Two-dimensional band structure of organic metals (BDT-TTP)2X (X = SbF6, AsF6) studied by polarized reflection spectroscopy

Polarized reflectance spectra were measured on the conductive (010) plane of metallic (BDT-TTP)(2)X (X=ShF(6), AsF6) single crystals at room and low temperatures. Well-defined plasma edges appeared in both directions parallel (parallel to a) and perpendicular (perpendicular to a) to the molecular stack, indicating two-dimensional band structures with a significant anisotropy. The intra- and interstack transfer integrals were estimated from the plasma frequencies in the framework of tight-binding model. Based on these transfer integrals, the energy dispersion, density of state; and Fermi surface were calculated. The Fermi surfaces of both compounds were open in the k(C) direction. The temperature dependence of the transfer integrals was determined by analyzing the low-temperature spectra. The intra-stack transfer integral increased by about 30% whereas the enhancement of inter-stack transfer integral was very weak. So the system became more anisotropic at low temperatures. The strongly appeared CH stretching mode in the perpendicular to a spectra suggested that the conjugated pi-electrons were extended over the hydrogen atoms of BDT-TTP molecule.