CHINESE JOURNAL OF PHYSICS VOL. 52, NO. 4 August 2014 The Direct Insulator-Quantum Hall Transition Chi-Te Liang 1, 2 and Shun-Tsung Lo 3 1 Department of Physics, National Taiwan University, Taipei 106, Taiwan 2 School of Electronic and Electrical Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Korea 3 Graduate Institute of Applied Physics, National Taiwan University, Taipei 106, Taiwan (Received April 27, 2013) The direct insulator-quantum Hall (I-QH) transition corresponds to a transition from an insulator to a high Landau-level-filling factor ν> 2 QH state, which is characterized by an approximately temperature T -independent longitudinal resistance of a few nm thick electron (or hole) layer. In this paper, we review both the experimental and theoretical results on the direct I-QH transition. In particular, we attempt to address several interesting yet unsettled issues in the field of the direct I-Q transition. We suggest that further studies are required for obtaining a thorough understanding of the direct I-QH transition observed in nano-scale charge layers. DOI: 10.6122/CJP.52.1175 PACS numbers: 73.20.Fz, 73.20.Jz, 73.43.Qt I. INTRODUCTION The simultaneous presence of a strong magnetic field B and disorder can result in many fascinating physical phenomena. For instance, the integer quantum Hall effect (IQHE) is probably one of the most studied effects in two-dimensional electron systems, in which the electrons can be confined in layers on the nano-scale [1]. It has been shown that with an appropriate amount of disorder, a two-dimensional electron system (2DES) may undergo a B-induced insulator to quantum Hall transition [2–5]. Experimental evidence for such an insulator-quantum Hall (I-QH) transition is an approximately temperature (T )- independent point in the measured longitudinal resistivity of a 2DES [3–5]. The I-QH transition continues to attract much interest as it may shed light on the fate of extended states [6–10], the true ground state of a non-interacting 2DES [2], and possibly on the metal-insulator transition in 2D [11, 12]. In the seminal work done by Kivelson, Lee, and Zhang [2], the only allowed I-QH transition in the global phase diagram (GPD) is a transition between an insulator and a Landau-level-filling factor ν = 1 QH state, as shown in Fig. 1. When the spin degeneracy is taken into consideration, the only allowed I-QH transitions are the 0-2-0 and 0-2-1-0 transitions. Here 0 corresponds to the insulating phase. Experimental evidence supporting the GPD has been reported by three independent groups in the early 90s [3–5]. In the late 90s, experimental evidence for a direct insulator to a high-Landau level filling factor QH state has been reported by two groups. These interesting results cannot be explained by the GPD and have been attracting a great deal of world-wide interest. The purpose of this paper is to review both the experimental and theoretical results on the http://PSROC.phys.ntu.edu.tw/cjp 1175 c ⃝ 2014 THE PHYSICAL SOCIETY OF THE REPUBLIC OF CHINA