Flat-Band-Induced Anomalous Anisotropic Charge Transport and Orbital Magnetism in Kagome Metal CoSn

For solids, the dispersionless flat band has long been recognized as an ideal platform for achieving intriguing quantum phases. However, experimental progress in revealing flat-band physics has so far been achieved mainly in artificially engineered systems represented as magic-angle twisted bilayer graphene. Here, we demonstrate the emergence of flat-band-dominated anomalous transport and magnetic behaviors in CoSn, a paramagnetic kagome-lattice compound. By combination of angle-resolved photoemission spectroscopy measurements and first-principles calculations, we reveal the existence of a kagome-lattice-derived flat band right around the Fermi level. Strikingly, the resistivity within the kagome lattice plane is more than one order of magnitude larger than the interplane one, in sharp contrast with conventional (quasi-) two-dimensional layered materials. Moreover, the magnetic susceptibility under the out-of-plane magnetic field is found to be much smaller as compared with the in-plane case, which is revealed to be arising from the introduction of a unique orbital diamagnetism. Systematic analyses reveal that these anomalous and giant anisotropies can be reasonably attributed to the unique properties of flat-band electrons, including large effective mass and self-localization of wave functions. Our results broaden the already fascinating flat-band physics, and demonstrate the feasibility of exploring them in natural solid-state materials in addition to artificial ones.

Automated summary

In this study, the emergence of flat-band-dominated anomalous transport and magnetic behaviors in a paramagnetic kagome-lattice compound, CoSn, was demonstrated. The existence of a kagome-lattice-derived flat band was confirmed through measurements and calculations. The resistivity within the kagome lattice plane was found to be significantly larger than the interplane resistivity, and the magnetic susceptibility under the out-of-plane magnetic field was much smaller due to the introduction of unique orbital diamagnetism. These anomalous and giant anisotropies were attributed to the properties of flat-band electrons, such as large effective mass and self-localization of wave functions.