Ball Philip
NSR from London.
Natl Sci Rev. 2022 Jan 13;9(4):nwac005. doi: 10.1093/nsr/nwac005. eCollection 2022 Apr.
Graphene is the building block of graphite, made of carbon atoms bonded into sheets of hexagonal rings just a single atom thick. Although such isolated sheets had been predicted for many decades to exist, and had been grown on other surfaces, interest in this material exploded after the discovery in 2004 that single sheets could be made easily and cheaply by separating them mechanically from graphite flakes (a process called exfoliation). Although graphene is often advertised as a 'wonder material'-electronically conducting, transparent and extremely strong and flexible-much of the interest in it is more fundamental. As a 2D conductor, graphene shows unusual electronic and magnetic properties that enable the study of quantum-mechanical effects of confinement and of correlations between electron motions-some of which might find applications in electronic devices. The excitement of this discovery was reflected in the award of the 2010 Nobel Prize in Physics to two pioneers in the field: Andre Geim and Konstantin Novoselov of the University of Manchester in the UK. This rich behavior is broadened still further when two graphene sheets are brought close enough to interact with one another. In particular, the electronic properties may then depend on the relative orientation of the sheets: how aligned the two 'honeycomb' lattices are. Two grids superimposed on one another may create 'superlattices': regularities at larger scales than the grid spacing, due to registry (commensurability) between the two at certain angles. This so-called moiré effect is sometimes evident for two closely spaced grid-like fences seen from afar. Experimentally exploring the electronic properties of such 'twisted bilayer graphene' requires an ability to precisely control the position and orientation of the two sheets. These phenomena are now recognized as generic to other 2D materials, such as hexagonal sheets of boron nitride. They have revealed a fertile playground for condensed-matter physics. In particular, striking electronic properties appear at certain 'magic-angle' twists of the layers. NSR spoke to two of the leading experts in the study of magic-angle twisted bilayer graphene (MATBG): experimentalist Pablo Jarillo-Herrero of the Massachusetts Institute of Technology and theorist Allan MacDonald of the University of Texas at Austin.
石墨烯是石墨的基本组成部分，由碳原子键合形成仅一个原子厚的六边形环片层。尽管这种孤立的片层早在几十年前就被预测存在，并且已在其他表面生长，但在2004年发现可以通过从石墨薄片中机械分离（一种称为剥离的过程）轻松且廉价地制备单一片层后，对这种材料的兴趣激增。尽管石墨烯常被宣传为一种“神奇材料”——具有导电、透明以及极强且灵活的特性，但对它的许多兴趣更多是基于其基础性。作为一种二维导体，石墨烯展现出不同寻常的电子和磁学性质，这使得人们能够研究限制的量子力学效应以及电子运动之间的相关性——其中一些可能会在电子设备中得到应用。这一发现的轰动效应体现在2010年诺贝尔物理学奖授予该领域的两位先驱：英国曼彻斯特大学的安德烈·海姆（Andre Geim）和康斯坦丁·诺沃谢洛夫（Konstantin Novoselov）。当两个石墨烯片层彼此靠近到足以相互作用时，这种丰富的行为会进一步扩展。特别是，电子性质可能随后取决于片层的相对取向：两个“蜂窝”晶格的对齐程度。两个相互叠加的网格可能会形成“超晶格”：由于在特定角度下两者的配准（可公度性），在比网格间距更大的尺度上出现规则性。从远处看，这种所谓的莫尔效应有时在两个间距很近的类似网格的围栏上很明显。通过实验探索这种“扭曲双层石墨烯”的电子性质需要能够精确控制两个片层的位置和取向。现在人们认识到这些现象对于其他二维材料，如六方氮化硼片层来说是普遍存在的。它们为凝聚态物理揭示了一个富有成果的研究领域。特别是，在层的某些“魔角”扭曲处会出现引人注目的电子性质。《国家科学评论》（NSR）采访了研究魔角扭曲双层石墨烯（MATBG）的两位顶尖专家：麻省理工学院的实验物理学家巴勃罗·贾里洛 - 埃雷罗（Pablo Jarillo - Herrero）和德克萨斯大学奥斯汀分校的理论物理学家艾伦·麦克唐纳（Allan MacDonald）。
