Pressure-dependent dielectric response of the frustrated Mott insulator κ−(BEDT−TTF)2Ag2(CN)3

We explore the dielectric properties of the molecular quantum-spin-liquid candidate $\ensuremath{\kappa}\text{\ensuremath{-}}{(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF})}₂{\mathrm{Ag}}₂{(\mathrm{CN})}₃$ as a function of frequency, pressure, and temperature. The transition from the incoherent semiconducting to the Mott-insulating state at the quantum Widom line yields a drop of the low-frequency dielectric constant to small positive values ${\ensuremath{\varepsilon}}₁\ensuremath{\approx}10$. The characteristic relaxor-type peak around $T=50$ K moves to lower temperatures when pressurized. An additional feature appears and quickly grows as the percolative first-order transition is approached with rising pressure. Above 4 kbar, it dominates the dielectric response and rapidly shifts to $T\ensuremath{\rightarrow}0$. Overall, the pressure-dependent dielectric response is remarkably similar to the less-correlated sister compound $\ensuremath{\kappa}\text{\ensuremath{-}}{(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF})}₂{\mathrm{Cu}}₂{(\mathrm{CN})}₃$. At the Mott transition, we identify a reentrant insulating behavior at ${T}^★=11$ K which might indicate a low-entropy ground state --- possibly with a spin gap.