Effect of Coulomb impurities on the electronic structure of magic angle twisted bilayer graphene

• Published in: NPJ 2D materials and applications Vol. 7; no. 1; pp. 49 - 8
• Main Authors: Ramzan, Muhammad Sufyan, Goodwin, Zachary A. H., Mostofi, Arash A., Kuc, Agnieszka, Lischner, Johannes
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 10.07.2023; Nature Publishing Group; Nature Portfolio
• Subjects: 639/301/1034/1038; 639/301/119/995; Bilayers; Chemistry and Materials Science; Conduction bands; Defects; Electronic properties; Electronic structure; Electrons; Graphene; Holes (electron deficiencies); Materials Science; Mathematical analysis; Monolayers; Nanotechnology; Surfaces and Interfaces; Thin Films; Unit cell; Wave functions
• ISSN: 2397-7132; 2397-7132
• DOI: 10.1038/s41699-023-00403-2

In graphene, charged defects break the electron-hole symmetry and can even give rise to exotic collapse states when the defect charge exceeds a critical value which is proportional to the Fermi velocity. In this work, we investigate the electronic properties of twisted bilayer graphene (tBLG) with charged defects using tight-binding calculations. Like monolayer graphene, tBLG exhibits linear bands near the Fermi level but with a dramatically reduced Fermi velocity near the magic angle (approximately 1.1°). This suggests that the critical value of the defect charge in magic-angle tBLG should also be very small. We find that charged defects give rise to significant changes in the low-energy electronic structure of tBLG. Depending on the defect position in the moiré unit cell, it is possible to open a band gap or to induce an additional flattening of the low-energy valence and conduction bands. Our calculations suggest that the collapse states of the two monolayers hybridize in the twisted bilayer. However, their in-plane localization remains largely unaffected by the presence of the additional twisted layer because of the different length scales of the moiré lattice and the monolayer collapse state wavefunctions. These predictions can be tested in scanning tunneling spectroscopy experiments.