  The understanding of density waves is a vital component of our insight into
electronic quantum matters. Here, we propose an additional mosaic to the
existing mechanisms such as Fermi-surface nesting, electron-phonon coupling,
and exciton condensation. In particular, we find that certain 2D spin
density-wave systems are equivalent to 3D Dirac nodal-line systems in the
presence of a magnetic field, whose electronic structure takes the form of
Dirac-fermion Landau levels and allows a straightforward analysis of its
optimal filling. The subsequent minimum-energy wave vector varies over a
continuous range and shows no direct connection to the original Fermi surfaces
in 2D. Also, we carry out numerical calculations where the results on model
examples support our theory. Our study points out that we have yet to attain a
complete understanding of the emergent density wave formalism.
