FlatMoi

The structure-property relation is a fundamental theme in condensed matter. In 2D materials this relation takes on an exceptional aspect since a structure as simple as a rotated bilayer can be associated with a remarkable electronic structure with flat bands, strong electronic correlations, and superconductivity. These properties, demonstrated in 2018 on rotated graphene bilayers with a “magic” rotation angle close to 1.1°, has reinforced the interest in these materials. Despite numerous theoretical and experimental studies, a full understanding of this electronic localization is still missing. This is the objective of the FlatMoi project, which brings together specialists in 2D systems and crystallography. The rotation angle is a crucial parameter in these systems. However, we have furthermore shown that a small dilatation or compression of one plane with respect to the other strongly modifies the electronic structure of the flat bands. A better understanding of the effects of the atomic structure of moiré patterns is therefore essential. Indeed, these bilayers are complex systems whose crystallographic properties are driven by a very large moiré cell compared to the single-layer crystal cell, inducing quantum interference at the moiré-cell scale that is still poorly understood. Thus, this new and exciting physics connects in many respects recent studies in the crystallography of moiré systems (bicrystals and quasicrystals). Our project proposes to explore new avenues opened by the control of crystallographic properties and the manipulation of important parameters such as rotation angle and strain in homogeneous or heterogeneous bilayers of graphene and/or semiconducting transition metal dichalcogenides. The aim is to better understand the origin and characteristics of electronic localization by strengthening the interaction between crystallography and solid-state physics research. The origin of electronic localization is an apparently simple question but still poorly understood. Answering it, even partially, would allow to understand better the relevant parameters that control the localization. For this purpose, it is essential to combine very different theoretical and experimental approaches to study the crystallographic structure and electronic properties. The consortium formed here gathers theoretical (LPTM, NEEL, IRCP) and experimental (PHILIQS, LPMS) French actors with complementary expertise (STM & photoemission measurements, crystallography, DFT & tight-binding methods, magnetism, quantum transport), essential to carry out such a study at the forefront of the international competition. A first major challenge is to control the deformations due to strain. For this, one of the partners (PHELIQS) will build uniaxial strain cells in order to create a one-dimensional moiré. The samples will be studied by the two experimental partners and the results will be compared with the electronic-structure calculations. The second challenge will be to continue the study of moirés formed by rotated bilayers, a topic where three partners (LPTM, NEEL, PHELIQS) are already recognized actors. We will conduct a systematic study of the combined effects of electronic correlations, stresses, and structural defects on electronic properties such as quantum transport in metallic and/or semiconductor bilayers. We will use realistic structures, deduced from experiments and analyzed by the tools of crystallography. The third issue will be a better understanding of the underlying bicrystallography, in particular the mathematical developments necessary to understand the limit of very small rotation angles.