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Breakdown of the Interlayer Coherence in Twisted Bilayer Graphene
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of the Interlayer Coherence
in Twisted Bilayer Graphene

Graphene, arguably an ideal two dimensional electron gas system, shows rich electronic properties depending on how it is stacked on top of another graphene. Recent studies on multilayer graphene emphasize that the interlayer interaction between graphene layers is one of the key parameters for tuning their electrical properties. Among various types of multilayer graphene, twisted bilayer graphene is of particular interest because several intriguing properties such as renormalization of Fermi velocity, van Hove singularities, and electronic localization were recently discovered. Experimental studies on twisted bilayer graphene have suggested that the layers are decoupled, and are often considered as being electrically isolated. So far, however, most of the studies on the transport properties of twisted bilayer graphene have focused on the in-plane conduction in a planar structure, but their vertical transport properties have never been investigated experimentally. Thus, it is not clear in what sense the layers are decoupled on an atomic length scale of the layer separation, and how strong the interlayer coupling is in twisted bilayer graphene.

In layered compounds, coherent motion of the electrons in the Bloch states often breaks down when the interlayer coupling is significantly reduced. The interlayer incoherence has been a long-standing puzzle in quasi 2-dimensional systems including high-Tc cuprates, organic crystals, dichalcogenides, graphite, and semiconductor superlattices. Common wisdom suggests that the interlayer coherence is destroyed by an increase of interlayer separation exceeding the mean-free-path across the layers, as often called as the interlayer version of the Mott-Ioffe- Regal limit. However, the underlying mechanism of the interlayer incoherence and the intriguing interlayer conduction are still under debate. In this respect, it is interesting to investigate whether or not the coherent interlayer conduction occurs in twisted bilayer graphene with an atomic length scale of the layer separation.

fig. 1: (a) Schematic diagram of a twisted bilayer graphene device. (b) Optical image of a twisted bilayer graphene device. (c) The momentum mismatch of two sets of Dirac cones (blue and red) from different layers in twisted bilayer graphene. (d) The interlayer resistivity of twisted bilayer graphene with variation of the gate voltages (Vg) at different temperatures. (e) The interlayer resistivity of twisted bilayer graphene at different gate voltages as a function of temperature. The inset shows schematic illustrations of the momentum mismatch between Fermi surfaces of neighboring layers at low temperatures. At high temperatures, phonon-assisted tunneling with a momentum exchange of q enhances the interlayer conduction.

Recently, Youngwook Kim et al. from POSTECH, in collaboration with research teams from Konkuk Univ., KIST, Norwegian Univ. of Sci. and Tech., and Sejong Univ., demonstrate that the interlayer coherence is completely suppressed in twisted bilayer graphene. They found that twisted bilayer graphene exhibits significant localization of electrons in interlayer conduction, which is in strong contrast to the metallic behavior of conventional bilayer graphene with Bernal stacking. The strong negative temperature dependence of the interlayer resistivity demonstrates that incoherent tunneling is the main conduction mechanism between the layers. This is attributed to momentum-mismatch of the 2 dimensional Fermi surfaces from each layer. When the twisted angle is incommensurate, the overlap between the Fermi surfaces from each layer is significantly suppressed, leading to incoherent interlayer conduction, which also results in strong sensitivity of the interlayer conduction to the twisted angles between the layers. They also found that the momentum-mismatch is relieved by phonon scattering with increasing temperatures, leading to enhancement of the interlayer conduction and its moderate dependence on the twisted angles at high temperatures. These findings clearly demonstrate that twisted bilayer graphene provides a rare example of layered systems showing the breakdown of the interlayer coherence even with an atomic scale of layer separation.


[1] Youngwook Kim, Hoyeol Yun, Seung-Geol nam, Minhyeok Son, Dong Su lee, Dong Chul Kim, S. Seo, Hee Cheul Choi, Hu-Jong Lee, Sang Wook Lee, and Jun Sung Kim, Phys. Rev. Lett. 110, 096602 (2013).

AAPPS Bulletin        ISSN: 2309-4710
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