TlInSe_2 with a nano-modulated structure as a thermoelectric material with a high efficiency _

发现一些TlInSe_2样品具有铁电性。我们观察到的电流-电压特性的记忆效应。研究了TlInSe_2晶体的光致发光（PL）和PL激发光谱。所观察到的板PL谱由中心在1.05和0.95 eV的两个峰重叠。两个分离的峰被认为是归因于自由到结合的跃迁根据激发强度的PL光谱的依赖性。还估计了这些峰的活化能，以评估与发射相关的供体或受体水平。在60 ~ 150℃温度范围内，观察到TlInSe_2的光致发光激发谱，在1.17和1.23eV处有类激子峰。从光谱中，发现该化合物具有间接带隙，并在温度范围内改变与跃迁相关的声子能量。用角分辨光电子能谱研究了TlInSe_2晶体的声子能谱。所得到的能带与计算的能带结构一致，并且在垂直于链的方向上表现出相当明显的色散。随着温度的升高，电子带明显向低结合能方向刚性移动，出现分裂，并形成微小带隙结构，导致Seebeck系数的破纪录值.

It is found that some TlInSe_2 samples indicate ferroelectricity. We observed memory effects of current-voltage characteristics. The phenomena are supposed to be the memory effects in an incommensurate phase which is often observed on other thallic compounds.Next, Photoluminescence (PL) and PL excitation spectra of TlInSe_2 crystals have been investigated. The observed board PL spectra are overlapped by two peaks centered at 1.05 and 0.95 eV. Both separated peaks are considered to be attributed to free-to-bound transition according to excitation intensity dependence of PL spectra. An activation energy of these peaks have also been estimated to evaluate donor or acceptor level associated with the emissions. Furthermore, we have observed PL excitation spectra with exciton-like peaks at 1.17 and 1.23 eV.The optical absorption of TlInSe_2 has been examined and the structure changes of the spectra were observed in the range of temperature between 60 and 150℃. From the spectra, the compound is found to has an indirect band gap and to change phonon energy associated with the transition in the temperature range. The decrease of phonon energy is attributed to the folding energy band due to a superlattice in the commensurate phase.Furthermore, TlInSe_2 crystals have been investigated by mean of angle-resolved photoemission spectroscopy. The obtained energy bands favorably agree with the calculated band structure and show quite noticeable dispersion in the direction normal to chains. A rigid shift toward lower binding energies, splitting and the formation of the mini-gap-like structures are clearly observed in the experimental electric bands with the temperature, and causing the record-breaking values of Seebeck coefficient.