[论文解读] Electron spin resonance and collective excitations in magic-angle twisted bilayer graphene

Erin Morissette, Jiangxiazi Lin+12 more|arXiv (Cornell University)|Jun 16, 2022

Quantum and electron transport phenomena被引用 7

一句话总结

本研究利用电阻检测电子自旋共振（ESR）技术，直接观测了魔角扭曲双层石墨烯（MATBG）在摩尔平带半满填充时的低能集体激发。作者识别出与谷间自旋耦合相关的微波诱导共振，提取了关键参数如谷间交换相互作用和自旋刚度，并确立该响应源于强关联的‘狄拉克复兴’态与同位旋序。

ABSTRACT

In a strongly correlated system, collective excitations contain key information regarding the electronic order of the underlying ground state. An abundance of collective modes in the spin and valley isospin channels of magic-angle graphene moiré bands has been alluded to by a series of recent experiments. However, direct observation of collective excitations has remained elusive due to the lack of a spin probe. In this work, we use a resistively-detected electron spin resonance technique to look for low-energy collective excitations in magic-angle twisted bilayer graphene. We report direct observation of collective modes in the form of microwave-induced resonance near half filling of the moiré flatbands. The frequency-magnetic field dependence of these resonance modes sheds light onto the nature of intervalley spin coupling, allowing us to extract parameters such as intervalley exchange interaction and spin stiffness. Two independent observations testify that the generation and detection of the microwave resonance relies on the strong correlation within the flat moiré energy band. First, the onset of robust resonance response coincides with the spontaneous flavor polarization at half moiré filling, and remains absent in the density range where the underlying Fermi surface is isospin unpolarized. Second, we performed the same resonance measurement on graphene monolayer and bilayer samples, including twisted bilayer with a large twist angle, where flatband physics is absent. We observe no indication of resonance response in these samples across a large range of carrier density, microwave frequency and power. A natural explanation is that the resonance response near the magic angle originates from "Dirac revivals" and the resulting isospin order.

研究动机与目标

• 直接探测强关联魔角扭曲双层石墨烯（MATBG）中的集体激发，此类激发因缺乏自旋探测手段而长期难以被观测。
• 确定谷间自旋耦合的本质，并提取谷间交换相互作用和自旋刚度等参数。
• 建立微波诱导共振与平带中同位旋序形成之间的关联。
• 通过与非平带体系对比，验证共振响应是强电子关联的特征信号。
• 解决理论模型中关于谷间洪德相互作用符号与大小的模糊性，该参数是MATBG中超导态与关联绝缘体理论的关键。

提出的方法

• 采用电阻检测电子自旋共振（ESR）技术，探测霍尔条形器件结构中微波诱导的纵向电阻（$R_{xx}$）变化。
• 通过同轴线施加微波辐射（1–30 GHz），在平面内磁场（$B_{ot}$）下测量低温（50 mK）下的$R_{xx}$响应。
• 采用高通滤波（$R_{xx}^{HP}$）将快速的微波驱动变化与缓慢的背景漂移分离。
• 在不同摩尔填充因子（$\nu$）下绘制频率与磁场依赖的共振响应，尤其关注$\nu = 2.8$和$\nu = -2$附近。
• 在大扭转角的单层与双层石墨烯上进行对照实验，其中不存在平带物理，以排除虚假响应。
• 通过分析共振频率与磁场的依赖关系，利用自旋模式耦合模型提取自旋刚度（$J$）与谷间交换相互作用（$J_H$）。

实验结果

研究问题

• RQ1 魔角扭曲双层石墨烯的平带中集体激发的本质是什么？能否被直接观测？
• RQ2 微波诱导的共振响应如何依赖于电子填充因子？其揭示了何种同位旋序的内在机制？
• RQ3 谷间自旋耦合在集体激发模式形成中扮演何种角色？能否实验提取谷间交换相互作用（$J_H$）？
• RQ4 所观测到的共振是否为强电子关联的特征信号？能否与‘狄拉克复兴’及味极化相联系？
• RQ5 为何在非平带体系中无共振现象？这对其信号起源有何启示？

主要发现

• 在$\nu = 2.8$处观测到稳健的微波诱导共振，与摩尔平带半满填充时自发味极化的出现同步。
• 共振模式的频率-磁场依赖关系显示出线性色散关系，与集体自旋激发一致，从而可提取自旋刚度（$J$）与谷间交换相互作用（$J_H$）。
• 在大扭转角的单层与双层石墨烯中无共振响应，证实该信号源于平带物理与强电子关联。
• 在$\nu = -2$处的共振表现出反铁磁性耦合特征，表现为$B=0$时负截距，与$g=2$和$g=4$模式结构一致。
• 在绝缘体单元（IU）区域或空穴侧的IF1区域均未观测到共振，与狄拉克复兴及同位旋序的缺失一致。
• 量子霍尔效应态在微波照射下保持稳健，表明加热效应极小，验证了共振源于多体集体激发，而非热效应。