Structural relaxation of twisted bilayer graphene approaching the magic angle by first-principles studies*

Twisted bilayer graphene has attracted a great amount of attention due to its various and peculiar electronic phases as the twisting angle becomes very small. As an example, alternating superconducting and correlated insulating phases depending on the band filling can be observed at the so-called magic angle around 1°. While the crucial role of structural relaxation in determining the electronic properties and the occurrence of flat bands in the electronic structure has already been pointed out, the involved system sizes have limited early theoretical studies to the out-of-plane deformation [1] or phenomenological models [2]. More recently, plane wave-based density functional theory (DFT) calculations [3, 4] showed oscillating relaxation patterns at low twisting angles, which is at odds with the earlier studies. Therefore, further investigation is required.

    In this presentation, DFT results for both the in-plane and out-of-plane relaxation of twisted bilayer graphene at small twisting angles will be presented and their role in determining the electronic structure will be discussed. Due to the aforementioned large system sizes, a linear scaling method with localized basis sets has been used to extend the picture towards the magic angle after careful validation.

 

[1] K. Uchida et al., Phys. Rev. B 90, 155451 (2014).

[2] N. N. T. Nam, M. Koshino, Phys. Rev. B 96, 075311 (2017).

[3] P. Lucignano et al., Phys. Rev. B 99, 195419 (2019).

[4] G. Cantele et al., Phys. Rev. Res. 2, 043127 (2020).