魔角扭曲双层石墨烯中的配对：声子和等离子体激元umklapp的作用
Pairing in magic-angle twisted bilayer graphene: role of phonon and plasmon umklapp
摘要：
确定魔角扭曲双层石墨烯（MATBG）超导电性的微观机制是一个悬而未决的问题。虽然MATBG表现出丰富的相图，驱动部分由相对于电子带宽的强相互作用，其单粒子性质是独特的，可能在一些现象学的复杂性中发挥重要作用。一些显着的特点包括电子带宽小于特征声子带宽和一个非平凡的基础布洛赫波函数的结构。我们进行了理论研究的合作效应，由于声子和等离子体对配对，以解开这些模式对超导性所发挥的独特作用。我们考虑一个变种的MATBG与扩大数量的费米子口味，$N \gg 1$，配对不稳定性的研究减少到传统的（弱耦合）Eliashberg框架。特别是，我们表明，某些umklapp过程涉及微型光学声子模式，这是物理上出现的结果，由于叠栅超晶格结构的原始声学分支的石墨烯的折叠，有助于显着提高配对。我们还研究了屏蔽库仑相互作用对配对的动力学所起的作用，这导致了填充物在窄窗口中的增强，并研究了由于金属栅极对超导电性的外部屏蔽的影响。在强耦合下，动力学对相互作用在单粒子隧穿态密度中留下谱标记。因此，我们预测这样的功能将出现在特定频率的umklapp声子对应的声速的石墨烯倍的布里渊区大小的整数倍。
英文摘要：
Identifying the microscopic mechanism for superconductivity in magic-angle twisted bilayer graphene (MATBG) is an outstanding open problem. While MATBG exhibits a rich phase-diagram, driven partly by the strong interactions relative to the electronic bandwidth, its single-particle properties are unique and likely play an important role in some of the phenomenological complexity. Some of the salient features include an electronic bandwidth smaller than the characteristic phonon bandwidth and a non-trivial structure of the underlying Bloch wavefunctions. We perform a theoretical study of the cooperative effects due to phonons and plasmons on pairing in order to disentangle the distinct role played by these modes on superconductivity. We consider a variant of MATBG with an enlarged number of fermion flavors, $N \gg 1$, where the study of pairing instabilities reduces to the conventional (weak-coupling) Eliashberg framework. In particular, we show that certain umklapp processes involving mini-optical phonon modes, which arise physically as a result of the folding of the original acoustic branch of graphene due to the moiré superlattice structure, contribute significantly towards enhancing pairing. We also investigate the role played by the dynamics of the screened Coulomb interaction on pairing, which leads to an enhancement in a narrow window of fillings, and study the effect of external screening due to a metallic gate on superconductivity. At strong coupling the dynamical pairing interaction leaves a spectral mark in the single particle tunneling density of states. We thus predict such features will appear at specific frequencies of the umklapp phonons corresponding to the sound velocity of graphene times an integer multiple of the Brillouin zone size.
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