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Understanding the chemical states and electronic states of superconducting materials can help researchers to grasp their superconducting mechanisms. Then, through doping, high pressure or other methods, the T c (Critical Temperature) can be boosted. Recently, a new layered carbide superconductor ThMo2Si2C was discovered. Its chemical states and electronic states are unknown. Here, the chemical states of ThMo2Si2C are investigated using X-ray photoelectron spectroscopy assisted with argon ion sputtering and Raman spectroscopy. The physical significance behind them is elucidated. Its electronic states are investigated by first-principles calculations. It is found that the major contribution to the total DOS (Density of States) is from Mo 4d state, which plays the dominant role in the superconductivity of ThMo2Si2C. The hybridization between Th 6d and Mo 4d is very weak. Thus, a separation of the structure into independent Mo-C-Si and Th subunits (layers) is justified from the point of view of electronic structure. A flat band along the A-M high-symmetry direction is observed near the Fermi level. The influence of the interlayer distance between Mo-C-Si units (layers) on the DOS and band structure is investigated by first-principles calculations too.

Using high-resolution angle-resolved and time-resolved photoemission spectroscopy, we have studied the low-energy band structures in occupied and unoccupied states of three ternary compounds GeBi2Te4, SnBi2Te4 and Sn0.571Bi2.286Se4 near the Fermi level. In previously confirmed topological insulator GeBi2Te4 compounds, we confirmed the existence of the Dirac surface state and found that the bulk energy gap is much larger than that in the first-principles calculations. In SnBi2Te4 compounds, the Dirac surface state was observed, consistent with the first-principles calculations, indicating that it is a topological insulator. The experimental detected bulk gap is a little bit larger than that in calculations. In Sn0.571Bi2.286Se4 compounds, our measurements suggest that this nonstoichiometric compound is a topological insulator although the stoichiometric SnBi2Se4 compound was proposed to be topological trivial.