The (DMe-DCQNI)2Cu system exhibits the low-dimensional mysterious reentrant metal-insulator-metal (M-IM)
transition driven by the competition between conduction electrons and localized electrons. We have conducted experiments
to study the thermodynamic properties of this reentrant M-I-M transition and have discussed the mechanism of the
transition in the DCNQI-Cu system. Measurements of specific heat and latent heat of samples at various effective pressures
were carried out with thermal relaxation (0.5 < T < 40 K), adiabatic (20 < T < 100 K), and differential thermal analysis
(1.5 < T < 200 K) methods. Using the results of the thermal experiments, we evaluated the Gibbs free energy difference
between the metallic and insulating phases. We succeeded in drawing the phase diagram that reproduced the experimental
results quite well. We conclude that the competition between the free energy of free electrons in the metallic phase
and that of localized electrons with spin s = 1/2 in the insulating phase essentially drives this characteristic reentrant M-IM
transition. The reentrant M-I-M transition observed in this composite system of conduction electrons and localized
electrons is a universal behavior of Fermion particles in which free Fermion particles localize to have spin freedom.
Keywords: AF order, CDW, DCNQI, free energy, low-dimensional, phase transition, specific heat, thermodynamical property,
reentrant metal-insulator-metal transition.
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