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Collective Spin and Charge Excitations in Planar Aromatic Molecules

Employing high accuracy fixed node diffusion Monte Carlo (DMC) method we calculated the lowest triplet collective excitation (spin gap), as well as an upper bound for the singlet excitations (charge gap) in a series of charge neutral planar non-ladder aromatic compounds. Both excitation energies lie below the continuum of particle-hole excitation energies obtained from Hartree-Fock orbitals. Hence they can be interpreted as genuine bound states in the particle-hole channel. Assuming a resonating valence bond (RVB) ground state which has been recently suggested for $sp^2$ bonded systems [ M. Marchi, {\em et. al.}, Phys. Rev. Lett. {\bf 107}, 086807 (2011)], offers a unified description of both excited states as two-spinon and doublon-holon bound states. We corroborate our interpretation, by Exact diagonalization study of a minimal model on finite honeycomb clusters.