近日，美国麻省理工学院的Long Ju及其研究团队取得一项新进展。经过不懈努力，他们揭示石墨烯/六方氮化硼莫尔超晶格中的扩展量子反常霍尔态。相关研究成果已于2025年1月22日在国际权威学术期刊《自然》上发表。

该研究团队报道了对菱形五层和四层石墨烯/六方氮化硼（hBN）莫尔超晶格在电子温度低至40毫开尔文以下的电输运测量结果。在五层器件中，研究人员观察到了另外两个分数量子反常霍尔（FQAH）态，且其纵向电阻（R_xx）值较之前报道的更小。在新的四层器件中，研究人员在莫尔填充因子v=3/5和2/3处观察到了FQAHE。在基底温度下，施加小电流时，研究人员观察到了一个新的扩展量子反常霍尔（EQAH）态和磁滞现象，其中横向电阻（R_xy）等于h/e²，且纵向电阻（R_xx）几乎为零，这一状态跨越了从0.5到1.3的广泛填充因子范围。当温度或电流增加时，EQAH态消失，并部分转变为FQAH液体。此外，研究人员还观察到了由位移场诱导的量子相变，从EQAH态转变为费米液体、FQAH液体以及可能的复合费米液体。这些观察结果确立了一种新的电子拓扑相，其在零磁场下具有量化的霍尔电阻，并丰富了具有拓扑平带材料中的新兴量子现象。

据悉，拓扑平带中的电子在关联效应的作用下可以形成新的拓扑态。研究表明，在约400毫开尔文的温度下，五层菱形石墨烯/六方氮化硼（hBN）莫尔超晶格中出现了分数量子反常霍尔效应（FQAHE），这引发了关于其背后机制及莫尔效应作用的广泛讨论。特别是，有人提出了具有非平凡拓扑性的新型电子晶体态。

Title: Extended quantum anomalous Hall states in graphene/hBN moiré superlattices

Author: Lu, Zhengguang, Han, Tonghang, Yao, Yuxuan, Hadjri, Zach, Yang, Jixiang, Seo, Junseok, Shi, Lihan, Ye, Shenyong, Watanabe, Kenji, Taniguchi, Takashi, Ju, Long

Issue&Volume: 2025-01-22

Abstract: Electrons in topological flat bands can form new topological states driven by correlation effects. The pentalayer rhombohedral graphene/hexagonal boron nitride (hBN) moiré superlattice was shown to host fractional quantum anomalous Hall effect (FQAHE) at approximately 400mK, triggering discussions around the underlying mechanism and role of moiré effects. In particular, new electron crystal states with non-trivial topology have been proposed. Here we report electrical transport measurements in rhombohedral pentalayer and tetralayer graphene/hBN moiré superlattices at electronic temperatures down to below 40mK. We observed two more fractional quantum anomalous Hall (FQAH) states and smaller R_xx values in pentalayer devices than those previously reported. In the new tetralayer device, we observed FQAHE at moiré filling factors v=3/5 and 2/3. With a small current at the base temperature, we observed a new extended quantum anomalous Hall (EQAH) state and magnetic hysteresis, where R_xy=h/e² and vanishing R_xx spans a wide range of v from 0.5 to 1.3. At increased temperature or current, EQAH states disappear and partially transition into the FQAH liquid. Furthermore, we observed displacement field-induced quantum phase transitions from the EQAH states to the Fermi liquid, FQAH liquid and the likely composite Fermi liquid. Our observations established a new topological phase of electrons with quantized Hall resistance at zero magnetic field and enriched the emergent quantum phenomena in materials with topological flat bands.

DOI: 10.1038/s41586-024-08470-1

Source: https://www.nature.com/articles/s41586-024-08470-1