Theory of insulating phase and superconductivity in twisted bilayer graphene

ORAL

Abstract

A correlated insulating phase and superconductivity have been recently observed in twisted bilayer graphene (tBG). We study theoretically these two phases in both the weak and strong coupling limit. In the weak coupling limit, using the random phase approximation, we find various possible combinations of density wave phases and pairing symmetries [1]. In the strong coupling limit, the insulating phase is anti-ferromagnetically (AFM) ordered, like in monolayer graphene [2]. However, we find there is no local magnetic moment in this AFM phase due to the special form of Wannier functions. Using the spin-fermion model, we find attractive electron-electron interaction mediated by the critical AFM fluctuations. This attractive interaction leads to chiral d+id superconductivity. Using a finite size tight-binding model, we show explicitly the existence of edge Majorana modes.

[1] E Laksono et al., Solid State Communications 282, 38-44 (2018)
[2] HK Tang et al., Science 361 (6402), 570-574 (2018)

*We acknowledge the Singapore Ministry of Education AcRF Tier 2 (MOE2017-T2-2-140) and National University of Singapore Department of Physics.

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Presenters

  • Xingyu Gu

    • Centre for Advanced 2D Materials and Graphene Research Centre, National University of Singapore
    • Department of Physics, National University of Singapore

Authors

  • Xingyu Gu

    • Centre for Advanced 2D Materials and Graphene Research Centre, National University of Singapore
    • Department of Physics, National University of Singapore

  • Evan Laksono

    • Centre for Advanced 2D Materials and Graphene Research Centre, National University of Singapore
    • Department of Physics, National University of Singapore

  • Chuan Chen

    • Department of Physics, National University of Singapore
    • National University of Singapore

  • Jia Ning Leaw

    • Department of Physics, National University of Singapore
    • National University of Singapore

  • Nimisha Raghuvanshi

    • Centre for Advanced 2D Materials and Graphene Research Centre, National University of Singapore
    • Department of Physics, National University of Singapore

  • Shaffique Adam

    • Department of Physics and Centre for Advanced 2D Materials, National University of Singapore
    • Centre for Advanced 2D Materials and Graphene Research Centre, National University of Singapore
    • Department of Physics, National University of Singapore
    • Yale-NUS College
    • National University of Singapore