Far-infrared optical properties of tetrathiofulvalene-tetracyanoquinodimethane (TTF-TCNQ)

Polarized far-infrared reflectance measurements of single-crystal mosaics of TTF-TCNQ have been made at several temperatures above and below the metal-insulator transition temperature range (54–38 K). The optical conductivity and dielectric function from 15 to 900 ${\mathrm{cm}}^{\mathrm{-}1}$ show that the metallic regime above 60 K departs significantly from Drude free-electron behavior; the optical conductivity below 300 ${\mathrm{cm}}^{\mathrm{-}1}$ is depressed before sharply increasing to a dc intercept value of 1700 ${\mathrm{\Omega }}^{\mathrm{-}1}$ ${\mathrm{cm}}^{\mathrm{-}1}$. This strongly suggests the presence of a pseudogap. Strong one-dimensional Peierls fluctuations and electron-electron interactions are discussed as possible causes of the pseudogap and it is suggested that interaction effects are observed at high temperature. Optical evidence of the phase transition between 60 and 37 K to the insulating state is a dramatic drop in the absolute reflectance below 30 ${\mathrm{cm}}^{\mathrm{-}1}$ as well as a broad absorption at 120 ${\mathrm{cm}}^{\mathrm{-}1}$ which gives rise to a sharp edge in the optical conductivity below 300 ${\mathrm{cm}}^{\mathrm{-}1}$. A sharp conductivity peak between 30 and 100 ${\mathrm{cm}}^{\mathrm{-}1}$ is interpreted as a collective antiphase oscillation of the electron charge-density wave on the TCNQ stack against the hole charge-density wave on the TTF stack. The pinned sliding mode is not observed directly.