Magnetic properties of narrow zig-zag graphene nanoribbons from ab initio calculations

Abstract

The graphene nanoribbons are one-dimensional strips of graphene, which are intensively studied because of their remarkable electronic and magnetic properties. In this work, we have studied the zig-zag type ribbons, because it has already been proposed that magnetic edges-states, present in the ground state at the Fermi, could be relevant for spintronic applications. In this report, we have tried to give a reliable description of the ground state of the zig-zag ribbons. At the density functional theory (DFT) level, it is believed that the antiferromagnetic phase is the ground state for both molecular and extended systems. However, the DFT results quantitatively depend on the chosen functional. Moreover, quantum Monte Carlo (QMC), a more accurate high-level theory, predicts the paramagnetic phase to be the ground state for the acene series, the molecular analogue of the narrowest zig-zag graphene nanoribbons. Since these systems are strongly correlated, DFT needs to be validated against a benchmark theory, such as QMC. In this report, we carry out extensive variational and diffusion QMC calculations, and we show that in the ribbon the antiferromagnetic phase is energetically more stable than the simple paramagnetic wavefunction. The QMC energy gains and the magnetic moments are comparable to those obtained by the DFT-GauPBE exchange-correlation (XC) functional. It turns out that the energetics of static magnetic configurations at the GauPBE level is more accurate than the one from the PBE XC-functional for such a strongly correlated system.