【科研进展】利用真空光学微腔调控转角石墨烯能带
转角双层石墨烯体系（twisted bilayer graphene, TBG）是凝聚态物理学近年来的重要发现之一，它已成为研究量子多体物理的极其丰富的实验平台。特别是在特定的扭转角度，即所谓的“魔角”（大约1.05度），TBG会发生超导相变。尽管TBG中奇异超导性的起源仍然是一个有争议的话题，但普遍认为平带效应在其中起着关键作用。然而，由于TBG在魔角处并非稳定构型，实验上常常难以精确制备出魔角石墨烯。实验观察到，当扭转角度偏离魔角0.1度时，超导相就会消失。这种不稳定性限制了TBG中超导性质的广泛研究。
在该项研究中，研究人员提出了一种新方法，即利用手性微腔中的量子涨落来调控TBG的能带，使得TBG即便在魔角之外也能形成平带（如图1所示）[1]。该效应的物理本质在于手性微腔破坏了时间反演对称性，因此其内部的真空量子涨落也具有出时间反演对称性破缺的特征。这些具有时间反演对称性破缺特性的量子涨落能够导致石墨烯的能带产生能隙，进而对魔角附近的能带结构产生显著影响。通过调节手性微腔的有效模式体积，可以有效地调控光子与电子的耦合强度，实现对能带结构及体系拓扑性质的精确调控，其研究成果近期发表在PRL期刊。该项工作建立在研究人员之前提出的量子大气效应[2]和手性真空分子选择效应[3]的理论基础之上。
Using Optical Cavity to Engineer Band Structure of Materials
The twisted bilayer graphene (TBG) system is one of the important discoveries in condensed matter physics in recent years. It has become an extremely rich platform for studying quantum many-body physics. Especially at a specific twist angle, the so-called "magic angle" (approximately 1.05 degrees), TBG undergoes a superconducting phase transition. Although the origin of exotic superconductivity in TBG remains a controversial topic, it is generally believed that the flat-band effect plays an essential role. However, since TBG is not a stable configuration at the magic angle, it is often difficult to accurately prepare magic-angle graphene experimentally. Experiments have observed that when the twist angle deviates from the magic angle by 0.1 degrees, the superconducting phase disappears. This instability has limited extensive research on superconducting properties in TBG.In this study, the researchers proposed a new method, namely, using quantum fluctuations in a chiral microcavity to engineer the band structure of TBG, so that TBG can form a flat band beyond the magic angle (see Figure 1)[1]. The physical picture is that the chiral microcavity breaks time-reversal symmetry, and the vacuum quantum fluctuations in the cavity inherit the characteristics of time-reversal symmetry breaking. The time-reversal symmetry broken quantum fluctuations can induce energy gaps in the band structure, leading to a significant impact on the band flatness near the magic angle. By controlling the effective mode volume of the chiral microcavity, one can effectively tune the coupling strength of electron-photon interaction, achieving precise control of the band structure and even topological properties of the system. This work is based on the previous studies on the quantum atmospheric effect [2] and the chiral vacuum molecule selection effect [3].
Figure 1：Schematic setup of using vacuum quantum fluctuations in chiral optical microcavities to engineer TBG band structure. The method of using vacuum microcavities to control quantum materials can avoid non-equilibrium processes and material heating effects that may be caused by traditional optical methods, and achieve new quantum states matter that cannot be achieved by classical electromagnetic radiation.
This first author of this work is Jiang Cunyuan, a graduate student in the School of Physics and Astronomy of Shanghai Jiao Tong University. Associate Professor Matteo Baggioli of the School of Physics and Astronomy and Associate Professor Qingdong Jiang of the Tsung-Dao Lee Institute are the co-corresponding authors [1]. This research was funded by the Shanghai Municipal Science and Technology Major Project, Innovation Program for Quantum Science and Technology, the National Natural Science Foundation of China, and Shanghai Jiao Tong University, for which we express our sincere gratitude.[1] Cunyuan Jiang, Matteo Baggioli, and Qing-Dong Jiang, Phys. Rev. Lett. 132, 166901 (2024)[2] Qing-Dong Jiang and Frank Wilczek, Phys. Rev. B 99, 201104(R) (2019) (also see the news report by Nature Materials 17, 951 (2018))[3] Yanzhe Ke, Zhigang Song, and Qing-Dong Jiang, Phys. Rev. Lett. 131, 223601 (2023)
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