Fermi surface electron-hole instability of the (TMTSF)₂PF₆Bechgaard salt revealed by the first-principles Lindhard...

We report the first-principles DFT calculation of the electron-hole Lindhard response function of the (TMTSF)₂PF₆Bechgaard salt using the real triclinic low-temperature structure. The Lindhard response is found to change considerably with temperature. Near the 2k_Fspin density wave (SDW) instability it has the shape of a broad triangular plateau as a result of the multiple nesting associated with the warped quasi-one-dimensional Fermi surface. The evolution of the 2k_Fbroad maximum as well as the effect of pressure and deuteration is calculated and analyzed. The thermal dependence of the electron-hole coherence length deduced from these calculations compares very well with the experimental thermal evolution of the 2k_Fbond order wave correlation length. The existence of a triangular plateau of maxima in the low-temperature electron-hole Lindhard response of (TMTSF)₂PF₆should favor a substantial mixing ofq-dependent fluctuations which can have important consequences in understanding the phase diagram of the 2k_FSDW ground state, the mechanism of superconductivity and the magneto-transport of this paradigmatic quasi-one-dimensional material. The first-principles DFT Lindhard response provides a very accurate and unbiased approach to the low-temperature instabilities of (TMTSF)₂PF₆which can take into account in a simple way 3D effects and subtle structural variations, thus providing a very valuable tool in understanding the remarkable physics of molecular conductors.