Reentrant Metal-Insulator-Metal Transition in (DMe-DCNQI)2Cu System - Competition between Spin and Free Electron Degrees of Freedom-

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.