Seo Yu-Seong, Eom Man Jin, Kim Jun Sung, Kang Chang-Jong, Min Byung Il, Hwang Jungseek
Department of Physics, Sungkyunkwan University, Suwon, Gyeonggi-do, 16419, Republic of Korea.
Department of Physics, Pohang University of Science and Technology, Pohang, 37673, Republic of Korea.
Sci Rep. 2018 Aug 10;8(1):11961. doi: 10.1038/s41598-018-30430-9.
An interesting van der Waals material, TaNiSe has been known one of strong excitonic insulator candidates since it has very small or zero bandgap and can have a strong exciton binding energy because of its quasi-one-dimensional crystal structure. Here we investigate a single crystal TaNiSe using optical spectroscopy. TaNiSe has quasi-one-dimensional chains along the a-axis. We have obtained anisotropic optical properties of a single crystal TaNiSe along the a- and c-axes. The measured a- and c-axis optical conductivities exhibit large anisotropic electronic and phononic properties. With regard to the a-axis optical conductivity, a sharp peak near 3050 cm at 9 K, with a well-defined optical gap ([Formula: see text] 1800 cm) and a strong temperature-dependence, is observed. With an increase in temperature, this peak broadens and the optical energy gap closes around ∼325 K ([Formula: see text]). The spectral weight redistribution with respect to the frequency and temperature indicates that the normalized optical energy gap ([Formula: see text]) is [Formula: see text]. The temperature-dependent superfluid plasma frequency of the excitonic condensation in TaNiSe has been determined from measured optical data. Our study may pave new avenues in the future research on excitonic insulators.
作为一种有趣的范德华材料，TaNiSe因其具有非常小或零带隙，且由于其准一维晶体结构而可能具有很强的激子结合能，一直被认为是强激子绝缘体的候选材料之一。在此，我们使用光谱学方法研究了单晶TaNiSe。TaNiSe沿a轴具有准一维链结构。我们获得了单晶TaNiSe沿a轴和c轴的各向异性光学性质。测量得到的a轴和c轴光学电导率展现出很大的各向异性电子和声学性质。关于a轴光学电导率，在9K时于3050 cm附近观察到一个尖锐的峰，其具有明确的光学带隙（1800 cm）且强烈依赖于温度。随着温度升高，这个峰变宽，并且光学能隙在约325 K时关闭。光谱权重随频率和温度的重新分布表明归一化光学能隙为。已根据测量的光学数据确定了TaNiSe中激子凝聚的温度依赖超流等离子体频率。我们的研究可能为激子绝缘体的未来研究开辟新途径。
