Optical conductivity studies in a one-dimensional organic metal: Tetrathiofulvalene tetracyanoquinodimethan (TTF) (TCNQ)

The optical properties of tetrathiofulvalene tetracyanoquinodimethan (TTF) (TCNQ) are reported in the visible and near infrared from room temperature to 4.2 K. The experimental results are analyzed in terms of simple Drude theory, with corrections made for anisotropic effects introduced by the anisotropic band structure. The inclusion of a frequency dependence in the electron scattering time is found to be unnecessary. The reflectance spectrum for polarization parallel to the conducting axis shows a plasma edge at about 1.4 μm. The plasma frequency is used to infer a value for the effective mass and the tight-binding transfer integral ($t\simeq 0.1$ eV). The plasma frequency and the scattering time lead to a conductivity consistent with the measured room-temperature dc value. All of this information confirms that at room temperature, (TTF)(TCNQ) is a highly anisotropic (pseudo-one-dimensional) metal. Studies of the related materials (asymmetric TTF)(TCNQ) and (tetramethyl TTF)(TCNQ) show similar behavior. The temperature dependence of the scattering rate is found to be consistent with single-phonon scattering, with a contribution from temperature-independent defect scattering which varies from sample to sample and which increases as the number of defects increases. The defect scattering is used to obtain an estimate of the single-particle residual resistivity. Comparison with the typical dc data shows that the measured dc resistivity regularly falls below the residual resistivity by an order of magnitude, implying that the dc conductivity is carried in a collective manner. From the linear temperature dependence of ${\tau }^{-1\mathrm{}}$, a value is obtained for the dimensionless electron-phonon coupling constant $\lambda \simeq 1.3$.