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

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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.