Crossover from localized to itinerant states in hydrocarbon Mott insulators
Crossover from an itinerant state to an isolated electronic one in electron doped polycyclic aromatic hydrocarbon (PAH) is studied for the two smallest zigzag-type molecules of naphthalene (NN) and anthracene (AN) by focusing on their 1:1 stoichiometric A1(NN) and A1(AN) with alkali metals (A = K and Rb). The competition between on-site Coulomb repulsion energy (U) and band width (W) is argued from magnetic and electrical properties upon lattice expansion, when A varies from K with smaller ionic radius to Rb with larger one. Temperature dependence of magnetic susceptibility shows a pronounced hump associated with antiferromagnetic (AFM) interactions for Rb1(NN), being similar to those of K1(NN) and K1(AN) in the earlier report. On the other hand, Rb1(AN) intriguingly exhibits paramagnetic susceptibility to be observed in a nearly localized electron system apart from an highly correlated Mott insulating state. Crystal structural analyses on X-ray diffraction profiles show a small difference in lattice parameters of the ab plane among of K1(NN), K1(AN), and Rb1(NN), while Rb1(AN) exhibits a significantly larger value than those of the others, being indicative of greatly modified interaction energies. The different magnetic properties observed in Rb1(AN) are interpreted from its modified intermolecular distance. Possibility of an emergent metallic state of K1(AN) under high pressure is also described referring to electrical transport under high pressure.