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arXiv 2201.10789cond-mat.mes-hall
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无磁场作用下$C_4$对称扭曲双层膜系统的Moiré Landau能级

Moiré Landau levels of a $C_4$-symmetric twisted bilayer system in the absence of a magnetic field

Y. Soeda, K. Asaga, T. Fukui

摘要:

众所周知,扭曲双层石墨烯(TBG)在魔角处显示出平坦带,这可以用Bistritzer和MacDonald(BM)推导的有效连续模型很好地描述。本文提出了一个类似的扭曲双层系统,但定义在正方形格子上,每个格元具有$π$ -通量,并使用BM哈密顿量研究了它的谱,其中质量项源于交错势。TBG和本模型之间的基本区别是简单的旋转对称性,$C_3$ vs $C_4$,以及质量项。然而,平坦带的特征是完全不同的:TBG的平坦带仅出现在魔角,而本模型显示出许多平坦带,这让人想起朗道能级,即使在没有磁场的情况下,除了保持时间反演(TR)对称性的$π$通量之外,在任何角度都非常稳定。此外,在狄拉克谱的质量隙中出现平坦带,并且构成这些平坦带的每个状态都很好地定域在形成莫尔晶格的位置。结果表明,莫尔势作为一个周期性的磁场,它可以提供比莫尔晶格位置周围的间隙更小的能量。我们推导出一个有效的局部哈密顿周围的莫尔晶格网站,并表明,它确实再现了质量间隙内的平带的能量。由于这些中间带隙态位于莫尔晶格中,因此它们形成简并能级,其可以被称为莫尔朗道能级,尽管简并的机制与传统的朗道能级不同。有趣的是,与两层相关联的BH哈密顿量的双费米子在与有效莫尔磁场耦合时具有相反的电荷,这涉及TR对称性。

英文摘要:

It is widely known that the twisted bilayer graphene (TBG) shows flat bands at magic angles, which can be well described by the effective continuum model derived by Bistritzer and MacDonald (BM). We propose in this paper a similar twisted bilayer system but defined on the square lattice with $π$-flux per plaquette, and study its spectrum using the BM Hamiltonian with a mass term which is originated from the staggered potential. The basic difference between the TBG and the present model is simply rotational symmetry, $C_3$ vs $C_4$, as well as a mass term. Nevertheless, the feature of the flat bands is quite different: Those of the TBG appear at magic angles only, while the present model shows many flat bands, which are reminiscent of Landau levels, quite stably at any angles even in the absence of a magnetic field other than $π$-flux which keeps time reversal (TR) symmetry. Moreover, flat bands emerge in the mass gap of the Dirac spectrum, and each state composing these flat bands is well-localized at the position forming the moiré lattice. It turns out that the moiré potential serves as a periodic magnetic field, which can give energies smaller that the gap around moiré lattice positions. We derive a local Hamiltonian valid around the moiré lattice sites and show that it indeed reproduces the energies of the flat bands within the mass gap. Since these mid-gap states are localized at the moiré lattice, they form degenerate levels, which may be referred to as moiré Landau levels, although the mechanism of degeneracies are different from the conventional Landau levels. Interestingly, doubled fermions of the BH Hamiltonian associated with two layers have opposite charges when they couple with the effective moiré magnetic filed, which concern TR symmetry.

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