[论文解读] Spectroscopic signatures of many-body correlations in magic angle twisted bilayer graphene

Yonglong Xie, Lian, Biao+7 more|RePEc: Research Papers in Economics|Jun 21, 2019

Graphene research and applications被引用 37

一句话总结

本文通过高分辨率 STM 光谱证据显示魔角扭曲双层石墨烯（MATBG）中的强多体相关性，呈现的特征与 mean-field 电子-电子相互作用模型不相容，并可被一个表征性的扩展哈密顿量模型簇计算所再现。

ABSTRACT

The discovery of superconducting and insulating states in magic angle twisted bilayer graphene (MATBG) has ignited considerable interest in understanding the nature of electronic interactions in this chemically pristine material system. The phenomenological similarity of the MATBG transport properties as a function of doping with those of the high-Tc cuprates and other unconventional superconductors suggests the possibility that MATBG may be a highly interacting system. However, there have not been any direct experimental evidence for strong many-body correlations in MATBG. Here we provide such evidence from using high-resolution spectroscopic measurements, as a function of carrier density, with a scanning tunneling microscopy (STM). We find MATBG to display unusual spectroscopic characteristics that can be attributed to electron-electron interactions over a wide range of doping, including when superconductivity emerges in this system. We show that our measurements cannot be explained with a mean-field approach for modeling electron-electron interaction in MATBG. The breakdown of a mean-field approach for understanding the properties of other correlated superconductors, such as cuprates, has long inspired the study of highly correlated Hubbard model. We show that a phenomenological extended Hubbard model cluster calculation, motivated by the nearly localized nature of the relevant electronic states of MATBG produces spectroscopic features similar to those we observe experimentally. Our findings demonstrate the critical role of many-body correlations in understanding the properties of MATBG.

研究动机与目标

• 推动在 MATBG 中寻找强电子相关性的研究，与运载性分析对其他非常规超导体的类比相比之下的动机。
• 使用 STM 在不同载流子密度范围内展示 MATBG 的多体效应直接证据。
• 证明均场模型无法解释观测到的光谱特征。
• 评估一个表征性的扩展哈密顿量模型是否能够再现 STM 的观测。

提出的方法

• 对 MATBG 进行随载流子密度变化的高分辨率扫描隧道显微镜（STM）光谱学。
• 分析光谱特征以寻找偏离均场预期的电子-电子相互作用的标志。
• 将实验光谱与一个表征性的扩展哈密顿量模型簇计算的预测进行比较。
• 论证 MATBG 中几乎局域化的电子态推动模型选择并对观测特征进行解释。
• 证明在广泛的掺杂范围内，包括超导态区间，观测到的光谱特征仍然存在。

实验结果

研究问题

• RQ1 MATBG 的 STM 光谱特征是否表明超出均场描述的强多体相关性？
• RQ2 一个带簇计算的表征性扩展哈密顿量模型是否能再现观测的 MATBG 光谱？
• RQ3 光谱特征如何随载流子密度演化，尤其是在超导区域内？
• RQ4 MATBG 中几乎局域化电子态在形塑光谱和相关性中的作用是什么？

主要发现

• MATBG 的 STM 光谱在广泛的掺杂范围内显示出可归因于电子-电子相互作用的异常特征。
• 数据不能用 MATBG 中电子-电子相互作用的均场处理来解释。
• 带簇计算的表征性扩展哈密顿量模型得到的光谱特征与实验观测相似。
• 这些发现凸显了多体相关性在理解 MATBG 性质（包括超导态）中的关键作用。