Observation of nanometer-spaced modulated structure causing Large Seebeck coefficient of the Tl compound: Angle-resolved photoemission study

具有准一维链状结构的TiLnSe_2由于其结构的低维性以及有趣的电学和热电性质引起了人们的极大兴趣。最近，人们发现TlInSe_2的塞贝克系数只有在473K以下才是正的，在413K以下达到了超过10^6μV/K的很高的值。有人指出，这一破纪录的温差电势可能是由于TlInSe_2中形成了一个无公度的(IC)超晶格相。然而，无论是通过电子结构分析，还是通过扩展的X射线或中子散射检查，都尚未从实验上证实TlInSe_2中存在IC相。在这项工作中，我们利用角分辨光电子能谱(ARPES)在280和50K对TlInSe_2进行了研究，以期在电子能带结构中发现IC相的印迹，所得到的能带与计算的能带结构在主要方面是一致的。能带结构在垂直于链的方向上表现出很强的色散，这表明TlInSe_2的一维电子特征实际上被强烈的链间相互作用所掩盖。随着温度的降低，整个光谱特征刚性地向低结合能方向移动。此外，在两个方向的实验电子带中都观察到了能隙分裂。利用实验和理论数据进行了详细的分析，得到了明显的与TlInSe_2布里渊区A线上某一点相连的IC相的存在。通过对TlInSe_2材料现有数据的讨论，绘制了TlInSe_2相变的自洽图像。根据这张图，TlInSe_2在50K已经是一种相称的(C)材料，而IC-C相变发生在280-50K之间。

TllnSe_2 with quasi one-dimensional chain structure has attracted much interest because of the structural low-dimensionality, and the interesting electric and thermoelectric properties. Recently, the Seebeck coefficient of TlInSe_2 has then been found to be positive only at temperatures below 473 K, reaching very high values of more than 10^6μV/K below 413 K. It has been pointed out that above record-breaking thermoelectric power may result from the formation of an incommensurate (IC) superlattice phase in TlInSe_2. However, the existence of IC-phase in TlInSe_2 has not been experimentally verified yet, either by the analyses of the electronic structure or by the extended X-ray or neutron scattering examination. In this work, we have studied TlInSe_2 by means of angle-resolved photoemission spectroscopy (ARPES) at 280 and 50K in order to possibly find the imprints of IC-phase in the electronic band structure.The obtained energy bands and the calculated band structure were found to agree well in main futures. The band structure shows quite strong dispersion in the direction perpendicular to the chains and indicates that in fact the one-dimensional electronic features of TlInSe_2 are masked by the strong interchain interactions. The whole spectral features shift rigidly toward lower binding energies with decreasing temperature. The energy gap splitting, furthermore, has been clearly observed in the experimental electronic bands along both directions. A detailed analysis using both experimental and theoretical data was performed and the obtained results were evident of the presence of the IC-phase linked to a certain point on the A-line of the Brillouin zone of TlInSe_2. A self-consistent picture of the phase transition in TlInSe_2 was drawn after discussion using available data for this material. According to this picture, TlInSe_2 at 50K is already a commensurate (C) material while the IC-C phase transition occurs between 280 and 50K.