DFT + U + V Study of Magnetic Ordering in Single-Layer Pentahexoctite: Implications for Magnetic Device Platforms

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.