zheng·Yujie Wang·Yubiao Wu·Zhijun Wang·Youguo Shi·Xianggang Qiu·Bing Xu

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Abstract

We report on the charge dynamics of Ta$_2$Pd$_3$Te$_5$ using temperature-dependent optical spectroscopy with polarized light. We observe a metal-insulator transition characterized by the collapse of Drude response and the emergence of sharp and narrow absorption peaks at low temperatures. Unlike previous excitonic insulator candidates such as TiSe$_2$ and Ta$_2$NiSe$_5$, where the excitonic order is intertwined with charge density wave or structural instabilities, the sharp features in Ta$_2$Pd$_3$Te$_5$ point to intrinsic excitonic excitations associated with ultra-flat bands driven by many-body renormalization of the band structure via spontaneous exciton condensation. Our findings thus provide clear-cut optical evidence for exciton condensation in a bulk crystal and establish Ta$_2$Pd$_3$Te$_5$ as a promising platform for exploring correlated quantum phases and novel excitonic phenomena.

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Shaohui Yi, Zhiyu Liao, Chenhao Liang, Sheng Zhang, Xiutong Deng, Yongjie Xie, Lincong Zheng, Yujie Wang, Yubiao Wu, Zhijun Wang, Youguo Shi, Xianggang Qiu, Bing Xu. 2025-06-23. Optical Excitations of Flat Bands Induced by Exciton Condensation in Ta$_2$Pd$_3$Te$_{5}$. https://arxiv.org/abs/2506.18350

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