Pere Alemany, Jean-Paul Pouget, and Enric Canadell.
Phys. Rev. B 89, 155124 – Published 17 April 2014
A first-principles density functional theory study of the Bechgaard salts (TMTSF)2ClO4 and (TMTSF)2NO3, with special emphasis on the anion ordering transition, is reported. It is found that the main effect of the anion ordering on the band structure occurs through the deformations induced in the TMTSF layers and not through the Coulomb potential of the anions, as it has been almost invariably assumed. It is shown that the anion ordering in (TMTSF)2ClO4 leads to a modulation of the dimerization in successive chains and to a noteworthy polarization of the two partially filled bands around the Fermi level into either the TMTSF A or TMTSF B donors. The calculated anion ordering half gap ΔAO is found to be small but definitely non-nil, ∼14 meV. The suppression of the SDW instability in the relaxed samples is attributed to the deterioration of the Fermi surface nesting in the anion ordered phase. The possible nodal structure of the superconducting gap is discussed in relation with the newly calculated split Fermi surface. Anion ordering in (TMTSF)2NO3 leads to a smaller half gap, 8.9 meV. The anion ordering in this salt leads to a larger charge transfer between the two donors but to a smaller polarization of the two partially filled electron and hole bands. The weak influence of anion ordering into the low-temperature physics of this salt is related to the different periodicity of the anion ordering which leaves the nested parts of the Fermi surface almost unaltered.