Pairing in graphene: A quantum Monte Carlo study

To address the issue of electron correlation driven superconductivity in graphene, we perform a systematic quantum Monte Carlo study of the pairing correlation in the t-U-V Hubbard model on a honeycomb lattice. For $V=0$ and close to half filling, we find that pairing with $d+id$ (d_x²-y²+idxy^' in its specific form) symmetry dominates pairing with extended-s symmetry. However, as the system size or the on-site Coulomb interaction increases, the long-range part of the $d+id$ pairing correlation decreases and tends to vanish in the thermodynamic limit. An inclusion of nearest-neighbor interaction V, either repulsive or attractive, has a small effect on the extended-s pairing correlation, but strongly suppresses the $d+id$ pairing correlation.