Band-structure parameters of a series of tetramethyltetraselenafulvalene [(TMTSF)2X] compounds

Abstract

Using semiempirical molecular-orbital calculations, we have obtained the principal transfer integrals which parametrize the band structure of tetramethyltetraselenafulvalene salts [${\left(\mathrm{TMTSF}\right)}_{2}X$] [$X=\mathrm{Cl}{\mathrm{O}}_4, \mathrm{Re}{\mathrm{O}}_4, \mathrm{FS}{\mathrm{O}}_3, \mathrm{P}{\mathrm{F}}_6 \left(300\mathrm{} \mathrm{and} 4 \mathrm{K}\right), \mathrm{and} \mathrm{As}{\mathrm{F}}_6$]. Contrary to what has generally been believed, we find the most important interaction determining interchain bandwidth dispersion to be between third-nearest-neighbor TMTSF cations with respect to Se-Se contact distances. Moreover, we find this interaction to yield a transverse bandwidth of order 47-52 meV for all values of the anion $X$ we considered. This band-width is large enough to sustain quasi-two-dimensional coherent transport behavior well above the metal-insulator transition and possibly at room temperature as well, but of not sufficient magnitude to create a closed Fermi surface within the first Brillouin zone. In addition, we determine the strain components for the major transfer integrals. From these we conclude that the experimental temperature and pressure behavior of the conductivity cannot arise solely from a simple effective-mass dependence on the ${\left(\mathrm{TMTSF}\right)}_{2}X$ transfer integrals.