Spectroscopic Study of Isostructural Charge-Transfer Salts: Non-metallic DMTTA-BF4and Metallic DMTSA-BF4

We present the polarized reflection spectra of non-metallic DMTTA-BF 4 and the isostructural metallic DMTSA-BF 4 . From these spectra, both compounds were regarded as quasi-one-dimensional conductors. From the analysis of these spectra we show that DMTTA-BF 4 is a Mott insulator with U /4 t ≈0.8–1.2 and DMTSA-BF 4 is a weakly correlated metal with structural fluctuation near the room temperature. Resistivity, thermopower, and ESR are consistent with the above picture, and demonstrate the phase transitions at c a . 100 K for DMTTA-BF 4 and at c a . 150 K for DMTSA-BF 4 . The low-temperature reflection spectra of both compounds strongly suggest the breaking of screw-axis symmetry along the conducting axis. These phase transitions are regarded as the spin Peierls transition (DMTTA-BF 4 ) and Peierls (DMTSA-BF 4 ) transition. Assuming a dimerized stack, we estimate the transfer integrals t 1 and t 2 as 0.25 and 0.21 eV from the 10 K spectrum of DMTSA-BF 4 . The coupling constants of molecular vibration of DMTSA with the inter-molecular charge-transfer excitation are obtained analyzing the polarized reflection spectrum of DMTSA-ClO 4 having a dimerized structure.