Strong Electron-Phonon Coupling in the Excitonic Insulator Ta2NiSe5

Jian Yan; Ruichun Xiao; Xuan Luo; Hongyan Lv; Ranran Zhang; Yan Sun; Peng Tong; Wenjian Lu; Wenhai Song; Xuebin Zhu; Yuping Sun

doi:10.1021/acs.inorgchem.9b00432

Strong Electron-Phonon Coupling in the Excitonic Insulator Ta₂NiSe₅

Inorg Chem. 2019 Jul 15;58(14):9036-9042. doi: 10.1021/acs.inorgchem.9b00432. Epub 2019 Jun 27.

Authors

Jian Yan^{1

2}, Ruichun Xiao^{1

2}, Xuan Luo¹, Hongyan Lv¹, Ranran Zhang³, Yan Sun⁴, Peng Tong¹, Wenjian Lu¹, Wenhai Song¹, Xuebin Zhu¹, Yuping Sun^{1

3

5}

Affiliations

¹ Key Laboratory of Materials Physics , Institute of Solid State Physics, Chinese Academy of Sciences , Hefei 230031 , People's Republic of China.
² University of Science and Technology of China , Hefei 230026 , People's Republic of China.
³ High Magnetic Field Laboratory , Chinese Academy of Sciences , Hefei 230031 , People's Republic of China.
⁴ Institute of Physical Science and Information Technology , Anhui University , Hefei 230601 , People's Republic of China.
⁵ Collaborative Innovation Center of Advanced Microstructures , Nanjing University , Nanjing 210093 , People's Republic of China.

PMID: 31246443
DOI: 10.1021/acs.inorgchem.9b00432

Abstract

An excitonic insulating (EI) state is a fantastic correlated electron phase in condensed matter physics, driven by screened electron-hole interaction. Ta₂NiSe₅ is an excitonic insulator with a critical temperature (T_C) of 328 K. In the current study, temperature-dependent Raman spectroscopy is used to investigate the phonon vibrations in Ta₂NiSe₅. The following observations were made: (1) an abnormal blue shift around T_C is observed, which originates from the monoclinic to orthorhombic structural phase transition; (2) the splitting of a mode and two new Raman modes at 147 and 235 cm^-1 have been observed with the formation of an EI state. With the help of first-principles calculations and temperature-dependent X-ray diffraction (XRD) experiments, it is found that the TaSe₆ octahedra are "frozen" and the NiSe₄ tetrahedra are greatly distorted below T_C. Thus, it seems that the distortion of NiSe₄ tetrahedra plays an important role in the strong electron-phonon coupling (EPC) in Ta₂NiSe₅, while the strong EPC, coupled with electron-hole interaction, opens the energy gap to form the EI state in Ta₂NiSe₅.