Anion ordering transition and Fermi surface electron-hole instabilities in the (TMTSF)2ClO4 and (TMTSF)2NO3 Bechgaard salts analyzed through the first-principles Lindhard response function.

The first-principles electron-hole Lindhard response function has been calculated and analyzed in detail for two (TMTSF)2X (X= ClO4 and NO3) Bechgaard salts undergoing different anion-ordering (AO) transitions. The calculation was carried out using the real triclinic low-temperature structures. The evolution of the electron-hole response with temperature for both relaxed and quenched salts is discussed. It is shown that the 2kF response of the quenched samples of both salts display a low temperature curved and tilted triangular continuum of maxima. This is not the case for the relaxed samples. (TMTSF)2ClO4 in the AO state exhibits a more quasi-1D response than in the non AO state and relaxed (TMTSF)2NO3 shows a sharp maximum. The curved triangular plateau of the quenched samples results from multiple nesting of the warped quasi-1D Fermi surface which implies the existence of a large q range of electron-hole fluctuations. This broad maxima region is around 1 % of the Brillouin zone area for the X= ClO4 salt (and X = PF6) but only 0.1 % for the X = NO3 salt. It is suggested that the strong reduction of associated SDWfluctuations could explain the non detection of the SDW-mediated superconductivity in (TMTSF)2NO3. The calculated maxima of the Lindhard response nicely account for the modulation wave vector experimentally determined by NMR in the SDW ground state of the two salts. The critical AO wave vector for both salts is located in regions where the Lindhard response is a minimum so that they are unrelated to any electron-hole instability. The present first-principles calculation reveals 3D effects in the Lindhard response of the two salts at low temperature which are considerably more difficult to model in analytical approaches.