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

研究发现，部分TlInSe_2样品具有铁电性。我们观察了电流-电压特性的记忆效应。这种现象被认为是在非公度相中的记忆效应，这是在其他金属化合物中经常观察到的。其次，研究了TlInSe_2晶体的光致发光和光致发光激发光谱。观测到的板状光致发光光谱由中心位于1.05 eV和0.95 eV的两个峰重叠。根据荧光光谱的激发强度依赖关系，认为这两个峰都属于自由-束缚跃迁。还估计了这些峰的激活能，以评估与发射相关的施主或受主水平。此外，我们还观察到了在1.17和1.23 eV处有类激子峰的荧光激发光谱。测试了TlInSe_2的光吸收，并观察到在60-150℃的温度范围内光谱的结构变化。从光谱中发现，该化合物具有间接带隙，并改变了与温度范围内的转变相关的声子能量。声子能量的降低归因于公度相中超晶格的折叠能带。此外，我们还用角度分辨光电子能谱研究了TlInSe_2晶体。得到的能带与计算的能带结构很好地吻合，并且在垂直于链的方向上表现出相当明显的弥散。随着温度的升高，在实验的电子能带中明显地观察到刚性向低结合能方向移动，分裂和微小带隙结构的形成，导致了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.