# Quantum-Hall to Insulator Transition in Ultra-low-carrier-density Topological Insulator Films and a Hidden Phase of the Zeroth Landau Level

## TL;DR

This paper reports the observation of a quantum-Hall-to-insulator transition in ultra-low-carrier-density topological insulator films, revealing a new universality class and a hidden phase at the zeroth Landau level, advancing understanding of topological surface states.

## Contribution

It introduces ultra-low-carrier-density Sb₂Te₃ films enabling exploration of the zeroth Landau level and uncovers a novel quantum-Hall-to-insulator transition with a distinct universality class.

## Key findings

- Observation of quantum-Hall-to-insulator transition near the zeroth Landau level
- Discovery of a hidden phase at the zeroth Landau level
- Identification of a new universality class for the transition

## Abstract

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.