Landau level spectroscopy of two-dimensional massive Dirac fermions in single-crystal ZrTe$_{\mathrm{5}}$ thin flakes

ZrTe$_{\mathrm{5}}$ has recently attracted much interest due to the possibility of hosting a large-gap quantum spin Hall insulator in its monolayer form. However, its electronic structure in the bulk is currently under heated debate, with interpretations ranging from weak/strong topological insulator to Dirac semimetal. Here, we report on a ``bulk-sensitive'' magneto-infrared transmission study of ZrTe$_{\mathrm{5}}$ thin flakes. At zero magnetic field, our samples exhibit graphene-like optical absorption, which signifies their two-dimensional (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 mass of \textasciitilde 4.7meV. More interestingly, we observed a four-fold splitting of low-lying LL transitions in our samples, which we attributed to the effect from finite mass, large Zeeman effects and the electron-hole asymmetry. Our results support a 2D Dirac semimetal interpretation, consistent with recent electronic transport studies.