近日，日本理化学研究所新兴物质科学中心的Yoshinori Tokura及其研究团队取得一项新进展。经过不懈努力，他们实现具有点费米表面的半金属外尔铁磁体的合成。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。

本文从理论上预测并在实验中观察到了一种范德华（Cr,Bi）₂Te₃中的半金属性外尔铁磁体。在输运性质方面，研究人员发现了创纪录的体反常霍尔角大于0.5，同时伴随着非金属性的电导率，这一特性与传统铁磁体截然不同。结合对称性分析，该研究数据表明，该材料的费米面由两个外尔点组成，它们之间的巨大分离距离超过了体布里渊区线性尺寸的75%，且不存在其他电子态。利用最先进的晶体合成技术，研究人员大范围调节了电子结构，从而能够湮灭外尔态，并直观地展示了一个独特的拓扑相图，其中包含了广泛的陈数绝缘体、外尔半金属和磁性半导体区域。研究人员观察到的半金属性外尔铁磁体为探索新的关联态和非线性现象，以及开发零磁场下的外尔自旋电子学和光学器件提供了新途径。

据悉，由新兴拓扑费米子支配的量子材料已成为物理学的基石。石墨烯中的狄拉克费米子构成了莫尔量子物质的基础，而磁性拓扑绝缘体中的狄拉克费米子则促成了量子反常霍尔（QAH）效应的发现。相比之下，几乎没有哪种材料的电磁响应是由新兴外尔费米子主导的。几乎所有已知的外尔材料都极具金属性，且主要受到无关紧要的常规电子的支配。

附：英文原文

Title: Synthesis of a semimetallic Weyl ferromagnet with point Fermi surface

Author: Belopolski, Ilya, Watanabe, Ryota, Sato, Yuki, Yoshimi, Ryutaro, Kawamura, Minoru, Nagahama, Soma, Zhao, Yilin, Shao, Sen, Jin, Yuanjun, Kato, Yoshihiro, Okamura, Yoshihiro, Zhang, Xiao-Xiao, Fujishiro, Yukako, Takahashi, Youtarou, Hirschberger, Max, Tsukazaki, Atsushi, Takahashi, Kei S., Chiu, Ching-Kai, Chang, Guoqing, Kawasaki, Masashi, Nagaosa, Naoto, Tokura, Yoshinori

Issue&Volume: 2025-01-22

Abstract: Quantum materials governed by emergent topological fermions have become a cornerstone of physics. Dirac fermions in graphene form the basis for moiré quantum matter and Dirac fermions in magnetic topological insulators enabled the discovery of the quantum anomalous Hall (QAH) effect. By contrast, there are few materials whose electromagnetic response is dominated by emergent Weyl fermions. Nearly all known Weyl materials are overwhelmingly metallic and are largely governed by irrelevant, conventional electrons. Here we theoretically predict and experimentally observe a semimetallic Weyl ferromagnet in van der Waals (Cr,Bi)₂Te₃. In transport, we find a record bulk anomalous Hall angle of greater than 0.5 along with non-metallic conductivity, a regime that is strongly distinct from conventional ferromagnets. Together with symmetry analysis, our data suggest a semimetallic Fermi surface composed of two Weyl points, with a giant separation of more than 75% of the linear dimension of the bulk Brillouin zone, and no other electronic states. Using state-of-the-art crystal-synthesis techniques, we widely tune the electronic structure, allowing us to annihilate the Weyl state and visualize a unique topological phase diagram exhibiting broad Chern insulating, Weyl semimetallic and magnetic semiconducting regions. Our observation of a semimetallic Weyl ferromagnet offers an avenue towards new correlated states and nonlinear phenomena, as well as zero-magnetic-field Weyl spintronic and optical devices.

DOI: 10.1038/s41586-024-08330-y

Source: https://www.nature.com/articles/s41586-024-08330-y