Infrared studies of the energy gap in tetrathiofulvalene-tetracyanoquinodimethane (TTF-TCNQ)

Abstract

Infrared studies of single crystals and thin films of the one-dimensional organic metal tetrathiofulvalene-tetracyanoquinodimethane (TTF-TCNQ) are presented. The frequency-dependent conductivity, ${\sigma }_1\left(\omega \right)$, and dielectric function, ${\epsilon }_1\left(\omega \right)$, are obtained by Kramers-Kronig analysis of the single-crystal reflectivity. The results indicate the existence of an energy gap ($E_g=0.14\mathrm{}$ eV) in the optical spectrum. Transmission and reflection measurements on sublimed thin films of TTF-TCNQ yield results for the response functions in excellent agreement with those obtained from single crystals. The thin-film data are presented for several temperatures, both above and below the metal-insulator transition. The energy gap is present at all temperatures; reduction of the temperature sharpens the structure, but the metal-insulator transition is all but invisible at these frequencies. At low temperatures the overall level of the conductivity in the far infrared decreases as the pinned collective mode appears, centered in all films measured to date at 80 ${\mathrm{cm}}^{-1\mathrm{}}$. The data are analyzed in terms of a simple two-fluid model described by a collective mode and a single-particle excitation spectrum. Quantitative estimates of the collective-mode oscillator strength, ${\Omega }_p\simeq 500$ ${\mathrm{cm}}^{-1\mathrm{}}$, effective mass, $M^{*}\simeq 300m^{*}$, where the band mass is $m^{*}\simeq 3m_e$, and lifetime, ${\tau }_c^{-1\mathrm{}}\simeq 0.5$ ${\mathrm{cm}}^{-1\mathrm{}}$ near 60 K, are obtained from the infrared conductivity and dielectric functions. A consistent picuture of TTF-TCNQ is developed in terms of a Peierls-Fröhlich conductor with the Peierls gap established and the Fröhlich mode sliding at temperatures above 58 K and pinned at temperatures below 58 K.