Topological aspects of the band structure of monolayer \beta-Sb in flat and buckled form : Nodal line to unpinned Dirac cones.

Monolayer antimonene in the beta structure has a buckled honeycomb
structure with a semiconducting band structure. DFT calculations show that
a flat honeycomb structure is a metastable state which has been stabilized
epitaxially on Ag. In the flat form of Sb, the Fermi level occurs near
linear dispersion band crossings, formed by the intersection of two warped
Dirac cones, one derived from p_z orbitals and the other from p_x, p_y orbitals.
This conical intersection corresponds to a nodal line
with a Lissajous pattern in k-space leading to an interesting Fermi
surface with electron and hole pockets in different in-plane
directions, in other words an in-plane angle dependent carrier
polarity (goniopolarity). Tight-binding as well as DFT
calculations show that the nodal line breaks
up into six individual symmetry protected Dirac points
in the slightly buckled form. These Dirac cones are unpinned and can
be moved with increasing buckling until Dirac points of
opposite winding number annihilate beyond a critical buckling angle.
Band structure calculations in the quasiparticle self-consistent GW approximation including
spin-orbit coupling show that a gap opens at the various Dirac crossings
but lead to an indirect zero gap situation.