Li Hui, Zhang Chengping, Zhou Chengjie, Ma Chen, Lei Xiao, Jin Zijing, He Hongtao, Li Baikui, Law Kam Tuen, Wang Jiannong
Department of Physics, the Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong, China.
Department of Physics, South University of Science and Technology of China, Shenzhen, Guangdong, China.
Nat Commun. 2024 Sep 5;15(1):7779. doi: 10.1038/s41467-024-52206-8.
The study of quantum geometry effects in materials has been one of the most important research directions in recent decades. The quantum geometry of a material is characterized by the quantum geometric tensor of the Bloch states. The imaginary part of the quantum geometry tensor gives rise to the Berry curvature while the real part gives rise to the quantum metric. While Berry curvature has been well studied in the past decades, the experimental investigation on the quantum metric effects is only at its infancy stage. In this work, we measure the nonlinear transport of bulk MnBiTe, which is a topological anti-ferromagnet We found that the second order nonlinear responses are negligible as required by inversion symmetry, the third-order nonlinear responses are finite. The measured third-harmonic longitudinal ( ) and transverse ( ) voltages with frequency 3 , driven by an a.c. current with frequency , show an intimate connection with magnetic transitions of MnBiTe flakes. Their magnitudes change abruptly as MnBiTe flakes go through magnetic transitions from an antiferromagnetic state to a canted antiferromagnetic state and to a ferromagnetic state. In addition, the measured is an even function of the applied magnetic field B while is odd in B. Amazingly, the field dependence of the third-order responses as a function of the magnetic field suggests that is induced by the quantum metric quadrupole and is induced by the Berry curvature quadrupole. Therefore, the quadrupoles of both the real and the imaginary part of the quantum geometry tensor of bulk MnBiTe are revealed through the third order nonlinear transport measurements. This work greatly advanced our understanding on the connections between the higher order moments of quantum geometry and nonlinear transport.
近几十年来，材料中的量子几何效应研究一直是最重要的研究方向之一。材料的量子几何由布洛赫态的量子几何张量表征。量子几何张量的虚部产生贝里曲率，而实部产生量子度量。虽然在过去几十年中对贝里曲率已经进行了充分研究，但对量子度量效应的实验研究仍处于起步阶段。在这项工作中，我们测量了体相MnBiTe（一种拓扑反铁磁体）的非线性输运。我们发现，根据反演对称性，二阶非线性响应可忽略不计，三阶非线性响应是有限的。由频率为(\omega)的交流电流驱动的频率为(3\omega)的测量三阶谐波纵向（ ）和横向（ ）电压，显示出与MnBiTe薄片的磁转变有密切联系。当MnBiTe薄片经历从反铁磁态到倾斜反铁磁态再到铁磁态的磁转变时，它们的大小会突然变化。此外，测量得到的 是外加磁场(B)的偶函数，而 是(B)的奇函数。令人惊讶的是，三阶响应随磁场的变化表明， 由量子度量四极矩诱导， 由贝里曲率四极矩诱导。因此，通过三阶非线性输运测量揭示了体相MnBiTe量子几何张量实部和虚部的四极矩。这项工作极大地推进了我们对量子几何高阶矩与非线性输运之间联系的理解。
