First-Principles Study on the Thermoelectric Properties in Topological Insulator BiSbTe₂S

Three-dimensional (3D) topological insulators have attracted significant interest due to their massless Dirac fermions and potential applications in spintronics and quantum devices. Research on Bi_2-xSb_xTe_3-ySey has been active in the field of 3D topological insulators. By replacing Se with S, Bi_1.1Sb_0.9Te₂S also exhibits a massless Dirac band dispersion, as observed through an angle-resolved photoemission spectroscopy [1]. Additionally, this compound has achieved a high positive Seebeck coefficient at 200K and has demonstrated the integer quantum Hall effect in its 3μm thin-film form [2]. In this study, we theoretically investigate the microscopic mechanism of the thermoelectric properties by considering the influences of the bulk and the surface states individually. To achieve this, we performed first-principles calculations for the bulk and the [0001] surface-slab structure of BiSbTeS₂ and evaluated the Seebeck coefficients. Our analysis clarified that the massless Dirac band dispersions appearing in the topological surface state do not significantly enhance the Seebeck coefficient. We also examined the thickness-dependent change in the thermoelectric properties and how the electronic state near the Fermi level determines them.

[1] S. K. Kushwaha. et al., Nat. Commun. 10, 1038 (2016).

[2] S. Y. Matsushita et al., Phys. Rev. Materials, 5, 014205 (2021).