Electrodynamics In Organic Dimer Insulators Close To Mott Critical Point

Organic layered charge-transfer salts kappa-(BEDT-TTF)(2)X form highly frustrated lattices of molecular dimers in which strong correlations give rise to Mott insulating states situated close to the metal-to-insulator phase boundary. The salts kappa-(BEDT-TTF)(2)Cu-2(CN)(3) and kappa-(BEDT-TTF)(2)Ag-2(CN)(3) have been considered as prime candidates for a quantum spin liquid, while kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]Cl has been suggested as a prototypical charge-order-driven antiferromagnet. In this paper, we summarize and discuss several key results, including some not reported previously, obtained in search to clarify the competition of these two ground states. The origin of anomalous dielectric response found at low temperatures in all three salts is also discussed. We conclude by pointing out the relevant new insights into the role of frustration and random disorder in the suppression of magnetic ordering and formation of the spin liquid state.