The Study of Metal-insulator transition in Highly conducting metal complexes

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Followings are main results during the term of the project :1. Theoretical study on molecular conductors with two-chain orbitals (12)We investigate a theoretical model applicable to molecular conductors, such as TTP and M(dmit)_2 salts, whose charge carriers originate from two kinds of molecular orbitals. The model Hamiltonian consists of two independent Hubbard chains. The degree of charge transfer between the two chains was calculated. The results qualitatively agree with some experimental data.2. The study of metal-insulator transition in Me_4N[Ni(dmit)_2]_2 (4)We investigate electrical resistivity, thermoelectric power, and magnetic susceptibility under ambient pressure for Me_4N[Ni(dmit)_2]_2 which exhibits superconductivity under applied pressure. We reexamine the metal-insulator (M-I) transition of this salt, and determine the M-I transition temperature to be approximately 40 K from the temperature dependence of the thermopower and electrical resistivity. We also find another ph … More ase transition at 20 K from paramagnetic insulator to diamagnetic insulator. We attribute this newly found transition to the spin-Peierls transition.3. The study of optical and electrical properties for (Me_2-DCNQI}_2Li_1-_xCu_x (15,18)We systematically study transport and optical properties of charge-transfer salts, (Me_2-DCNQI)_2Li_1-_<ax>Cu_x (O<less than or equal>x<less than or equal>1). The temperature dependence of electrical resistivity exhibits semiconducting behavior at x=0, a metal-semiconductor transition for 0.08<less than or equal>x<less than or equal>0.29, and metallic behavior down to 4.2 K for x<greater than or equal>0.39. It also exhibits a semiconductor-semiconductor transition for x<less than or equal>0.14 around 60 K.The reflectance spectra at room temperature reveal two optical excitations in the infrared region. The first excitation appears m salts with x*0 far below 650 cm^<-1>. The second excitation is the so-called mid-infrared band, which appears around 3000 cm^<-1> for O<less than or equal>x<less than or equal>0.14, and shifts downward for x<less than or equal>0 17. We propose a hypothesis which provides an explanation both for the metallic conduction and for the mid-infrared band in a one-dimensional conductor having a quarte Less