Antiferromagnetism of the Hubbard Model on a Layered Honeycomb Lattice – Is MgB2 a Nearly-Antiferromagnetic Metal? –

As a model for the B 2 p z bands of MgB 2 , a half-filled Hubbard model on a honeycomb lattice is studied, and a possibility of an antiferromagnetism (AF) at the ground state is examined by Monte Carlo calculations as well as a random phase approximation method. On a single-layer honeycomb lattice, a paramagnetic to AF phase transition occurs at a finite value of the repulsion energy U c / t ≃3.6, due to the loss of the density of states at the fermi level. On a layered honeycomb lattice, however, inter-layer hopping term creates a perfect nesting of the fermi surface, and leads the AF with U c =0. From these facts, we discuss that MgB 2 is possibly a nearly-AF metal, where the AF of the 2 p z bands is destroyed by carrier doping.