Temperature effects in topological insulators of transition metal dichalcogenide monolayers

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We investigate the role of temperature on the topological insulating state of metal dichalcogenide monolayers, 1T′−MX2 (M=W, Mo and X=S, Se). Using first principles calculations based on density functional theory, we consider three temperature-related contributions to the topological band gap: electrons coupling with short-wavelength phonons, with long-wavelength phonons Fröhlich coupling, and thermal expansion. We find that electron-phonon coupling promotes the topology of the electronic structures of all 1T′−MX2 monolayers, while thermal expansion acts as a counteracting effect. Additionally, we derive the band renormalization from Fröhlich coupling in the two-dimensional context and observe its relatively modest contribution to 1T′−MX2 monolayers. Finally, we present a simplified physical picture to understand the “inverse Varshni” effect driven by band inversion in topological insulators. Our work reveals that, among the four 1T′−MX2 studied monolayers, MoSe2 is a promising candidate for room-temperature applications because its band gap displays remarkable resilience against thermal expansion, while the topological order of WS2 can be tuned under the combined influence of strain and temperature. Both materials represent novel examples of temperature-promoted topological insulators. Published by the American Physical Society 2024