近日，美国华盛顿大学的Xiaodong Xu及其研究小组取得一项新进展。经过不懈努力，他们成功观察到分数量子反常霍尔效应。相关研究成果已于2023年8月17日在国际权威学术期刊《自然》上发表。

该研究团队在扭曲双层MoTe₂的电测量中直接观察到整数和分数QAH效应。在零磁场时，在填充因子ν=-1（每摩尔晶胞具有一个空穴）附近观察到QAH平台，霍尔电阻R_xy量子化为h/e²±0.1%，而纵向电阻R_xx消失。值得注意的是，在ν=-2/3和-3/5时，研究人员观察到R_xy分别在3/2h/e²±1%和5/3h/e²±3%处呈现平台特征，而R_xx仍然很小。所有特征随外加磁场线性变化，斜率与相应的Chern数-1、-2/3和-3/5相匹配，符合整数和分数QAH态的预期。

此外，在零磁场时，R_xy接近半填充(ν=-1/2)并随ν调谐而线性变化，类似于高磁场下二维电子气体半填充的最低朗道能级中的复合费米液体。直接观测FQAH及其相关效应，为零磁场下电荷分数化和任意子统计的研究铺平了道路。

据悉，整数量子反常霍尔效应(QAH)是零磁场下量子霍尔效应的晶格模拟。这种惊人的现象发生在具有拓扑非平凡带和自发时间反演对称性破缺的系统中。在存在强电子关联性的情况下发现其分数对应物，即分数量子反常霍尔(FQAH)效应，这将打开凝聚态物理学的新篇章。

附：英文原文

Title: Observation of Fractionally Quantized Anomalous Hall Effect

Author: Park, Heonjoon, Cai, Jiaqi, Anderson, Eric, Zhang, Yinong, Zhu, Jiayi, Liu, Xiaoyu, Wang, Chong, Holtzmann, William, Hu, Chaowei, Liu, Zhaoyu, Taniguchi, Takashi, Watanabe, Kenji, Chu, Jiun-haw, Cao, Ting, Fu, Liang, Yao, Wang, Chang, Cui-Zu, Cobden, David, Xiao, Di, Xu, Xiaodong

Issue&Volume: 2023-08-17

Abstract: The integer quantum anomalous Hall (QAH) effect is a lattice analog of the quantum Hall effect at zero magnetic field. This striking phenomenon occurs in systems with topologically nontrivial bands and spontaneous time-reversal symmetry breaking. Discovery of its fractional counterpart in the presence of strong electron correlations, i.e., the fractional quantum anomalous Hall (FQAH) effect, would open a new chapter in condensed matter physics. Here, we report direct observation of both integer and fractional QAH effects in electrical measurements on twisted bilayer MoTe₂. At zero magnetic field, near filling factor ν=-1 (one hole per moiré unit cell) we see an integer QAH plateau in the Hall resistance R_xy quantized to h/e²±0.1% while longitudinal resistance R_xx vanishes. Remarkably, at ν=-2/3 and -3/5 we see plateau features in R_xy at 3/2h/e²±1% and 5/3h/e2±3%, respectively, while R_xx remains small. All features shift linearly versus applied magnetic field with slopes matching the corresponding Chern numbers -1, -2/3, and -3/5, precisely as expected for integer and fractional QAH states. Additionally, at zero magnetic field, R_xy is approximately 2h/e² near half filling (ν=-1/2) and varies linearly as ν is tuned. This behavior resembles that of the composite Fermi liquid in the half-filled lowest Landau level of a two-dimensional electron gas at high magnetic field Direct observation of the FQAH and associated effects paves the way for researching charge fractionalization and anyonic statistics at zero magnetic field.

DOI: 10.1038/s41586-023-06536-0

Source: https://www.nature.com/articles/s41586-023-06536-0