A. Mailman, S.M. Winter, X. Yu, C.M. Robertson, W. Yong, J.S. Tse, R.A. Secco, Z. Liu, P.A. Dube, J.A. Howard, R.T. Oakley.
In principle the unpaired electrons provided by a solid state array of neutral radicals should be capable of serving as charge carriers, giving rise to a half-filled (f = ½) energy band and a metallic ground state. The problem with the idea lies in the fact that there is an intrinsically high Coulombic barrier (U) to charge transfer in such a system. Overcoming the onsite charge repulsion requires radicals with extensive spin delocalization to lower U, and strong intermolecular resonance interactions to increase the electronic bandwidth W. When W > U, the Mott-Hubbard gap ΔE = U - W should vanish and a metallic state prevail.