Antiferromagnetism of the Hubbard Model on a Layered Honeycomb Lattice - Is MgB$_2$ a Nearly-Antiferromagnetic Metal? -

As a model for the B 2pz 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 $U_c/t \simeq 3.6$, 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 2pz bands is destroyed by carrier doping.