Far-infrared reflectance of β″-(BEDT-TTF)2AuBr2: co-existence of free carriers and a single-particle gap at 2Δ=130cm−1

We have measured the temperature dependence of far-infrared reflectance of β″-(BEDT-TTF)2AuBr2 to investigate the possibility of SDW transition. The polarized reflectance in two-dimensional BEDT-TTF layers shows a highly anisotropic feature of conduction band. Both the E⊥c and the E∥c reflectance line-shapes, which are Drude-like at room temperature, demonstrate a clear dip below 100 cm−1 even at 40 K. The frequency-dependent conductivity at 7 K indicates the presence of a single-particle gap at 2Δ=130 cm−1 assignable to the SDW nesting along c. The nesting fluctuation exists already at room temperature because of a strongly one-dimensional nature of the band dispersion.