Parity law of the singlet-triplet gap in graphitic ribbons
  This work explores the possibility to transfer the parity law of the singlet-triplet gap established for square ladders (gapped for even number of legs, gapless for odd number of legs) to fused polyacenic 1-D systems, i.e., graphite ribbons. Qualitative arguments are presented in favor of a gapped character when the number $n\_{\omega}$ of benzene rings along the ribbon width is odd. A series of numerical calculations (quantitative mapping on spin 1/2 chains, renormalized excitonic treatments and Quantum Monte Carlo) confirm the parity law and the gapless character of the ribbon for even $n\_{\omega}$. 
Mohamad Al Hajj (LPQ), Fabien Alet (LPT), Sylvain Capponi (LPT), Marie   Bernadette Lepetit (CRISMAT), Jean-Paul Malrieu (LP...