Finding a large band gap topological insulator (TI) is crucial to future room-temperature (corresponding to ~25 meV) device applications. Recently, monolayer ZrTe₅ has been predicted to be new functional two-dimensional (2D) TI [1], with a large (bulk) band gap far surpassing their predecessors, e.g., HgTe/CdTe and InAs/GaSb quantum wells. However, its electronic structure in the bulk is currently under heated debate, with interpretations ranging from weak/strong TI to Dirac semimetal.

Here, we report on a “bulk-sensitive” magneto-infrared transmission study of ZrTe₅ thin flakes in magnetic fields up to 17T. At zero magnetic field, our samples exhibit graphene-like optical absorption, which signifies their 2D nature. In a magnetic field, we observed a series of inter-band Landau level (LL) transitions that can be described by a massive Dirac fermion model with a small mass. More interestingly, we observed a four-fold splitting of low-lying LL transitions. Thanks to the newly developed circular polarized magneto-infrared capability, we were able to separate the σ⁺ and σ^- active transitions. These observations enable further exploration of the origin in the splitting: (i) the band asymmetry breaks the degeneracy of the dipole allowed inter-band LL transitions, and (ii) the remaining two-fold degeneracy is due to a combined effect of large g-factor and a small energy gap in this system. Our results support a 2D Dirac semimetal interpretation, consistent with recent electronic transport studies [2,3]. Finally, we compare the LL structure of Dirac fermions in ZrTe₅ and in other topological materials that we systematically investigated over last few years, such as HgCdTe and InAs/GaSb [4, 5, 6].

1. H. Weng, X. Dai, and Z. Fang, Physical Review X 4, 011002 (2014).

2. W. Yu et al. Scientific Reports 6, 35357 (2016).

3. X. Yuan et al. NPG Asia Mater 8, e325 (2016).

4. J. Ludwig et al. Physical Review B 89, 241406(R) (2014).

5. F. Teppe et al. Nature Communications 7, 12576 (2016).

6. Y. Jiang et al. Physical Review B 95, 045116 (2017).