Absence of critical fluctuations at the bandwidth-controlled Mott transition in a molecular conductor

The universality class of the Mott metal-insulator transition in electronic systems strongly coupled to the lattice degrees of freedom has been the subject of recent debate. While transport scaling exponents as well as a critical slowing down of resistance fluctuations point towards Ising-type criticality, a strong involvement of the lattice elasticity suggests a mean-field scenario instead. We investigate the low-frequency resistance noise in the model system κ-(ET)₂Cu[N(CN)₂]Cl0.16em0ex(κ-Cl)---selected because it is known to exhibit critical elasticity---tuned to the Mott end point by hydrostatic pressure. Contrary to previous reports for the related, deuterated κ-Br compound, we do not find any indication of a critical slowing down, but observe a strong and anisotropic change of the relative resistance noise level in the metallic compared to the insulating phase. We speculate within the framework of isostructural criticality that a critical slowing down might emerge in the presence of structural disorder.