Quantum-Hall to insulator transition in ultraclean topological insulator films*

Under a magnetic field, many two dimensional electronic systems including semiconductor 2DEGs, graphene, and topological insulators (TIs) exhibit quantum Hall effect (QHE), where R_xy is quantized at h/(νe²) while R_xx→0. In the high field limit, the QH state gives way to an insulating state in semiconductor 2DEGs as the Fermi level falls into localized states at the tail of the lowest Landau level. However, for a Dirac system such as TIs, the presence of the zeroth Landau level makes it questionable whether such a QH-to-insulator-transition (QIT) should occur at all. Here, in newly-developed ultraclean TI films, we report the first observation of QIT in TIs, with a well-defined scaling behavior. Surprisingly, the scaling analysis revealed the first example of an unconventional QIT behavior distinct from those in 2DEGs. This observation raises new questions on the origin of the insulating phase near the center of the zeroth Landau level in TIs and other Dirac materials as well.