Temperature-dependent reflection spectra of metallic (BEDT-ATD)2X(THF) (X=PF6, AsF6)

Metallic (BEDT-ATD)2X(THF) (X=PF6, AsF6) exhibits a reflectivity dispersion only along the b-axis (stacking axis), thereby the inter-stack transfer integral being very small. The shape of R(ω) is non-Drude-like. The integrated intensity of this conduction electron absorption suggests a markedly narrow bandwidth W=0.2 eV. At low temperaturue, especially below 50 K, strong vibronic modes appears in the //b spectrum, which suggests the screw-axis symmetry is broken below 50 K. It should be noted that this broken symmetry is not correlated to the electrical transport properties.