image: (a) Summary of anomalous Hall conductivity and anomalous Hall angle in various topological magnets. (b) Summary of anomalous Nernst coefficients in various topological magnets.
Credit: ©Science China Press
The anomalous Hall effect and anomalous Nernst effect are two important physical phenomena in magnetic materials. They manifest as transverse electric potentials arising from the interplay between spontaneous magnetization and an external electric field (or a temperature gradient). These effects not only serve as powerful probes of the Berry curvature in materials but also hold promise for future applications in electronic devices and thermoelectric energy conversion.
However, most previous studies have focused on weakly correlated systems. A key open question is whether stronger anomalous responses can be achieved in strongly correlated electron systems. This question is of interest not only for enhancing material performance but also for deepening the understanding of strongly correlated topological physics. Researchers from the group of Prof. Jia Shuang at Peking University and the group of Prof. Xu Gang at Huazhong University of Science and Technology have recently collaborated to investigate the heavy-fermion ferromagnet CeCrGe3. They observed giant anomalous Hall and anomalous Nernst effects and revealed that these remarkable responses originate from Kondo-driven topological flat bands. The results were published under the title “Giant Anomalous Hall and Nernst Effects in a Heavy Fermion Ferromagnet” in Science Bulletin.
In this work, CeCrGe₃ exhibits exceptionally large anomalous responses. The anomalous Hall angle reaches up to 33%, representing one of the largest intrinsic values reported in bulk materials, while the anomalous Nernst coefficient is approximately 10 μV K⁻¹, ranking second among known topological ferromagnets. Both values are significantly larger than those observed in LaCrGe₃.
What is the origin of such giant anomalous responses? Density functional theory calculations performed by the Xu Gang group provide a clear explanation. The results show that CeCrGe₃ hosts a series of topological Kondo flat bands with strong Ce-4f orbital character. These flat bands carry highly concentrated Berry curvature, leading to the giant anomalous responses. In contrast, LaCrGe₃, which lacks f electrons, exhibits highly dispersive and topologically trivial bands. This comparison directly demonstrates that Kondo flat bands are the key origin of the enhanced anomalous effects, highlighting the cooperative role of electronic correlations and band topology in strongly correlated systems.
Further studies reveal a more complex behavior of scaling relations at elevated temperatures. Experimentally, the scaling relation of the anomalous Hall effect breaks down in CeCrGe₃, while the anomalous Mott relation exhibits pronounced nonlinearity. These observations collectively indicate a gradual destruction of the Kondo flat bands at high temperatures, explaining why the giant anomalous responses mainly emerge at low temperatures.
This work suggests that heavy-fermion materials may serve as an important platform for topology, providing new insights for discovering and designing strongly correlated systems with large anomalous responses. Through material tuning and band engineering, such systems may find practical applications in low-power electronics and efficient thermoelectric energy conversion.