Mott Transition in a Valence-Bond Solid Insulator with a Triangular Lattice

We have investigated the Mott transition in a quasi-two-dimensional Mott insulator ${\mathrm{EtMe}}_3\mathrm{P}[\mathrm{Pd}(\mathrm{dmit}{)}_2{]}_2$ with a spin-frustrated triangular-lattice in hydrostatic pressure and magnetic-field [Et and Me denote ${\mathrm{C}}_2{\mathrm{H}}_5$ and ${\mathrm{CH}}_3$, respectively, and $\mathrm{Pd}(\mathrm{dmit}{)}_2$ ($\mathrm{dmit}=1,3\mathrm{\text{-dithiole-}}2\mathrm{\text{-thione-}}4,5\mathrm{\text{-dithiolate}},\mathrm{\text{dithiolate}}$) is an electron-acceptor molecule]. In the pressure-temperature ($P\mathrm{\text{−}}T$) phase diagram, a valence-bond solid phase is found to neighbor the superconductor and metal phases at low temperatures. The profile of the phase diagram is common to those of Mott insulators with antiferromagnetic order. In contrast to the antiferromagnetic Mott insulators, the resistivity in the metallic phase exhibits anomalous temperature dependence, $\rho ={\rho }_0+A{T}^{2.5}$.

Figure 1

$P\mathrm{\text{−}}T$ phase diagram of ${\mathrm{EtMe}}_3\mathrm{P}[\mathrm{Pd}(\mathrm{dmit}{)}_2{]}_2$ obtained from resistivity measurements. Open circle: metal-insulator transition or crossover temperatures; solid circle: insulator-metal transition temperatures (averages between the cooling and warming processes); square: VBS transitions; triangle: onset superconducting transitions. The solid and doted lines denote the first-order and second-order transitions, respectively. Inset: the schematic VBS state on the triangular lattice of $[\mathrm{Pd}(\mathrm{dmit}{)}_2{]}_2^{-}$.

Figure 2

Resistivity profile of ${\mathrm{EtMe}}_3\mathrm{P}[\mathrm{Pd}(\mathrm{dmit}{)}_2{]}_2$ in hydrostatic pressures ($a=4.08$, $b=4.10$, $c=4.12$, and $d=4.15\mathrm{kbar}$). Inset: temperature dependence of the activation gap obtained as $\Delta =d(\ln \rho )/d(1/T)$.

Figure 3

Magnetic-field dependence of resistivity at 4.08 kbar for (a) cooling and (b) warming processes. Inset: $H\mathrm{\text{−}}T$ phase diagram, where solid and open circles are the $M\mathrm{\text{−}}I$ transition temperatures for cooling and warming processes, and the square is the transition point of the field sweep measurement.

Figure 4

Magnetoresistivity measured at 4.2 K and 4.08 kbar for zero-field-cooling (ZFC) and field-cooling (FC) at 16 T.

Figure 5

Resistivity plotted as a function of $T^{2.5}$ in the metallic state. The inset shows a $\log (\rho -{\rho }_0)$ vs $\log T$ plot at pressures below 30 K.