Theoretical Study of Surface Electronic States of Layered Transition-Metal Dichalcogenides and Scanning Tunneling Spectroscopy

作为获得层状过渡金属双硫族化合物TX_2（T：过渡金属原子，X：硫族原子）的表面电子态或电子态的第一步，我们计算了TX_2本身及其过渡金属层间化合物的体电子能带结构，首先采用基于APW方法的Muffin-tin（MT）近似计算了1 T-TiS_2，1 T-TiSe_2，2 H-TaS_2和2 H-NbS_2的体电子能带结构。作为结果，它已被澄清，修正MT近似是非常重要的，因为它们的强各向异性性质的这些材料。还已经阐明，为了对MT近似进行校正，由于计算时间的问题，我们必须使用带计算的线性化方法，例如LAPW方法，特别是当计算在晶胞中包含许多原子的系统的电子带结构时。用LAPW方法计算了过渡金属层间化合物2 H-TaS_2，Mn_（1/4）TaS_2，它的晶胞里有26个原子非磁性态的能带结构清楚地表明，Mn 3d电子的局域化矩模型和嵌入效应的刚性带模型是无效的。此外，从铁磁态的能带计算中估计的每个Mn原子的磁矩的大小与观察结果吻合得很好。我们正在编制LAPW程序，对MT近似进行修正，并计算TX_2的电子结构。

As a first step toward obtaining surface electronic states or electronic states of a slab of layered transition-metal dichalcogenides TX_2 (T:transition-metal atoms, X:chalcogen atoms), we have calculated the bulk electronic band structures of TX_2 itself and its transition-metal intercalation compounds.We first calculated the bulk electronic band structures of 1T-TiS_2, 1T-TiSe_2, 2H-TaS_2 and 2H-NbS_2 on the basis of the APW method with use of the muffin-tin (MT) approximation. As the result it has been clarified that corrections to the MT approximation are very important for these materials because of their strong anisotropic nature. It has been also clarified that in order to make correction to the MT approximation we have to use linearized methods of the band calculation such as the LAPW method because of a problem of computing time, in particular when calculating the electronic band structures of systems containing many atoms in the unit cell.Next, by using the LAPW method we calculated the electronic band structures of transition-metal intercalation compounds of 2H-TaS_2, Mn_<1/4>TaS_2, which has 26 atoms in the unit cell. The band structures obtained for the non-magnetic states have clearly shown that the localized moment model for the Mn 3d electrons and the rigid band model for intercalation effects are not valid. Further the magnitude of the magnetic moment per Mn atom evaluated from the band calculation for the ferromagnetic states is in good agreement with the observation. Thus it has been clarified that the magnetic properties of Mn_<1/4>TaS_2 can be understood from the view point of itinerant Mn 3d electrons.We are now developing LAPW computer programs for making corrections to the MT approximation and for calculating electronic structures of a slab of TX_2.