Seminars Archive

Modelling band structure and ARPES spectra of van der Waals heterostructures: impact of incommensurate interfaces and interlayer twists

Abstract
Since the isolation of graphene in 2004 scientists have produced many other atomically thin crystals. These can now be assembled into stacks, resulting in new artificial materials with unique properties. Because of the nature of the weak interlayer coupling, lattice matching at an interface between two two-dimensional crystals is not necessary for the whole structure to be stable. As a result, not only is any combination of materials possible - they can also be assembled with any relative angle between their respective crystallographic directions, with small rotations making the difference between, for example, the structure being superconducting or not [1]. Photoemission spectroscopy is a powerful tool to study two-dimensional crystals and I will present some examples of how it helps us understand the impact of interlayer coupling and atomic registry at a van der Waals interface on the properties of a stack. I will start by discussing the reconstruction of the electronic dispersion of graphene on hexagonal boron nitride (hBN), arising due to the incommensurability of the periods of the two lattices and leading to the appearance of secondary Dirac points and minigaps [2,3]. However, changes in local stacking driven by lattice mismatch and angular misalignment between two crystals affect not only the charge carriers but also the lattice as atoms try to minimise their interlayer interaction energy at a cost of deforming bonds within the same layer. As a result, in the graphene/hBN heterostructure, when the crystallographic directions of the two materials are aligned, the valence hBN electrons couple to a zone-centre graphene phonon [4]. Finally, I will discuss the electronic band structure of bulk layered rhenium dichalocogenides ReSe2 and ReS2, in which a Jahn-Teller distortion breaks the in-plane trigonal symmetry usually present in transition metal dichalcogenides. Because of the markedly lower symmetry than most other two-dimensional crystals, these compounds were referred to as "bulk monolayer materials" and I will provide some insight into their bulk band structure, in particular their kz-dispersion and the question of direct vs indirect band gap [5,6].

REFERENCES
[1] Y. Cao et al., Nature 556, 43 (2018).
[2] J. R. Wallbank et al., Physical Review B 87, 245408 (2013).
[3] M. Mucha-Kruczynski et al., Physical Review B 93, 085409 (2016).
[4] C. Chen et al., Nano Letters 18, 1082 (2018).
[5] L. S. Hart et al., Sci. Rep. 7, 5145 (2017).
[6] S. M. Gunasekera et al., J. Electron. Mater. 47, 4314 (2018).