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Rare Earth Element Flavors Quantum Kagome Magnet

The Japanese word kagome(笼目) means a bamboo basket (kago) woven pattern (me) that is composed of interlaced triangles whose lattice points each have four neighboring points. This word has become more and more popular in the community of condensed matter physics as the kagome lattice of magnetic atoms, known as kagome magnet, often exhibits exotic quantum properties such as correlated topological band structures and spin liquid state. Kagome magnets have been thought as tantalizing quantum materials which have potential applications for next-generation electronic technology. Of particular, the inclusion of spin-orbit coupling and out-of-plane ferromagnetic ordering in the kagome lattice can effectively realize the Chern gapped topological fermions with chiral edge modes. However, such ideal kagome magnet with strong out-of-plane magnetization has not been found until now as the magnetic moment on the lattice is lack of anisotropy in general.

The researchers from Peking University, Princeton University and the collaborators report a new kagome magnet TbMn6Sn6, which is distinct from others previously reported. The difference comes from the introduction of rare earth element, terbium (Tb), which hosts a strongly localized, anisotropic magnetic moment. The Tb moment points to the out-of-plane direction and strongly couples with the manganese (Mn) moment on the kagome lattice. As a result, the researchers obtain a topological kagome magnet which is close to ideally satisfying the criteria of the Chern gapped quantum magnet even up to room temperature.

In the work published in Nature (DOI:10.1038/s41586-020-2482-7), the researchers report the  observation of distinct Landau quantization in the kagome lattice of TbMn6Sn6, with the Landau fan exhibiting a large spin-polarized Dirac gap. The topological nature of the gap is further supported by the emergence of pronounced in-gap edge states with no backscattering. Moreover, the researchers evaluate the gap Chern number by matching the Berry curvature expected from the tunneling data with the anomalous Hall scaling. These combined results point to the realization of a quantum-limit kagome Chern magnet in TbMn6Sn6, opening up an avenue for discovering hitherto unknown topological quantum phenomena.

Figure 1.  The topological fermions in TbMn6Sn6. a, Landau fan diagram of the kagome lattice. b, Fitting the Landau fan data with spin-polarised and topologically gapped Dirac dispersions. c, Comparison of the Dirac dispersion obtained from tunneling and photoemission. d, Dirac gap size and bulk out-of-plane magnetization as a function of the magnetic field. e, dI/dV maps taken within the Chern gap energy (130meV) show pronounced step edge states.