Insulators at fractional fillings in twisted bilayer graphene partially aligned to hexagonal boron nitride
Insulators at fractional fillings in twisted bilayer graphene partially aligned to hexagonal boron nitride
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DOI：
10.1063/10.0019422
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发表时间：
2023-03
影响因子：
0.8
通讯作者：
Dillon Wong;Kevin P. Nuckolls;Myungchul Oh;Ryan L. Lee;Kenji Watanabe;T. Taniguchi;A. Yazdani
中科院分区：
文献类型：
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作者：
Dillon Wong;Kevin P. Nuckolls;Myungchul Oh;Ryan L. Lee;Kenji Watanabe;T. Taniguchi;A. Yazdani
魔角扭曲双层石墨烯（MATBG）在其平坦电子能带的部分填充处具有丰富多样的竞争相关相，这些相显示出样品间的变化。MATBG中的发散相图通常归因于亚晶格极化能量标度，其通过通常用于货车德瓦耳斯器件中的六方氮化硼（hBN）衬底的对准程度来调谐。未对齐的MATBG表现出非常规的超导体和相关的绝缘体相，而几乎完美对齐的MATBG/hBN表现出零场陈氏绝缘相，缺乏超导性。在这里，我们使用扫描隧道显微镜和光谱学（STM/STS），以观察在一个新的中间制度的亚晶格极化的MATBG的平带的部分填充的间隙相，观察时，MATBG仅部分对齐（θGr-hBN <$1.65 °）到下面的hBN基板。在这种条件下，MATBG主机不仅自然插入两个亚晶格势的限制之间的现象，但也意外的间隙相位不存在于这些限制。在电荷中性时，我们观察到一个具有小能隙（Δ < 5 meV）的绝缘相，这可能与hBN衬底的弱亚晶格对称性破缺有关。此外，我们观察到新的带隙相附近分数填充ν = ±1/3和ν = ±1/6，这是以前没有观察到的MATBG。重要的是，能量分辨STS明确地识别出这些分数填充态是单粒子起源的，可能是由两个莫尔超晶格形成的超超晶格的结果。我们的观察强调的权力STS在区分单粒子间隙阶段从多体间隙阶段的情况下，可以很容易混淆的电输运测量，并展示了使用基板工程修改的莫尔平带材料的电子结构。
At partial fillings of its flat electronic bands, magic-angle twisted bilayer graphene (MATBG) hosts a rich variety of competing correlated phases that show sample-to-sample variations. Divergent phase diagrams in MATBG are often attributed to the sublattice polarization energy scale, tuned by the degree of alignment of the hexagonal boron nitride (hBN) substrates typically used in van der Waals devices. Unaligned MATBG exhibits unconventional superconductor and correlated insulator phases, while nearly perfectly aligned MATBG/hBN exhibits zero-field Chern insulating phases and lacks superconductivity. Here we use scanning tunneling microscopy and spectroscopy (STM/STS) to observe gapped phases at partial fillings of the flat bands of MATBG in a new intermediate regime of sublattice polarization, observed when MATBG is only partially aligned (θGr-hBN ≈ 1.65°) to the underlying hBN substrate. Under this condition, MATBG hosts not only phenomena that naturally interpolate between the two sublattice potential limits, but also unexpected gapped phases absent in either of these limits. At charge neutrality, we observe an insulating phase with a small energy gap (Δ < 5 meV) likely related to weak sublattice symmetry breaking from the hBN substrate. In addition, we observe new gapped phases near fractional fillings ν = ±1/3 and ν = ±1/6, which have not been previously observed in MATBG. Importantly, energy-resolved STS unambiguously identifies these fractional filling states to be of single-particle origin, possibly a result of the super-superlattice formed by two moiré superlattices. Our observations emphasize the power of STS in distinguishing single-particle gapped phases from many-body gapped phases in situations that could be easily confused in electrical transport measurements, and demonstrate the use of substrate engineering for modifying the electronic structure of a moiré flat-band material.