Quantum Materials by Twistronics

由具有共价层内键和弱范德华(VDW)层间耦合的大块层状晶体衍生而来的二维材料(2DM)为创造具有特定应用特性的量子材料提供了一个多功能的游乐场。这是通过将不同的原子薄的2 DM晶体按选定的顺序逐层组合成异质结构来实现的。与传统的晶体生长不同，这种技术不受晶格匹配或界面化学的限制，因此，它使我们能够从几十种具有不同物理性质(电、光或磁)的现成VDW晶体中构建异质结构。这一平台为实现纳米器件和设计者超材料提供了广泛的潜力，具有由相邻层之间的耦合决定的新的特性和功能，包括层间能带杂化和强邻近效应。控制VDW异质结构特性的新自由度是组成2D晶体的相互晶体旋转-扭曲。再加上相邻2D晶体的晶格失配，产生了莫尔超晶格(MSL)：局域原子注册表的周期性变化，周期由扭角控制。即使是一个很小的扭曲也可以导致异质结构性质的显著变化-例如，在2 DM的同质双层中，它导致了强烈的光谱重建和电子和空穴微带的形成。到目前为止，莫尔超晶格的突破性研究主要集中在六方氮化硼的石墨烯异质结构和扭曲的石墨烯双层结构上。最近，对过渡金属二卤代化合物扭曲层的初步探索已经开始，在2019年3月的一期《自然》杂志上刊登了四封信(该联盟的成员在其中一期中报告了激子的莫尔微带)。毫不奇怪，这些最新的发展推动了一场世界范围内的竞赛，以发展这一新的材料科学和固体物理学领域，被称为“双电子学”。该项目将在新型2 DM异质结构中开创双电子科学的新领域，绘制出通过层间邻近效应和莫尔超晶格效应来控制其性质的极限。使用这种方法，我们的目标是在半导体2 DM异质结构中设计平坦的电子能带，促进量子多体效应，我们将通过量子输运和光学研究来探索这一点。此外，我们将实现世界上第一个以自然形式((反)铁磁、电荷密度波或超导)表现出强关联状态的新出现的2DM的扭曲双层，并探索这些具有实际应用前景的系统中的新物理。在所有的材料组合中，我们将研究两种不同的情况：(1)中等扭角，其中晶格预期表现为刚性固体，在层间注册中产生平滑的变化；(2)小扭角，我们最近发现扭曲的2D材料重建形成由堆叠断层隔开的扩展的公度域。为了实现这项提议的雄心勃勃和改变游戏规则的目标，该财团将采用最近投入使用的世界首个纳米制造设施，该设施允许在超高真空中组装范德华异质结构。这一独特的仪器将提供这个项目所需的改变游戏规则的高质量材料。这项提议的资金将使我们能够充分利用这一新工具的潜力，并提供突破性的新研究和颠覆性技术。

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.