  In this work, we investigate the electronic and magnetic properties of
single-layer pentahexoctite, a two-dimensional carbon allotrope patterned by
pentagons, hexagons, and octagons. Using density functional theory (DFT)
calculations incorporating on-site and intersite Coulomb interactions, we find
that type-II Dirac Fermions are formed by a nearly flat band intersecting with
a dispersive band at the Fermi level. We further examine the physical origin of
nearly flat bands of pentahexoctite. Constructing ab initio tight-binding
Hamiltonian based on Wannier functions, we reveal that nearly flat bands of
pentahexoctite originate from quantum-mechanical destructive interference.
Remarkably, our DFT calculations including extended Hubbard interactions show
that hole doping induces a ferrimagnetic phase transition driven by the
enhanced density of states in the nearly flat band. This finding highlights
that monolayer pentahexoctite is a promising candidate for pristine all-carbon
magnetic materials to serve as a platform for future magnetic and spintronic
devices.
