Antiferromagnetism of the Hubbard model on a layered honeycomb lattice -: Is MgB2 a nearly-antiferromagnetic metal?

As a model for the B 2p(z) bands of MgB2, a half-filled Hubbard model on a honeycomb lattice is studied, and a possibility of all antiferromagnetism (AF) at the ground state is examined by Monte Carlo calculations as well as a random phase approximation method. On a singlelayer honeycomb lattice, a paramagnetic to AF phase transition occurs at a finite value of the repulsion energy U-c/t similar or equal to 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 MgB2 is possibly a nearly-AF metal, where the AF of the 2p(z) bands is destroyed by carrier doping.