Thermodynamic properties of the Mott insulator-metal transition in a triangular lattice system without magnetic order

The organic system ${κ}{{-}}{[{(BEDT{{-}}TTF)}_{1{-}x}{(BEDT{{-}}STF)}ₓ]}₂{Cu}₂{(CN)}₃$, showing a Mott transition between a nonmagnetic Mott insulating (NMI) state and a Fermi liquid (FL), is systematically studied using calorimetric measurements. An increase in the electronic heat capacity at the transition from the NMI state to the FL state which keeps the triangular dimer lattice demonstrates that the charge sector lost in the Mott insulating state is recovered in the FL state. We observed that the remaining low-energy spin excitations in the Mott insulating state show a unique temperature dependence and that the NMI state has a larger lattice entropy originating from the frustrated lattice, which leads to a Pomeranchuk-like effect on the electron localization. Near the Mott boundary, an unexpected enhancement and magnetic field dependence of the heat capacity are observed. This anomalous heat capacity is different from the behavior in a typical first-order Mott transition and shows similarities with quantum critical behavior. To reconcile our results with previously reported scenarios about a spin gap and the first-order Mott transition, further studies are desired.