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Mapping the twist-angle disorder and Landau levels in magic-angle graphene

机译：映射魔角石墨烯中的扭曲角紊乱和Landau能级

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摘要

The recently discovered flat electronic bands and strongly correlated and superconducting phases in magic-angle twisted bilayer graphene (MATBG)(1,2) crucially depend on the interlayer twist angle, theta. Although control of the global theta with a precision of about 0.1 degrees has been demonstrated(1-7), little information is available on the distribution of the local twist angles. Here we use a nanoscale on-tip scanning superconducting quantum interference device (SQUID-on-tip)(8) to obtain tomographic images of the Landau levels in the quantum Hall state(9) and to map the local theta variations in hexagonal boron nitride (hBN)-encapsulated MATBG devices with relative precision better than 0.002 degrees and a spatial resolution of a few moire periods. We find a correlation between the degree of theta disorder and the quality of the MATBG transport characteristics and show that even state-of-the-art devices-which exhibit correlated states, Landau fans and superconductivity-display considerable local variation in theta of up to 0.1 degrees, exhibiting substantial gradients and networks of jumps, and may contain areas with no local MATBG behaviour. We observe that the correlated states in MATBG are particularly fragile with respect to the twist-angle disorder. We also show that the gradients of theta generate large gate-tunable in-plane electric fields, unscreened even in the metallic regions, which profoundly alter the quantum Hall state by forming edge channels in the bulk of the sample and may affect the phase diagram of the correlated and superconducting states. We thus establish the importance of theta disorder as an unconventional type of disorder enabling the use of twist-angle gradients for bandstructure engineering, for realization of correlated phenomena and for gate-tunable built-in planar electric fields for device applications.SQUID-on-tip tomographic imaging of Landau levels in magic-angle graphene provides nanoscale maps of local twist-angle disorder and shows that its properties are fundamentally different from common types of disorder.
机译：最近发现的扁平电子带以及魔术角扭曲双层石墨烯（MATBG）（1,2）中的强相关和超导相关键取决于层间扭曲角theta。尽管已经证明以约0.1度的精度控制全局θ（1-7），但是关于局部扭曲角分布的信息很少。在这里，我们使用纳米级尖端扫描超导量子干涉装置（SQUID-on-tip）（8）获得处于量子霍尔态的Landau能级的断层图像（9）并绘制六方氮化硼中的局部θ变化图（hBN）封装的MATBG器件，其相对精度优于0.002度，并且空间分辨率为几个莫尔周期。我们发现theta紊乱程度与MATBG传输特性的质量之间存在相关性，并表明，即使是显示相关状态，Landau风扇和超导性的最先进的设备，也显示出高达θ的相当大的局部变化。 0.1度，具有较大的坡度和跳跃网络，并且可能包含没有本地MATBG行为的区域。我们观察到MATBG中的相关状态相对于扭曲角异常特别脆弱。我们还表明，θ的梯度会产生大的门可调平面内电场，即使在金属区域也不会被屏蔽，通过在样品的大部分中形成边缘通道来深刻改变量子霍尔态，并且可能会影响相图相关和超导状态。因此，我们确立了theta紊乱作为一种非常规类型的紊乱的重要性，从而可以将扭曲角梯度用于带状结构工程，实现相关现象以及用于设备应用的门可调内置平面电场。魔术角石墨烯中Landau的尖端X线断层扫描成像提供了局部扭曲角疾病的纳米级图，并显示了其性质与常见类型的疾病根本不同。

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  • 来源
    《Nature》 |2020年第7806期|47-52|共6页
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    Weizmann Inst Sci Dept Condensed Matter Phys Rehovot Israel|Osaka Univ Dept Phys Toyonaka Osaka Japan;

    Weizmann Inst Sci Dept Condensed Matter Phys Rehovot Israel;

    MIT Dept Phys Cambridge MA 02139 USA;

    NYU Shanghai Div Arts & Sci Shanghai Peoples R China|NYU Shanghai NYU ECNU Inst Phys Shanghai Peoples R China;

    Natl Inst Mat Sci Tsukuba Ibaraki Japan;

    NYU Shanghai Div Arts & Sci Shanghai Peoples R China|NYU Shanghai NYU ECNU Inst Phys Shanghai Peoples R China|NYU Dept Phys 4 Washington Pl New York NY 10003 USA|East China Normal Univ State Key Lab Precis Spect Shanghai Peoples R China;

    Osaka Univ Dept Phys Toyonaka Osaka Japan;

  • 收录信息 美国《科学引文索引》(SCI);美国《工程索引》(EI);美国《生物学医学文摘》(MEDLINE);美国《化学文摘》(CA);
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