Reflectance Spectra and Electrical Resistivity of (Me2-DCNQI)2Li1-xCux

We systematically study transport and optical properties of charge-transfer salts, (Me 2 -DCNQI) 2 Li 1- x Cu x (0≤ x ≤1). The temperature dependence of electrical resistivity exhibits semiconducting behavior at x =0, a metal-semiconductor transition for 0.08≤ x ≤0.29, and metallic behavior down to 4.2 K for x ≥0.39. A semiconductor-semiconductor transition is observed around 60 K for x ≤0.14. The reflectance spectra at room temperature reveal two optical excitations in the infrared region. The first excitation appears in 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 0≤ x ≤0.14, and shifts downward for x ≥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 quarter-filled band.