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

发现部分TlInSe_2样品具有铁电性。我们观察到电流-电压特性的记忆效应。这一现象被认为是在不相称期的记忆效应，这在其他的thalic化合物中也经常观察到。其次，研究了TlInSe_2晶体的光致发光（PL）和激发光谱。观测到的板PL光谱有两个以1.05和0.95 eV为中心的峰重叠。根据PL光谱的激发强度依赖性，认为两个分离峰都属于自由束缚跃迁。还估计了这些峰的活化能，以评估与排放物有关的供体或受体水平。此外，我们还观察到在1.17和1.23 eV处具有激子样峰的PL激发光谱。研究了TlInSe_2在60 ~ 150℃范围内的光吸收，并观察了光谱的结构变化。从光谱上发现，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。声子能量的降低是由于相应相中存在超晶格导致的能带折叠。此外，利用角分辨光谱学对TlInSe_2晶体进行了研究。得到的能带与计算的能带结构吻合良好，并在链的正方向上表现出相当明显的色散。随着温度的升高，在实验电带中明显观察到向低结合能的刚性转移、分裂和小间隙结构的形成，并导致塞贝克系数破纪录值。

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.