近日，美国麻省理工学院的Pablo Jarillo-Herrero及其研究团队取得一项新进展。经过不懈努力，他们揭示了螺旋三层石墨烯的拓扑带和关联态。相关研究成果已于2025年1月7日在国际知名学术期刊《自然—物理学》上发表。

该研究团队证实尽管螺旋三层石墨烯（即由三层石墨烯依次以相同角度扭转堆叠而成的系统）保持了全局面内反演对称性，但其中仍表现出了这种物理现象。研究人员在1.8°的魔角下揭示了关联态和磁性态的相图，这可通过晶格弛豫来解释，晶格弛释导致形成了大的周期性畴区，在这些畴区内，面内反演对称性在莫尔尺度上被破坏。

每个畴区都包含在两个谷中具有相反陈数的平拓扑能带。研究人员发现，在每个莫尔原胞的多个整数和分数电子填充下存在关联态，并且在其中一部分填充下观察到了反常霍尔效应。

当每个晶胞有一个电子时，反常霍尔效应在超过临界电位移场后消失，这表明发生了拓扑相变。研究人员确定，螺旋三层石墨烯由于其新兴的莫尔尺度对称性，为拓扑工程提供了一个平台机遇。

据悉，本征反常霍尔效应（AHE）由非零贝里曲率和自发的时间反演对称性破缺所驱动。这一效应可在具有平电子带的二维莫尔系统中实现，但通常在具有反演对称性的材料中并不常见。

附：英文原文

Title: Topological bands and correlated states in helical trilayer graphene

Author: Xia, Li-Qiao, de la Barrera, Sergio C., Uri, Aviram, Sharpe, Aaron, Kwan, Yves H., Zhu, Ziyan, Watanabe, Kenji, Taniguchi, Takashi, Goldhaber-Gordon, David, Fu, Liang, Devakul, Trithep, Jarillo-Herrero, Pablo

Issue&Volume: 2025-01-07

Abstract: The intrinsic anomalous Hall effect (AHE) is driven by non-zero Berry curvature and spontaneous time-reversal symmetry breaking. This effect can be realized in two-dimensional moiré systems hosting flat electronic bands but is not usually seen in inversion-symmetric materials. Here, we show that this physics is manifested in helical trilayer graphene—three graphene layers, each twisted in sequence by the same angle—although the system retains global in-plane inversion symmetry. We uncover a phase diagram of correlated and magnetic states at a magic twist angle of 1.8°, which is explained by a lattice relaxation that leads to the formation of large periodic domains where in-plane inversion symmetry is broken on the moiré scale. Each domain harbours flat topological bands with opposite Chern numbers in the two valleys. We find correlated states at multiple integer and fractional electron fillings per moiré unit cell and an AHE at a subset of them. The AHE disappears above a critical electric displacement field at one electron per unit cell, indicating a topological phase transition. We establish helical trilayer graphene as a platform that presents an opportunity to engineer topology due to its emergent moiré-scale symmetries.

DOI: 10.1038/s41567-024-02731-6

Source: https://www.nature.com/articles/s41567-024-02731-6