Extended Hubbard model on a C20 molecule

The electronic correlations on a C20 molecule, as described by an extended Hubbard Hamiltonian with a nearest-neighbor Coulomb interaction of strength V , are studied using quantum Monte Carlo and exact diagonalization methods. For electron-doped C20, it is known that pair binding arising from a purely electronic mechanism is absent within the standard Hubbard model (V = 0). Here we show that this is also the case for hole doping for 0 < U/t 3 and that, for both electron and hole doping, the effect of a non-zero V is to work against pair binding. We also study the magnetic properties of the neutral molecule, and find transitions between spin singlet and triplet ground states for either fixed U or V values. In addition, spin, charge and pairing correlation functions on C20 are computed. The spin–spin and charge–charge correlations are very short-range, although a weak enhancement in the pairing correlation is observed for a distance equal to the molecular diameter. (Some figures in this article are in colour only in the electronic version)