Parting the Fermi Sea at the Mott Point: Dynamics of Correlated Electrons Reveals the Mechanism Underpinning Mottness

By increasing the interaction among conduction electrons, a Fermi-liquid-type metal turns into a Mott insulator. This first-order phase transition should exhibit a regime where the adjacent ground states coexist, leading to electronic phase separation, but the range near $T = 0$ remained unexplored because it is commonly concealed by antiferromangetism. Here we map the genuine low-temperature Mott transition by applying dielectric spectroscopy under pressure to quantum-spin-liquid compounds. The dielectric permittivity uniquely distinguishes all conduction regimes around the Mott point, allowing us to reliably detect insulator-metal phase coexistence below the critical endpoint. Via state-of-the-art theoretical modeling we establish the coupling between segregated metallic puddles as the driving source of a colossal peak in the permittivity reaching $\epsilon_1\approx 10^5$ within the coexistence region. Our results indicate that the observed inhomogeneities are the consequence of phase separation emerging from strong correlation effects inherent to Mottness, suggesting a similar 'dielectric catastrophe' in other correlated materials.