Quantum Spin Liquids Unveil the Genuine Mott State

The Widom line identifies the supercritical crossover from a gas-like to a more liquid-like behavior. A similar transition exists in correlated electron liquids, where the interplay of Coulomb repulsion, bandwidth and temperature triggers between the Mott insulating state and an incoherent conduction regime. Here we explore the electrodynamic response of three organic quantum spin liquids, where the absence of magnetic order enables unique insight into the nature of the genuine Mott state. Combining optical spectroscopy with pressure-dependent dc transport and theoretical calculations, we succeeded to construct a phase diagram valid for all Mott insulators on a quantitative scale. Our findings reveal the Pomeranchuk-like anomaly of the Mott transition and metallic fluctuations within the Mott gap, previously predicted but never observed.