Seminars

  • Towards the ultimate van der Waals heterostructure quantum Hall device

  • Speaker : 김영욱/Youngwook Kim
    Affiliation : Max Planck Institute for Solid State Research, Germany
    Date : April 19, 2018 2:00 PM
    Place : Buliding 110 Room N106
    Contact : noejung@unist.ac.kr
    Host : Prof.Noejung Park
  • Abstract

  • For a long time, the study on delicate fractional quantum Hall states was a privilege of the GaAs community. These days, new class of materials, which are two dimensional atomic sheets isolated from bulk crystals, is discovered. Due to multiple degrees of freedom and wide density tunabiltiy, 2D crystals are expected to host new quantum Hall ground states. Hunting new states in the quantum Hall regime requires device of extreme carrier mobility, hence, device fabrication techniques for 2D crystals have been intensively developed over the past few years. A powerful van der Waals pick-up method has been established. This not only enables the fabrication of high quality device but also build multilayer 2D crystal device, van der Waals heterostructure. Despite this successful effort, there are only few experimental reports about delicate quantum Hall states in 2D crystals. Clearly, further work is needed to obtain even higher quality van der Waals device in order to unlock their full potential.

    Here, we present recent progress of our simple but strong fabrication technique; high temperature annealing and mechanical ironing. This method significantly reduces disorder potential fluctuations which often obscure very fragile ground states. We systematically improve the quality of graphene and MoS2 devices, resulting in steeper zero-field transport curves, increased Hall coefficient at room temperature, and quantum Hall effect appearing in very low magnetic field. We successively achieve unprecedented quality of monolayer graphene devices that for the first time show evidence of new non-Abelian fractional quantum Hall states and reentrant quantum Hall states. In addition, we also observe hidden Landau levels and their competition in graphene superlattice device.