Quantum Materials by Twistronics

Two-dimensional materials (2DM), derived from bulk layered crystals with covalent intra-layer bonding and weak van der Waals (vdW) interlayer coupling, offer a versatile playground for creating quantum materials with properties tailored for particular applications. This is achieved by combining different atomically thin 2DM crystals into heterostructures layer-by-layer in a chosen sequence. Unlike conventional crystal growth, this technique is not limited by lattice matching or interface chemistry, hence, it enables us to build heterostructures from several dozens of readily available vdW crystals with diverse physical properties (electronic, optical or magnetic). This platform offers broadly acknowledged potential for the realisation of nano-devices and designer meta-materials with new properties and functionalities determined by the coupling of adjacent layers, including interlayer band hybridisation and strong proximity effects. A new degree of freedom for controlling the properties of vdW heterostructures is the mutual crystal rotation - twist - of the constituent 2D crystals. Together with the lattice mismatch of the adjacent 2D crystals it gives rise to the moiré superlattice (mSL): a periodic variation of the local atomic registry, with the period controlled by the twist angle. Even a small twist can lead to remarkable changes in the properties of heterostructures - for instance, in homobilayers of 2DM it leads to strong spectrum reconstruction and formation of electron and hole minibands. So far, the breakthrough studies of moiré superlattices have been focused on graphene heterostructures with hexagonal boron nitride and on twisted graphene bilayers. Recently, initial exploration of twisted layers of transition metal dichalcogenides have begun, featuring four letters in a single issue of Nature in March 2019 (in one of those the members of this consortium have reported moire minibands for excitons). Not surprisingly, these recent developments have fuelled a world-wide race to develop this new field of materials science and solid state physics, branded as 'twistronics'. This project will pioneer the new scientific area of twistronics in novel types of 2DM heterostructures, mapping out the limits to which one can control their properties through the interlayer proximity and moiré superlattice effects. Using this approach, we aim to engineer flat electronic bands in semiconducting 2DM heterostructures, promoting quantum many-body effects, which we will explore through quantum transport and optical studies. Furthermore, we will realise the world-first twisted bilayers of new emerging 2DMs that exhibit strongly correlated states in their natural form ((anti)ferromagnetic, charge-density waves, or superconductivity) and explore novel physics in those system with an outlook for practical applications. In all material combinations, we will look into two distinct cases of (1) intermediate twist angles, where lattices are expected to behave as rigid solids, producing smooth variation in interlayer registry and (2) small twist angles where we have recently found that twisted 2D materials reconstruct to form extended commensurate domains separated by stacking faults. To achieve the ambitious and game-changing goals of this proposal, the consortium will employ a recently commissioned world-first nanofabrication facility, which allows assembly of van der Waals heterostructures in ultra-high vacuum. This unique instrument will provide the game-changing quality materials necessary for this project. Funding of this proposal will allow us to fully employ the potential of this new instrument and deliver ground-breaking new research and disruptive technologies.