Chern bands of twisted bilayer graphene: Fractional Chern insulators and spin phase transition
Chern bands of twisted bilayer graphene: Fractional Chern insulators and spin phase transition
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DOI：
10.1103/physrevresearch.2.023238
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发表时间：
2020-05-27
影响因子：
4.2
通讯作者：
Senthil, T.
中科院分区：
文献类型：
--
作者：
Repellin, Cecile;Senthil, T.
当魔角扭曲双层石墨烯的其中一个石墨烯层几乎与其六方氮化硼基底（一种称为 TBG/hBN 的配置）对齐时，活性电子能带几乎平坦，并且陈数 C = +/- 1。最近的实验证明了该系统中导带整数填充 V-T = 3 时的量子反常霍尔效应和自发谷极化。受这一发现的启发，我们询问分数量子反常霍尔态 (FQAH) 是否也可能出现在 TBG/hBN 中。我们重点关注通过实验观察到谷铁磁性的填充分数范围。使用精确对角化，我们发现 V-T = 10/3 和 V-T = 17/5 处的基态是平带极限中的分数陈绝缘体态（在孔图中，这些是 FQH 分数 2/3 和 3/5）。基态要么是自旋极化，要么是自旋单线态，具体取决于能带参数。对于名义上实际的能带参数，自旋极化是有利的。通过改变能带参数来展平贝里曲率让位于自旋单线态相。我们对平带极限中电荷间隙的估计表明，FQAH 状态可能在实验中可达到的温度下出现。我们还研究了非零带宽的影响，并表明存在一个合理的参数范围，其中 FQAH 状态是基态。
When one of the graphene layers of magic-angle twisted bilayer graphene is nearly aligned with its hexagonal boron nitride substrate (a configuration dubbed TBG/hBN), the active electronic bands are nearly flat and have a Chern number C = +/- 1. Recent experiments demonstrated a quantum anomalous Hall effect and spontaneous valley polarization at integer filling V-T = 3 of the conduction band in this system. Motivated by this discovery, we ask whether fractional quantum anomalous Hall states (FQAHs) could also emerge in TBG/hBN. We focus on the range of filling fractions where valley ferromagnetism was observed experimentally. Using exact diagonalization, we find that the ground states at V-T = 10/3 and V-T = 17/5 are fractional Chern insulator states in the flatband limit (in the hole picture, these are the FQH fractions 2/3and 3/5). The ground state is either spin polarized or a spin singlet, depending sensitively on band parameters. For nominally realistic band parameters, spin polarization is favored. Flattening the Berry curvature by changing a band parameter gives way to the spin singlet phase. Our estimation of the charge gap in the flatband limit shows that the FQAH state may be seen at accessible temperatures in experiments. We also study the effect of a nonzero bandwidth and show that there is a reasonable range of parameters in which the FQAH state is the ground state.