Yan Jian, Xiao Ruichun, Luo Xuan, Lv Hongyan, Zhang Ranran, Sun Yan, Tong Peng, Lu Wenjian, Song Wenhai, Zhu Xuebin, Sun Yuping
Key Laboratory of Materials Physics , Institute of Solid State Physics, Chinese Academy of Sciences , Hefei 230031 , People's Republic of China.
University of Science and Technology of China , Hefei 230026 , People's Republic of China.
Inorg Chem. 2019 Jul 15;58(14):9036-9042. doi: 10.1021/acs.inorgchem.9b00432. Epub 2019 Jun 27.
An excitonic insulating (EI) state is a fantastic correlated electron phase in condensed matter physics, driven by screened electron-hole interaction. TaNiSe is an excitonic insulator with a critical temperature () of 328 K. In the current study, temperature-dependent Raman spectroscopy is used to investigate the phonon vibrations in TaNiSe. The following observations were made: (1) an abnormal blue shift around 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 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 TaSe octahedra are "frozen" and the NiSe tetrahedra are greatly distorted below . Thus, it seems that the distortion of NiSe tetrahedra plays an important role in the strong electron-phonon coupling (EPC) in TaNiSe, while the strong EPC, coupled with electron-hole interaction, opens the energy gap to form the EI state in TaNiSe.
激子绝缘（EI）态是凝聚态物理中一种奇妙的关联电子相，由屏蔽的电子 - 空穴相互作用驱动。TaNiSe是一种临界温度（）为328 K的激子绝缘体。在当前研究中，利用温度依赖拉曼光谱研究TaNiSe中的声子振动。得到了以下观察结果：（1）在处观察到异常蓝移，其源于单斜晶系到正交晶系的结构相变；（2）随着EI态的形成，观察到一个模式的分裂以及在147和235 cm处出现两个新的拉曼模式。借助第一性原理计算和温度依赖X射线衍射（XRD）实验发现，在以下温度下TaSe八面体“冻结”，NiSe四面体严重畸变。因此，似乎NiSe四面体的畸变在TaNiSe的强电子 - 声子耦合（EPC）中起重要作用，而强EPC与电子 - 空穴相互作用相结合，打开能隙以在TaNiSe中形成EI态。
