Magnetic Mott criticality in a κ-type organic salt probed by NMR

Near a Mott transition(1), which can be tuned by controlling either the charge-carrier density(`filling') or the correlation strength ('bandwidth'), lies fascinating emergent behaviour, such as unconventional superconductivity(2,3), and the understanding of the underlying Mott criticality is a longstanding challenge. Recent studies have showed that the bandwidth-controlled Mott criticality (BCMC) involves critical fluctuations in charge(4,5) and lattice(6,7) degrees of freedom. Spin is another degree of freedom and its antiferromagnetic fluctuations are ubiquitous in strongly correlated electrons(8,9). However, the magnetic aspects of BCMC are unexplored. Here, we report on the magnetic criticality brought about by BCMC. Through NMR investigations on a kappa=type organic salt, we observe critical suppression of antiferromagnetic fluctuations accompanied by the critical enhancement of conductance. The two criticalities show the same exponent within experimental error. Site-to-site electron hopping introduces doubly occupied and empty sites, which extinguish stroboscopically the local spins, probably resulting in the identical criticality in charge and spin.