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arXiv 2205.06830cond-mat.mes-hallcond-mat.other
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扭曲多层石墨烯晶格振动模的对称性起源:相位子与莫尔声子

Symmetry Origin of Lattice Vibration Modes in Twisted Multilayer Graphene: Phasons vs Moiré Phonons

Qiang Gao, Eslam Khalaf

摘要:

晶格动力学在莫尔系统的物理学中起着至关重要的作用。在扭曲双层石墨烯(TBG)中,除了石墨烯声子之外,还存在另一组称为Moiré Phonons的无间隙激发[Phys. Rev. B,075416,2019],反映了Moire超晶格水平的晶格动力学。由于两个石墨烯层的不相称堆叠,这些模式后来被认为是相子[Phys. Rev. B,155426,2019]。在这项工作中,我们阐明了这两个看似不同的观点的等价性,确定一个潜在的对称性,我们称之为失配对称性,存在于任何扭曲角。对于公度角,这是离散对称,而对于非公度角,它相当于产生相子模式的连续相位对称。在小角度极限下,这种对称性变成连续的局部对称性,其自发破缺产生莫尔声子作为其戈德斯通模式。我们推导出一个有效场论,这些集体模式在TBG中精确协议的完整模型,并讨论了它们的不同性质。我们的分析,然后推广到扭曲的多层石墨烯(TMG),我们确定了高阶失配,推导出无隙模式,包括石墨烯声子,莫尔声子和相位子的计数。特别地,我们研究了具有交替扭曲角$θ$的扭曲镜像对称三层石墨烯,并发现它可以映射到具有重新缩放的扭曲角$\sqrt{2/3}θ$的TBG,在偶数镜像扇区中具有相同的Moiré声子模式,在奇数扇区中具有额外的一组带隙模式。我们的工作提出了一个系统的研究晶格对称性TMG提供其独特的晶格动力学的见解。

英文摘要:

Lattice dynamics play a crucial role in the physics of Moiré systems. In twisted bilayer graphene (TBG), it was shown that, in addition to the graphene phonons, there is another set of gapless excitations termed Moiré Phonons [Phys. Rev. B, 075416, 2019] reflecting the lattice dynamics at the Moire superlattice level. These modes were later suggested to be phasons due to the incommensurate stacking of the two graphene layers [Phys. Rev. B, 155426, 2019]. In this work, we elucidate the equivalence of these two seemingly distinct perspectives by identifying an underlying symmetry, which we dub mismatch symmetry, that exists for any twist angle. For commensurate angles, this is a discrete symmetry whereas for incommensurate angles, it is equivalent to a continuous phase symmetry giving rise to phason modes. In the small angle limit, such symmetry becomes a continuous local symmetry whose spontaneous breaking gives rise to Moiré phonons as its Goldstone mode. We derive an effective field theory for these collective modes in TBG in precise agreement with the full model and discuss their different properties. Our analysis is then generalized to twisted multilayer graphene (TMG) where we identify higher order mismatch and deduce the count of gapless modes including graphene phonons, Moiré phonons and phasons. Especially, we study twisted mirror-symmetric trilayer graphene with an alternating twist angle $θ$ and find that it can be mapped to a TBG with the re-scaled twist angle $\sqrt{2/3}θ$, hosting the same Moiré phonon modes in the even mirror sector with an additional set of gapped modes in the odd sector. Our work presents a systematic study of lattice symmetries in TMG providing insights into its unique lattice dynamics.

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