Crossover from localized to itinerant states in hydrocarbon Mott insulators

Crossover from an itinerant state to an isolated electronic state in electron-doped polycyclic aromatic hydrocarbon (PAH) was studied for the two smallest zigzag-type molecules of naphthalene (NN) and anthracene (AN) by focusing on their 1 : 1 stoichiometry, A₁(NN) and A₁(AN), with alkali metals (A = K and Rb). The competition between on-site Coulombic repulsion energy (U) and bandwidth (W) was argued in terms of their magnetic and electrical properties upon lattice expansion, when A varies from K, with a smaller ionic radius, to Rb, with a larger ionic radius. The 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, observed in a nearly localized electron system, being apart from an highly correlated Mott insulating state. Crystal structural analyses of the X-ray diffraction profiles show a small difference in lattice parameters of the ab plane among K₁(NN), K₁(AN), and Rb₁(NN), whereas 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. The possibility of an emergent metallic state of K₁(AN) under high pressure has also been described, referring to electrical transport measured under high pressure.