Indications of a Metallic Antiferromagnetic Phase in the 2 Dimensional $U-t-t'$ Model

We present mean-field and quantum Monte Carlo results that suggest the existence of an itinerant antiferromagnetic ground state in the half-filled $U-t-t'$ model in two dimensions. In particular, working at $t'/t=-0.2$ we found that antiferromagnetic long range order develops at $U_{c_1}/t\approx 2.5$, while a study of the density of states $N(\omega)$ and the response to an external magnetic field indicates that the system becomes insulating at a larger coupling $4<U_{c_2}/t<6$.