Probing local electronic structure in 2D heterostructures

从半金属石墨烯开始，二维材料（2DM）库迅速扩展到包括金属，半导体，绝缘体等。强相关二维材料正在成为一个迷人的研究领域，有报道称二维超导体，磁体甚至二维拓扑绝缘体（2D TI）。与此相关，2D中的相变与三维中的相变有着根本的不同，这导致了新的物理学的探索。随着2017年诺贝尔物理学奖因物质拓扑相变和拓扑相的研究而获得，以及二维模型的持续兴起，这凸显了令人兴奋的新研究途径。简单地堆叠不同的2DM以产生具有新功能特性的复杂异质结构的能力不仅增加了器件设计的重要自由度，而且也为实现真实的技术进步带来了巨大希望。因此，探测2D材料的局域电子结构和特性是一个突出的挑战和机遇。2D堆栈通常只有几微米宽，需要本地探针。我们已经表明，角分辨光电子能谱与亚微米空间分辨率（nanoARPES）是能够直接测量这些堆栈中的层相关的电子结构。这开辟了新的研究途径，使我们能够直接询问相互作用现象，如莫尔效应和重正化。为了补充nanoARPES实验，扫描探针显微镜（SPM）可以在一系列环境条件下以纳米精度解析地形，电子，铁磁和磁性结构。对于2DM，这使得研究局部电子和铁电性质如何随着层数而变化，区分边缘和“体积”效应，并探测空间不均匀性。例如，通过2D TI，SPM将能够探测体绝缘和边缘导电状态，并检查相变时的空间相关性。因此，该项目是一个令人兴奋的机会，利用新技术研究新材料中的新物理。主要重点将是应用高分辨率光电发射和SPM技术来研究纳米级二维材料的电子和功能特性。这将利用沃里克独特的SPM设施，用于低温（基础温度< 2 K）原子力显微镜，具有矢量磁场（9x1x1 T）和光学通道，用于时间分辨的光依赖测量，并辅之以新的最先进的环境AFM。虽然一些2D异质结构样品将在内部制造，但其他样品将来自我们的国际合作者。

Starting with the semi-metallic graphene, the library of two-dimensional materials (2DMs) has expanded rapidly to include metals, semiconductors, insulators and more. Strongly correlated 2D materials are emerging as a fascinating area of research, with reports of 2D superconductors, magnets and even 2D topological insulators (2D TIs). Associated with this, phase transitions in 2D are fundamentally different from in three dimensions, leading to new Physics to explore. With the award of the Nobel Prize in Physics 2017 for the study of topological phase transitions and topological phases of matter, and the continued rise of 2DMs, this highlights exciting new avenues of research. The ability to simply stack different 2DMs to create complex heterostructures with new functional properties adds not only important degrees of freedom in designing devices, but also great hope for achieving real technological advances.Probing the local electronic structure and properties of 2D materials is thus an outstanding challenge and opportunity. The 2D stacks are typically only a few micrometres across, necessitating local probes. We have shown that angle resolved photoemission spectroscopy with submicrometre spatial resolution (nanoARPES) is capable of directly measuring the layer dependent electronic structure in these stacks. This opens new avenues of research, allowing us to directly interrogate interaction phenomena such as moire effects and renormalisation. To complement the nanoARPES experiments, scanning probe microscopy (SPM) can resolve topographic, electronic, ferroic and magnetic structure with nanometre precision and under a range of environmental conditions. For 2DM, this enables studying how local electronic and ferroic properties change with the number of layers, distinguishing edge and 'bulk' effects, and probing spatial inhomogeneities. For example, with a 2D TI, SPM will be able to probe the bulk insulating and edge conducting states and examine spatial correlations at the phase transition.This project is thus an exciting opportunity to study new Physics in new materials using new technology. The primary focus will be to apply high-resolution photoemission and SPM techniques to study electronic and functional properties of 2D materials at the nanoscale. This will capitalise on the unique SPM facility at Warwick for low temperature (base temperature < 2 K) atomic force microscopy with vectoral magnetic field (9x1x1 T) and optical access for time-resolved photodependent measurements, complemented by a new state-of-the-art ambient AFM. Although some of the 2D heterostructure samples will be fabricated in house, others will be sourced from our international collaborators.