Phase coexistence at the first-order Mott transition revealed by pressure-dependent dielectric spectroscopy of κ − ( BEDT − TTF ) 2 − Cu 2 ( CN ) 3

The dimer Mott insulator $\\ensuremath{\\kappa}\\text{\\ensuremath{-}}{(\\mathrm{BEDT}\\text{\\ensuremath{-}}\\mathrm{TTF})}_{2}{\\mathrm{Cu}}_{2}{(\\mathrm{CN})}_{3}$ can be tuned into metallic and superconducting states on applying pressure of 1.5 kbar and more. We have performed dielectric measurements (7.5 kHz to 5 MHz) on $\\ensuremath{\\kappa}\\text{\\ensuremath{-}}{(\\mathrm{BEDT}\\text{\\ensuremath{-}}\\mathrm{TTF})}_{2}{\\mathrm{Cu}}_{2}{(\\mathrm{CN})}_{3}$ single crystals as a function of temperature (down to $T=8$ K) and pressure (up to $p=4.3$ kbar). In addition to the relaxor-like dielectric behavior seen below 50 K at $p=0$, that moves toward lower temperatures with pressure, a second peak emerges in ${\\ensuremath{\\varepsilon}}_{1}(T)$ around $T=15$ K. When approaching the insulator-metal boundary, this peak diverges rapidly reaching ${\\ensuremath{\\varepsilon}}_{1}\\ensuremath{\\approx}{10}^{5}$. Our dynamical mean-field theory calculations substantiate that the dielectric catastrophe at the Mott transition is not caused by closing the energy gap, but due to the spatial coexistence of correlated metallic and insulating regions. We discuss the percolative nature of the first-order Mott insulator-to-metal transition in all details.