Electronic interactions in the organic conductors (TMTSF${)}_2$X (X=${\mathrm{ClO}}_4$ and ${\mathrm{PF}}_6$) and (TMTTF${)}_2$X (X=Br and ${\mathrm{PF}}_6$) from their infrared spectra

Accurate measurements of the polarized reflectance spectra of the series of isostructural organic conductors (TMTSF${)}_2$ ${\mathrm{ClO}}_4$, (TMTSF${)}_2$ ${\mathrm{PF}}_6$, (TMTTF${)}_2$Br, and (TMTSF${)}_2$ ${\mathrm{PF}}_6$ (where TMTSF denotes tetra-methy tetraselenafulvalene and TMTIF denotes tetra methyletrathiafulvalene) at room temperature are reported. A dimerized molecular chain model is used to analyze these spectra as well as those previously reported [C. C. Homes and J. E. Eldridge, Phys. Rev. B 42, 9522 (1990); J. E. Eldridge and C. C. Homes, ibid. 43, 13 971 (1991)] for (TMTSF${)}_2$ ${\mathrm{BF}}_4$ and (TMTSF${)}_2$ ${\mathrm{ReO}}_4$ and the corresponding conductivity spectra obtained by Kramers-Kronig transformation. The complex structures observed in the spectra are successfully accounted for by assuming that the double occupancy of the band states is effectively excluded so that the Fermi level lies inside a narrow gap induced in these materials by a small dimerization of the molecular stacks. The spectral changes observed among different members of the series are reproduced by varying a limited number of model parameters in a way that follows closely the known changes of the crystal structural properties. Fitting of the experimental data enables us to estimate the parameters of the band structure, namely the transfer integral and the dimerization gap amplitude. Use of self-consistent relations inherent in the adopted model allows us to conclude that a static potential rather than a Peierls-type phonon induced mechanism plays the dominant role in driving the formation of charge-density waves and the opening of the gap. The same model analysis accounts for the presence of vibronic structures induced by the coupling of the conduction electrons with intramolecular vibrational modes of TMTSF or TMTTF. It has been thereby possible to evaluate the coupling constants for the individual vibrational modes with a greater degree of reliability than in previous attempts. The general physical picture of the studied materials at room temperature as narrow gap correlated semiconductors is briefly discussed.