Quantum Hall effect and optical magnetoconductivity of two-dimensional topological nodal-line semimetals

We investigate a gapped two-dimensional nodal-line semimetal subjected to a perpendicular magnetic field. We identify an unusual pattern for Landau levels, where the energy of Landau levels first decreases and then starts increasing versus the level index. These descending/ascending Landau levels form a stretched checkmark shape. Tuning the magnitude of the magnetic field changes the spacing between Landau levels. At fixed chemical potential in the low-doped regime, the Landau levels move below the chemical potential and get filled from one side, while they move out and get vacant from the other side. Using the Kubo formalism in the linear response theory, we analytically study a gapped nodal-line semimetal's longitudinal and Hall conductivities. In the dc limit, we observe that as the magnetic field is varied at a fixed chemical potential, the Chern number swings back and forth between two consecutive integer numbers, giving rise to bumpy Hall plateaus in a low-doped system. We also find that the longitudinal optical conductivity displays different series of the Shubnikov--de Haas oscillations, corresponding to interband and intraband transition among the descending and ascending sets of the Landau levels.