Experimental evidence for direct insulator-quantum Hall transition in multi-layer grapheme

We have performed magnetotransport measurements on a multi-layer graphene flake. At the crossing magnetic field B_c, an approximately temperature-independent point in the measured longitudinal resistivity _xx, which is ascribed to the direct insulator-quantum Hall (I-QH) transition, is observed. By analyzing the amplitudes of the magnetoresistivity oscillations, we are able to measure the quantum mobility μ_qof our device. It is found that at the direct I-QH transition, μ_qB_c≈ 0.37 which is considerably smaller than 1. In contrast, at B_c, _xxis close to the Hall resistivity _xy, i.e., the classical mobility μB_cis ≈ 1. Therefore, our results suggest that different mobilities need to be introduced for the direct I-QH transition observed in multi-layered graphene. Combined with existing experimental results obtained in various material systems, our data obtained on graphene suggest that the direct I-QH transition is a universal effect in 2D.