Parity law of the singlet-triplet gap in graphitic ribbons M. Al Hajj, F. Alet, S. Capponi, M. B. Lepetit, J.-P. Malrieu and S. Todo Eur. Phys. J. B, 51 4 (2006) 517-522 Published online: 28 June 2006 DOI: 10.1140/epjb/e2006-00259-9