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TllnSe_2由于其结构的低维性以及有趣的电学和热电性质而引起了人们的广泛兴趣。最近，人们发现TlInSe_2的塞贝克系数仅在473 K以下为正值，在413 K以下达到超过10^6μV/K的非常高的值。研究表明，上述热电势的突破可能是由于TlInSe_2中形成了无公度（IC）超晶格相所致。然而，TlInSe_2中IC相的存在还没有得到实验证实，无论是电子结构分析，还是扩展的X射线或中子散射检验。本工作利用角分辨光电子能谱（ARPES）在280和50 K下研究了TlInSe_2的电子能带结构，以期发现IC相在能带结构中的印记，所得能带与计算的能带结构基本一致。能带结构在垂直于链的方向上表现出相当强的色散，表明实际上TlInSe_2的一维电子特征被强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较低的结合能移动。此外，在实验电子带中沿沿着两个方向都清楚地观察到能隙分裂。利用实验和理论数据进行了详细的分析，所获得的结果是明显的IC相的存在下，连接到A线上的某个点上的布里渊区的TlInSe_2。利用已有的数据讨论了TlInSe_2的相变，并绘制了自洽的相变图象。根据这幅图，TlInSe_2在50 K时已经是一个相称的（C）材料，而IC-C相变发生在280和50 K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.