Magnifying the Landau level transition via resonant coupling in topological node-line semimetal

Since the discovery of Dirac and Weyl semimetals, topological properties in semimetals have stimulated unprecedented research interests as the extension of topological insulators. Among them, node-line semimetals remain largely untapped in experiments, in spite of the numerous progresses made in theory. The difficult lies in the fact that the node-line semimetals lack a distinct signature for photoemission such as Fermi arcs, and typically show very high carrier density and complex band structure, which hinders the detection of Landau levels. Here we report a new way to visualize the Landau level transition in node-line semimetal InBi by utilizing the strong resonant coupling with the gap from another band in infrared spectroscopy. Combining with the density functional calculations and model calculations, we find that this dip corresponds to the gap induced by spin-orbit coupling in another unprotected Dirac band. Interestingly, the observed nodal lines are found to be close to the Fermi level, but cannot be clearly resolved through conventional transport or photoemission experiments. Our result demonstrates InBi as a promising platform for studying node-line fermions and provides an effective experimental approach to magnify the Landau level transition via resonant coupling.