As an important electrical transport phenomenon in magnetic materials, the anomalous Hall effect is thought to originate from two different mechanisms: one is the outer process caused by the scattering of impurity atoms, and the other is driven by the Bailey curvature of the crystal energy band. Intrepid behavior. As a pseudo magnetic field in momentum space, Bailey's curvature originates from the inter-band interaction of Bloch electrons. The resulting intrinsic anomalous Hall conductance is more resistant to defects and thermal disturbances, and has higher stability and stability. Conducive to device applications. At the beginning of this century, physicists discovered that magnetic monopoles in the momentum space of magnetic materials can produce intrinsic anomalous Hall effects [Science 302 (2003) 92]. It now appears that the magnetic monopole is the Weir point in the Weir semi-metal later discovered. This further provides a new understanding of band topology theory for the study of the anomalous Hall effect.
As an important parameter of the anomalous Hall effect, the anomalous Hall angle, that is, the ratio of the transverse Hall conductance to the longitudinal charge conductance, reflects the efficiency with which the longitudinal drive current can be converted into a transverse Hall current. Materials with large anomalous Hall angles are expected to achieve the quantum anomalous Hall effect at two-dimensional limits. This is particularly important for new generation Hall devices, especially quantum anomalous Hall devices. To obtain a large anomalous Hall angle, a substance needs to satisfy two conditions at the same time, namely (1) high anomalous Hall conductance and (2) low carrier concentration. No suitable magnetic material has been found to satisfy such conditions in the past.
In recent years, in the research progress of topological physics, people have found various topological insulators and topological semimetals represented by Dirac, Weyl semimetals, etc. In Weyl semimetals, massless chiral Weyl fermions were discovered as quasi-particles in 2015 [Phys. Rev. X 5 (2015) 011029, Phys. Rev. X 5 (2015) 031013], thus Stimulate the research boom of Weyl semimetals. However, in the non-magnetic Weyl system, the time inversion symmetry makes the anomalous Hall effect zero; in the magnetic Weyl semimetal, non-zero Bailey curvature may produce a strong anomalous Hall effect. As a semimetal material, Weyl semimetal has a lower carrier concentration, and its topologically protected electronic state can dominate the transport operation of the system. Therefore, the magnetic Weyl semimetal provides an ideal carrier for obtaining the large abnormal Hall effect, especially the large abnormal Hall angle. However, several magnetic Weyl semimetals (Re 2 Ir 2 O 7, HgCr 2 Se 4, Co-based Heusler compounds, etc.) that have been proposed so far are still being confirmed experimentally, and the ideal anomalous Hall effect has not been observed.
The M05 group of the State Key Laboratory of Magnetism, Institute of Physics, Chinese Academy of Sciences / Beijing National Research Center for Condensed Matter Physics has been engaged in the exploration and research of new magnetic functional materials. Associate researcher Liu Enke of this group, with the support of the Humboldt Foundation, went to the Max-Planck Institute for Solid Chemical Physics in Dresden, Germany, to conduct research on magnetic topological materials and magnetoelectric transport. Recently, Liu Enke, in collaboration with Professor Claudia Felser, and several international research teams, discovered a new class of magnetic Weyl semimetals Co 3 Sn 2 S 2 in the Shandite family of compounds. The large abnormal Hall conductances and giant Anomalous Hall angle.
The Shandite compound has a chemical formula of M 3 M ′ 2 X 2 (M is a transition group metal, M ’is a main group metal, and X is a chalcogen element), and includes a two-dimensional kagome lattice structure composed of a transition group metal. The magnetic kagome lattice is an important platform for the generation of new states such as spin liquid, quantum anomalous Hall effect, magnetic Skyrmion in condensed matter physics, and the magnetic Shandite compound Co 3 Sn 2 S 2 is also considered as a single-spin ferromagnet ( half metal), this work has conducted in-depth research on the system. The system has the highest Curie temperature (175 K) in the Shandite family. Its magnetic properties are derived from the kagome lattice of Co atoms, and the magnetic moment is perpendicular to the two-dimensional lattice plane. Liu Enke calculated his electronic band structure, observed the single-spin property of the energy gap at the spin-down channel at the Fermi level, and found that there is a strong band inversion and linear crossing in the spin-up channel This is a possible key feature of the magnetic Weyl state.
Subsequently, Liu Enke grew high-quality single crystal samples by using self-service solvents and other methods. Large anomalous Hall conductance (~ 1130Ω -1 cm -1) and giant anomalous Hall angle (~ 20%) were observed in a wide temperature range of 150 K, which are both an order of magnitude higher than conventional magnetic materials. Electrical transport measurements show that Co 3 Sn 2 S 2 has unsaturated positive magnetoresistance and low carrier concentration, and exhibits a compensated semi-metal characteristic. At the same time, the anomalous Hall conductance has no dependence on temperature and longitudinal conductance, which makes the system occupy the intrinsic "Bayley phase" region in the unified model of the anomalous Hall effect, indicating that the anomalous Hall effect in the system originates from Bailey curvature in momentum space. Co 3 Sn 2 S 2 has a huge abnormal Hall angle due to its stable Bailey curvature and low carrier concentration semi-metal characteristics in a wide temperature range.
This study cooperated with Dr. Sun Yan, who is engaged in theoretical calculations by Max Planck, and conducted in-depth research on the Weyl phase. The results show that the system opens a small energy gap on the nodal ring caused by band inversion, but 3 Weyl nodes with opposite chirality are generated near it. The node is only ~ 60 meV away from the Fermi level, which makes it easy to observe Weyl-related transmission operations. The anomalous Hall conductivity (~ 1100Ω -1 cm -1) calculated based on the Bailey curvature is highly consistent with the experimental measurement. Negative magnetoresistance caused by Shubnikov-de Haas quantum oscillation and chiral anomaly of magnetic resistance in a strong magnetic field was observed experimentally, while ARPEs also measured the theoretically expected band dispersion and Fermi surface distribution. These together prove the accuracy and reliability of the theoretical calculations and provide experimental evidence for the existence of magnetic Weyl topological phases.
This work combines theoretical calculations and electrical transport measurements to discover a new class of magnetic Weyl semimetal systems. A topological magnet with both a large anomalous Hall conductance and a giant anomalous Hall angle is obtained, which tightly associates topological physics with spin electronics, providing an important foundation for basic research and device applications. This research is also the first time that the existence of Weyl fermions was discovered on a magnetic kagome lattice, which provided an ideal carrier for the realization of the high-temperature quantum anomalous Hall effect.