Disordered conduction in single-crystalline dimer Mott compounds

We study the temperature and frequency dependence of the dielectric constant and the ac conductivity in the dimer Mott insulator family beta'-type Pd(dmit)(2) (where dmit denotes 1,3-dithiole-2-thione-4,5-dithiolate). We find that all compounds within this family have relaxorlike dielectric relaxation with a Curie-Weiss increase in the dielectric constant with decreasing temperature. An analysis of the dielectric data shows a clear relation between electronic correlation and the dipole-dipole interaction; also, geometrical frustration appears to weaken this interaction. Furthermore, the dc and ac conductivities are shown to be closely correlated and follow the same universal ac conductivity as disordered solids.