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

Abstract

We explore the dielectric properties of the molecular quantum-spin-liquid candidate κ-(BEDT-TTF)2Ag2(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 ɛ1≈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→0. Overall, the pressure-dependent dielectric response is remarkably similar to the less-correlated sister compound κ-(BEDT-TTF)2Cu2(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.