Kamat Rupini V, Sharpe Aaron L, Pendharkar Mihir, Hu Jenny, Tran Steven J, Zaborski Gregory, Hocking Marisa, Finney Joe, Watanabe Kenji, Taniguchi Takashi, Kastner Marc A, Mannix Andrew J, Heinz Tony, Goldhaber-Gordon David
Department of Physics, Stanford University, Stanford, CA 94305.
Stanford Institute for Materials and Energy Science, SLAC National Accelerator Laboratory, Menlo Park, CA 94025.
Proc Natl Acad Sci U S A. 2024 Oct;121(40):e2410993121. doi: 10.1073/pnas.2410993121. Epub 2024 Sep 27.
The electronic properties of moiré heterostructures depend sensitively on the relative orientation between layers of the stack. For example, near-magic-angle twisted bilayer graphene (TBG) commonly shows superconductivity, yet a TBG sample with one of the graphene layers rotationally aligned to a hexagonal Boron Nitride (hBN) cladding layer provided experimental observation of orbital ferromagnetism. To create samples with aligned graphene/hBN, researchers often align edges of exfoliated flakes that appear straight in optical micrographs. However, graphene or hBN can cleave along either zig-zag or armchair lattice directions, introducing a [Formula: see text] ambiguity in the relative orientation of two flakes. By characterizing the crystal lattice orientation of exfoliated flakes prior to stacking using Raman and second-harmonic generation for graphene and hBN, respectively, we unambiguously align monolayer graphene to hBN at a near-[Formula: see text], not [Formula: see text], relative twist angle. We confirm this alignment by torsional force microscopy of the graphene/hBN moiré on an open-face stack, and then by cryogenic transport measurements, after full encapsulation with a second, nonaligned hBN layer. This work demonstrates a key step toward systematically exploring the effects of the relative twist angle between dissimilar materials within moiré heterostructures.
莫尔异质结构的电子特性敏感地取决于堆叠层之间的相对取向。例如，近魔角扭曲双层石墨烯（TBG）通常表现出超导性，然而，其中一个石墨烯层与六方氮化硼（hBN）包层旋转对齐的TBG样品提供了轨道铁磁性的实验观察结果。为了制备具有对齐的石墨烯/hBN的样品，研究人员通常会对齐在光学显微镜下看起来是直边的剥离薄片的边缘。然而，石墨烯或hBN可以沿着之字形或扶手椅形晶格方向裂开，在两个薄片的相对取向上引入了[公式：见原文]的模糊性。通过分别使用拉曼光谱和二次谐波产生来表征堆叠前剥离薄片的晶格取向，其中拉曼光谱用于石墨烯，二次谐波产生用于hBN，我们在接近[公式：见原文]而非[公式：见原文]的相对扭转角下将单层石墨烯与hBN明确对齐。我们通过对开放面堆叠上的石墨烯/hBN莫尔进行扭转力显微镜观察来确认这种对齐，然后在用第二个未对齐的hBN层完全封装后通过低温输运测量来确认。这项工作展示了朝着系统探索莫尔异质结构中不同材料之间相对扭转角的影响迈出的关键一步。
