||鹿野田 一司 教授
||NMR Study of Uniaxial-Strain-Induced Metal-Insulator Transition in the Charge-Frustrated System θ-(ET)2I3
||The θ-(ET)2X family of layered organic conductors, composed of conducting ET layers with triangular lattices and insulating layers of anion molecules X, is the representative charge-frustrated system. The strength of charge frustration is varied by X and determines ground states; for weak frustration, the system exhibits a charge ordering (CO) transition upon cooling, whereas the long-range CO is prevented in a strongly frustrated system, yielding a charge glass (CG) state.
θ-(ET)2I3, which is the most charge-frustrated system with a nearly isotropic triangular lattice, however, does not show the CG or CO transition and remains in the metallic state down to very low temperature, invoking a quantum melting of charges caused by charge frustration. Remarkably, under a uniaxial strain of 2 kbar applied in a direction to reduce the frustration, the electrical resistivity abruptly increases around 140 K, possibly suggesting that the insulating state is induced by the uniaxial strain. However, the microscopic electronic state of this phase has not been uncovered.
Thus, in the present study, we examine the microscopic electronic state of θ-(ET)2I3 under uniaxial strains using the 13C NMR spectroscopy. Under the uniaxial pressure of 2 kbar, the four sharp lines emerge in addition to the spectra that are almost unchanged from those at ambient pressure. The spin-lattice relaxation rates of the additional lines at 50 K, where the resistivity shows an insulating-like behavior, is about one order of magnitude smaller than that in the metallic state at ambient pressure. These results suggest that the metallic and insulating states coexist under uniaxial strain and the latter is a CO state, not CG with the broad spectra. The volume fraction of CO to metal is increased with pressure.