  The electronic band topology of monolayer $\beta$-Sb (antimonene) is studied
from the flat honeycomb to the equilibrium buckled structure using
first-principles calculations and analyzed using a tight-binding model and low
energy Hamiltonians. In flat monolayer Sb, the Fermi level occurs near the
intersection of two warped Dirac cones, one associated with the $p_z$-orbitals,
and one with the $\{p_x,p_y\}$-orbitals. The differently oriented threefold
warping of these two cones leads to an unusually shaped nodal line, which leads
to anisotropic in-plane transport properties and goniopolarity. A slight
buckling opens a gap along the nodal line except at six remaining Dirac points,
protected by symmetry. Under increasing buckling, pairs of Dirac points of
opposite winding number annihilate at a critical buckling angle. At a second
critical angle, the remaining Dirac points disappear when the band structure
becomes a trivial semiconductor. Spin-orbit coupling and edge states are
discussed.
