论文标题
配对魔术角扭曲双层石墨烯：声子和等离子体umklapp的角色
Pairing in magic-angle twisted bilayer graphene: role of phonon and plasmon umklapp
论文作者
论文摘要
识别魔法扭曲双层石墨烯（MATBG）中超导性的显微镜机制是一个杰出的开放问题。尽管MATBG表现出丰富的相数，部分是由相对于电子带宽的强相互作用驱动的，但其单粒子性质是独一无二的，并且可能在某些现象学复杂性中起重要作用。某些显着特征包括小于特征性声子带宽的电子带宽和基础Bloch波函数的非平凡结构。我们对由于声子和等离子对配对而进行的合作效应进行了理论研究，以解散这些模式对超导性的独特作用。我们考虑了一种MATBG的变体，并具有扩大的费米风味，$ n \ gg 1 $，其中配对不稳定性的研究减少到传统（弱耦合）Eliashberg框架。特别是，我们表明，由于Moiré超晶格结构而导致的原始声学分支的折叠，涉及迷你光音子模式的某些UMKLAPP过程是在物理上出现的，这对增强配对产生了显着贡献。我们还研究了筛选的库仑相互作用在配对上的动力学的作用，这会导致较窄的填充窗口增强，并研究金属栅极对超导性的外部筛选的影响。在强耦合下，动态配对相互作用在状态的单个粒子隧道密度中留下了光谱标记。因此，我们预测此类特征将出现在与石墨烯时时间的声速相对应的umklapp声子的特定频率上，是布里鲁因区域大小的整数倍数。
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.
