Nematicity with a twist: Rotational symmetry breaking in a moire superlattice
Nematicity with a twist: Rotational symmetry breaking in a moire superlattice
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DOI：
10.1126/sciadv.aba8834
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发表时间：
2020-08-01
期刊：
影响因子：
13.6
通讯作者：
Venderbos, Jorn W. F.
中科院分区：
文献类型：
--
作者：
Fernandes, Rafael M.;Venderbos, Jorn W. F.
受最近关于扭曲双层石墨烯（TBG）中向列序的报道的启发，我们研究了三角形莫尔超晶格自由度对向列性的影响。在 TBG 中，向列有序参数不像四方晶体中的 Ising 那样，而是具有与莫尔超晶格的三重旋转对称性 (C-3z) 相关的三态 Potts 特征。我们发现，即使存在明显破坏 C-3z 对称性的静态应变，系统仍然可以经历向列翻转相变，自发地破坏面内双重旋转。此外，表现为声声子的弹性波动介导向列轨道耦合，将向列指向矢方向与动量空间中的某些软方向联系起来，从而呈现波茨向列转变平均场和一阶。与刚性晶体的情况相反，与这些软方向相关的费米表面热点最大限度地与莫尔超晶格情况中的低能向列波动耦合。
Motivated by recent reports of nematic order in twisted bilayer graphene (TBG), we investigate the impact of the triangular moire superlattice degrees of freedom on nematicity. In TBG, the nematic order parameter is not Ising like, as in tetragonal crystals, but has a three-state Potts character related to the threefold rotational symmetry (C-3z) of the moire superlattice. We find that, even in the presence of static strain that explicitly breaks the C-3z symmetry, the system can still undergo a nematic-flop phase transition that spontaneously breaks in-plane twofold rotations. Moreover, elastic fluctuations, manifested as acoustic phonons, mediate a nemato-orbital coupling that ties the nematic director orientation to certain soft directions in momentum space, rendering the Pottsnematic transition mean field and first order. In contrast to the case of rigid crystals, the Fermi surface hot spots associated with these soft directions are maximally coupled to low-energy nematic fluctuations in the moire superlattice case.