Temperature dependence of the energy band gap in ZrTe5: Implications for the topological phase

Using Landau-level spectroscopy, we determine the temperature dependence of the energy band gap in zirconium pentatelluride (ZrTe5). We find that the band gap reaches E-g = (5 & PLUSMN; 1) meV at low temperatures and increases monotonically when the temperature is raised. This implies that ZrTe5 is a weak topological insulator, with noninverted ordering of electronic bands in the center of the Brillouin zone. Our magnetotransport experiments performed in parallel show that the resistivity anomaly in ZrTe5 is not connected with the temperature dependence of the band gap.