  A key feature of the topological surface state under a magnetic field is the
presence of the zeroth Landau level at the zero energy. Nonetheless, it has
been challenging to probe the zeroth Landau level due to large electron-hole
puddles smearing its energy landscape. Here, by developing ultra-low-carrier
density topological insulator Sb$_2$Te$_3$ films, we were able to reach an
extreme quantum limit of the topological surface state and uncover a hidden
phase at the zeroth Landau level. First, we discovered an unexpected
quantum-Hall-to-insulator-transition near the zeroth Landau level. Then,
through a detailed scaling analysis, we found that this
quantum-Hall-to-insulator-transition belongs to a new universality class,
implying that the insulating phase discovered here has a fundamentally
different origin from those in non-topological systems.
