Flat optical conductivity in the topological kagome magnet TbMn₆Sn₆

The kagome magnet TbMn6Sn6 is a new type of topological material that is known to support exotic quantum magnetic states. Experimental work has identified that TbMn6Sn6 hosts Dirac electronic states that could lead to topological and Chern quantum phases, but the optical response of the Dirac fermions of TbMn6Sn6 and its properties remain to be explored. Here, we perform an optical spectroscopy measurement combined with first -principles calculations on a single-crystal sample of TbMn6Sn6 to investigate the associated exotic phenomena. TbMn6Sn6 exhibits frequency-independent optical conductivity spectra in a broad range from 1800 to 3000 cm(-1) (220-370 meV) in experiments. The theoretical band structures and optical conductivity spectra are calculated with several shifted Fermi energies to compare with the experiment. The theoretical spectra with a 0.56 eV shift for Fermi energy are well consistent with our experimental results. In addition, massive quasi-two-dimensional (quasi-2D) Dirac bands, which have a linear band dispersion in the kx-ky plane and no band dispersion along the kz direction, exist close to the shifted Fermi energy. According to a tight-binding model analysis, the quasi-2D Dirac bands give rise to a flat optical conductivity, while its value is smaller than (about one tenth of) that from the calculations and experiments. It indicates that the other trivial bands also contribute to the flat optical conductivity.