Observation of Berry curvature and chirality in Mn3Ge Weyl semimetal using polarized SANS

Antiferromagnetic (AFM) Weyl semimetals, such as Mn3Ge, are topological materials exhibiting anomalous transport properties, including the anomalous Hall effect (AHE), anomalous magneto-optic Kerr effect (MOKE), and anomalous Nernst effect, all driven by nonvanishing Berry curvature. Mn3Ge stands out for its noncollinear AFM structure in the a-b plane, which breaks time-reversal symmetry, resulting in a large Berry curvature. This Berry curvature, arising from broken time-reversal symmetry, acts like an intrinsic magnetic field, leading to real-space effects similar to ferromagnetic (FM) materials. Since neutrons also sense the Berry phase, we propose that small-angle neutron scattering (SANS) of AFM Weyl semimetals, such as Mn3Ge, will show field-dependent patterns akin to those observed in FM systems. This experiment could provide new insights into the interaction between neutrons and Berry phase. Moreover, when a small magnetic field (at least 20 mT) is applied in the x-y crystallographic plane, an imbalance of magnetic domains with specific (net) chirality is created, leading to nonzero Berry curvature and observable effects like AHE and MOKE. We anticipate that the net chirality of magnetic domains can be detected using polarized SANS (SANSPOL), revealing asymmetry in the SANS cross-section when the magnetic field is applied within the x-y plane. This experiment will enhance the understanding of Berry curvature and chiral magnetism in AFM systems.

反铁磁外尔半金属（Antiferromagnetic Weyl semimetals, AFM）如Mn3Ge，是一类具有反常输运特性的拓扑材料，其涵盖反常霍尔效应（Anomalous Hall effect, AHE）、反常磁光克尔效应（Anomalous Magneto-Optic Kerr Effect, MOKE）与反常能斯特效应，所有效应均由非零贝里曲率（Berry curvature）驱动。Mn3Ge因其在a-b晶面内的非共线反铁磁结构而备受瞩目，该结构打破了时间反演对称性（Time-reversal symmetry），进而产生极强的贝里曲率。
这种源于时间反演对称性破缺的贝里曲率可类比为内禀磁场，能够带来与铁磁（Ferromagnetic, FM）材料相似的实空间效应。
由于中子同样会与贝里相位（Berry phase）发生相互作用，我们提出：针对反铁磁外尔半金属如Mn3Ge的小角中子散射（Small-angle neutron scattering, SANS）实验，将呈现出与铁磁体系中观测到的类似的场依赖散射图样。该实验可为中子与贝里相位之间的相互作用提供全新的研究视角。
此外，当在x-y晶面内施加至少20 mT的小型磁场时，具有特定（净）手性的磁畴会出现分布失衡，进而产生非零贝里曲率，并可观测到反常霍尔效应与反常磁光克尔效应等现象。
我们预期可通过极化小角中子散射（Polarized small-angle neutron scattering, SANSPOL）探测磁畴的净手性：当磁场沿x-y晶面施加时，小角中子散射截面将呈现出不对称性。该实验将有助于深化对反铁磁体系中贝里曲率与手性磁性的理解。