Optical and infrared properties of tetramethyltetraselenafulvalene [(TMTSF)2X] and tetramethyltetrathiafulvalene [(TMTTF)2X] compounds

The electronic structure of the organic conductors bis-tetramethyltetraselenafulvalene-X [(TMTSF)2X] and bis-tetramethyltetrathiafulvalene-X [(TMTTF)2X] has been investigated by means of polarized optical and infrared reflectance measurements. Analysis of plasma edges in reflectance is used to extract information on transfer integrals. Measurements of infrared reflectance provide information on the energy of charge-transfer processes and on electron-molecular vibration coupling. Far-infrared measurements allow comparison with low-frequency transport properties, and give clues to the transport mechanisms. The main results may be summarized as follows: The (TMTSF)2X class of materials has chain-axis transfer integrals of order 0.25 eV at 300 K and 0.28 eV at 30 K. The b-axis transfer integral is found to vary from 18 to 24 meV for different X. The (TMTTF)2X class has a chain-axis transfer integral of the order 0.18-0.20 eV. No b-axis plasma edge is observable. The infrared conductivity spectra of the materials consist of a broad electronic band with superimposed vibrational fine structure. The band is centered at 300 cm-1 in the best (TMTSF)2X conductors and at 2200 cm-1 in (TMTTF)2PF6, an organic conductor of moderate conductivity. The electron-molecular vibration coupling constants for TMTSF and TMTTF appear to be qualitatively similar to those of TTF (tetrathiafulvalene). A new feature is the observation of considerable coupling to modes involving methyl groups, suggesting that a sizable charge density is located near these groups. The electronic band in (TMTSF)2PF6 sharpens at low temperature, and a pseudogap at 180 cm-1 is formed at temperatures above the metal-insulator transition. This behavior is discussed in terms of a possible spin-density-wave contribution to the conductivity. The spin-density-wave amplitude is estimated to be 0.02μB.