Dirac revivals drive a resonance response in twisted bilayer graphene
Dirac revivals drive a resonance response in twisted bilayer graphene
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DOI：
10.1038/s41567-023-02060-0
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发表时间：
2022-06
期刊：
影响因子：
19.6
通讯作者：
E. Morissette;Jiangxiazi Lin;Song Liu;D. Rhodes;K. Watanabe;T. Taniguchi;J. Hone;M. Scheurer-
中科院分区：
文献类型：
--
作者：
E. Morissette;Jiangxiazi Lin;Song Liu;D. Rhodes;K. Watanabe;T. Taniguchi;J. Hone;M. Scheurer-
在一个强关联系统中，集体激发包含了关于底层基态电子序的关键信息。最近的一系列实验已经暗示了魔角石墨烯莫尔带的自旋和谷区同位旋通道中丰富的集体模式。然而，由于缺乏自旋探测器，对集体激发的直接观察仍然是难以捉摸的。在这项工作中，我们使用一个连续检测的电子自旋共振技术来寻找低能量集体激发魔角扭曲双层石墨烯。我们报告直接观察到的集体模式的形式微波诱导共振附近的半填充的莫尔平带。这些共振模式的频率-磁场依赖性揭示了谷间自旋耦合的性质，使我们能够提取参数，如谷间交换相互作用和自旋刚度。两个独立的观测结果证明，微波共振的产生和检测依赖于平坦莫尔能带内的强关联。首先，稳健的共振响应的开始与半莫尔填充时的自发味极化相一致，并且在底层费米表面是同位旋非极化的密度范围内仍然不存在。其次，我们对石墨烯单层和双层样品进行了相同的共振测量，包括具有大扭曲角的扭曲双层，其中不存在平带物理。我们没有观察到在这些样品中的共振响应的迹象，在大范围的载流子密度，微波频率和功率。一个自然的解释是，在魔角附近的共振响应来自“狄拉克复兴”和由此产生的同位旋序。
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 moir\'e 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 moir\'e 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 moir\'e energy band. First, the onset of robust resonance response coincides with the spontaneous flavor polarization at half moir\'e 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.