Thermodynamic Properties of the Mott Insulator-Metal Transition in a Triangular Lattice System Without Magnetic Order

TL;DR

This study investigates the thermodynamic properties of the Mott insulator-metal transition in a triangular lattice organic system, revealing unique entropy behaviors and quantum critical-like phenomena near the transition.

Contribution

It provides new calorimetric evidence of charge and spin entropy changes across the Mott transition without magnetic order in a triangular lattice system.

Findings

Electronic heat capacity increases at the transition, indicating charge sector recovery.

Remaining spin excitations show unique temperature dependence.

Anomalous heat capacity near the Mott boundary suggests quantum critical behavior.

Abstract

The organic system, $\kappa$-[(BEDT-TTF)$_{1-x}$(BEDT-STF)$_x$]$_2$Cu$_2$(CN)$_3$, showing the Mott transition between a nonmagnetic Mott insulating (NMI) state and a Fermi liquid (FL), is systematically studied by calorimetric measurements. An increase of 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 unique temperature dependence, and that the NMI state has a larger lattice entropy originating from the frustrated lattice, which leads to the Pomeranchuk-like effect on the electron localization. Near the Mott boundary, an unexpected enhancement and magnetic-field dependence of heat capacity are observed. This anomalous heat capacity is different from the behavior in the 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.