  Several intriguing electronic phenomena and electric properties were
discovered in three-dimensional Dirac nodal line semimetals (3D-DNLSM), which
are, however, easy to be perturbed under strong spin-orbit coupling (SOC).
While two-dimensional (2D) layers are an emerging material category with many
advantages, 2D-DNLSM against SOC is yet to be uncovered. Here, we report a
2D-DNLSM in odd-atomic-layer Bi (the brick phase, another Bi allotrope), whose
robustness against SOC is protected by the little co-group C_2v \times Z^T_2,
the unique protecting symmetry we found in 2D.Specially, (4n+2) valence
electrons fill the electronic bands in the brick phase, so that the Dirac nodal
line with fourfold degeneracy locates across the Fermi level. There are almost
no other low energy states close to the Fermi level; this allows to feasibly
observe the neat DNLSM-induced phenomena in transport measurements without
being affected by other bands. In contrast, Other VA-group elements also form
the brick phases, but their DNL states are mixed with the extra states around
the Fermi level. This unprecedented category of layered materials allows for
exploring nearly isolated 2DDNL states in 2D.
