via Magneto-infrared Spectroscopy*

We have systematically studied the 3D band structure of ZrTe$_5$ up to its parabolic band components using magneto-infrared spectroscopy. By comparing the Landau level transitions in different magnetic field orientations, we find that the band velocities are highly anisotropic and the dominant contribution along the layer stacking direction is parabolic with a record-low band velocity. More interestingly, with such a low band velocity, we have both theoretically and experimentally demonstrated that the band inversion can lead to a second extremum (band gap), allowing for a direct probe of the material’s topological nature. Our work not only provides an alternative way to identify the band inversion in 3D layered topological materials, but also yields important implications for understanding the exotic behavior of ZrTe$_5$ as well as that in similar (layered) materials.