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 ${\beta }^{\prime }$-type $\mathrm{Pd}{\left(\mathrm{dmit}\right)}_2$ (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.

Figure 1

Frequency and temperature dependence of the real part of the relative dielectric constant of (cation) [Pd(dmit)${}_2$]${}_2$, where the frequency range and the cation are specified within the graph. The inset shows the ac conductivity of the respective compounds.

Figure 2

Taylor-Isard scaling of the ac conductivity data of Me${}_4$P[Pd(dmit)${}_2$]${}_2$, Me${}_4$Sb[Pd(dmit)${}_2$]${}_2$, and EtMe${}_3$Sb[Pd(dmit)${}_2$]${}_2$ with frequencies from 500 Hz to 50 kHz with 31, 5, and 80 steps, respectively, with ${\log }_{10}$ scaling. The top inset shows the frequency dependence at 35 K of the same data set with frequencies from 177 Hz to 1 MHz with 150 steps spread out on a ${\log }_{10}$ scale and Debye and KWW fits with $\beta =0.49$ at the dielectric level. The bottom inset shows a plot of the 21.1-Hz conductivity and ${\epsilon }_0\Delta \epsilon {\omega }_{\mathrm{m}}$.

Figure 3

The top is a plot of the Curie temperature from the dielectric constant as a function of on-site correlation.[20] The top inset shows the relation between the electronic correlation and the frustration parameter $t^{\prime }/t$ in the ${\beta }^{\prime }$-Pd(dmit)${}_2$ salts.[20] The bottom is an estimate of the charge disproportionation as a function of on-site correlation from the Curie constant, where 0 indicates equal charge occupancy of the two molecules of a dimer structure. The lower (blue) dashed line is related to the minimum dipole moment value for a three-dimensional state converted to charge disproportionation.[28] The bottom inset shows a plot of the inverse dielectric constant in EtMe${}_3$Sb[Pd(dmit)${}_2$]${}_2$ used to estimate the Curie parameters (red dashed straight line).