A. Böhme | Searcharxiv

arXiv2021

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 $κ$-(BEDT-TTF)$_2$Cu$_2$(CN)$_3$ can be tuned into a metallic and superconducting state upon applying pressure of 1.5 kbar and more. We have performed dielectric spectroscopy measurements (7 kHz to 5 MHz) on $κ$-(BEDT-TTF)$_2$Cu$_2$(CN)$_3$ single crystals as a function of temperature (down to $T=8$ K) and pressure (up to $p=4$ kbar). At ambient conditions, a relaxor-like dielectric behavior develops below 50 K that shifts toward lower temperatures as the crystal is pressurized. Interestingly, a second peak emerges in $\varepsilon_{1}(T)$ around $T=15$ K, which becomes strongly enhanced with pressure and is attributed to a small volume fraction of metallic puddles in the insulating host phase. When approaching the phase boundary, this peak diverges rapidly reaching $\varepsilon_{1} \approx 10^{5}$. 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.

cond-mat.str-el↗

arXiv2019

Low-Temperature Dielectric Anomalies at the Mott Insulator-Metal Transition

The correlation-driven Mott transition is commonly characterized by a drop in resistivity across the insulator-metal phase boundary; yet, the complex permittivity provides a deeper insight into the microscopic nature. We investigate the frequency- and temperature-dependent dielectric response of the Mott insulator $κ$-(BEDT-TTF)$_{2}$-Cu$_2$(CN)$_3$ when tuning from a quantum spin liquid into the Fermi-liquid state by applying external pressure and chemical substitution of the donor molecules. At low temperatures the coexistence region at the first-order transition leads to a strong enhancement of the quasi-static dielectric constant $ε_1$ when the effective correlations are tuned through the critical value. Several dynamical regimes are identified around the Mott point and vividly mapped through pronounced permittivity crossovers. All experimental trends are captured by dynamical mean-field theory of the single-band Hubbard model supplemented by percolation theory.

cond-mat.str-el↗