Recently, the research group of Professor Zeng Changgan of the University of Science and Technology of China and the research group of Professor Wu Tao has made important progress in the study of the flat-belt physical properties of cage lattice materials. The team observed the flat-band electron structure near the Fermi energy level in the paramagnetic cage lattice material CoSn and revealed the transport and magnetic anomalies caused by the flat-band electrons. The study was published In the journal Physical Review Letters on Feb. 28 under the title "Flat-Band-Induced Anomalous Anisotropic Charge Transport and Orbital Magnetism in Kagome Metal CoSn."

The electromagnetic properties of solid materials depend largely on their electronic band structure. Compared with the electronic energy bands of the common parabolic dispersion relationship, the linear dispersive energy bands and the non-dispersive flat bands at the two extremes are important platforms for generating new states and new effects. For example, linear dispersive energy bands often lead to novel relativistic effects and topological properties, while the kinetic energy of electrons in the flat belt is quenched, and the Coulomb interaction between electrons dominates, which is an ideal candidate for studying strong correlation effects. The cage lattice is a two-dimensional lattice structure composed of triangular lattices shared by vertices, and theoretical studies have shown that such a special lattice system will have both a linear dispersive energy band with zero electrons effective mass and a flat band with an infinite effective mass. Professor Zeng Changgan's research group researched the special electronic structure and physical properties of the system's materials earlier internationally. For example, the ferromagnetic cage material Fe3Sn2 has experimentally confirmed The presence of a natural non-dispersive flat-band electronic structure, and further revealed that its high-temperature ferromagnetism stems from the flat-band electron association (Phys. Rev. Lett. 121, 096401 (2018), cover article, editor's recommendation), and the new topological state of the antiferromagnetic Dirac state predicted by previous theories (Phys. Rev. B 102, 155103 (2020), editor's recommendation).

Based on the above research, the research team combined multi-scale experimental characterization methods and theoretical analysis to systematically study the flat-band and flat-band physical properties in paramagnetic cage material CoSn. Combining first-principles calculations and angle-resolved photoelectron spectroscopy, the researchers first confirmed the existence of a flat-band electron structure near the Fermi energy level in CoSn, which occupies a large range of momentum space. Electronic transport testing has found that the resistivity of current along the lattice surface of the two-dimensional cage can reach 60 times that of the out-of-plane direction, which is a clear contrast to traditional (quasi)two-dimensional materials. In addition, macroscopic magnetic measurements show that the material susceptibility of the material when the magnetic field is perpendicular to the cage lattice is much smaller than along the in-plane direction, and further NMR experiments show that it originates from the anisotropy of the orbital magnetism. Combined with theoretical analysis, they found that the anomalous anisotropy of these electromagnetic properties can be attributed to the flat-band electron characteristics: the localization of the flat-band electron wave function and the larger effective mass it results in produces a large in-plane resistivity, and the ring current formed by the localized flat-band electron under the vertical magnetic field contributes additional orbital anti-magnetism. This work successfully reveals the macroscopic electronic behavior caused by the flat band in cage lattice materials, which provides a new idea for experiments to further explore the associated electron physical properties induced by flat bands. Compared with the artificial structure represented by magic angle double-layer graphene, which has attracted widespread attention, natural flat belt materials such as CoSn provide another type of material candidate for exploring flat belt physics.

 Figure: (a, b) Flat band near the Fermi energy level in CoSn from experimental observations and theoretical calculations. (c) The ring current generated by the flat-band electrons in the cage lattice under the vertical magnetic field and the corresponding orbital anti magnetism

Huang hao, a doctoral candidate at the university of science and technology of china, is the first author of the paper, and professor Zeng chengguan, professor wu tao and associate researcher li lin are the corresponding authors of the paper. this work was completed in cooperation with professor wang zhengfei of the Hefei national research center for microscale physical sciences of the university of science and technology of china, professor sun zhe of the national synchrotron radiation laboratory, and weng hongming, a researcher at the institute of physics of the Chinese academy of sciences, and received funding from the national natural science foundation of china, the Chinese academy of sciences and Anhui province.

Link:

https://link.aps.org/doi/10.1103/PhysRevLett.128.096601