Fermi surface electron–hole instability of the (TMTSF)₂PF₆ Bechgaard salt revealed by the first-principles Lindhard response function

We report the first-principles DFT calculation of the electron–hole Lindhard response function of the (TMTSF)₂PF₆ Bechgaard salt using the real triclinic low-temperature structure. The Lindhard response is found to change considerably with temperature. Near the 2k _F spin 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 _F broad 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 _F bond order wave correlation length. The existence of a triangular plateau of maxima in the low-temperature electron–hole Lindhard response of (TMTSF)₂PF₆ should favor a substantial mixing of q-dependent fluctuations which can have important consequences in understanding the phase diagram of the 2k _F SDW 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)₂PF₆ which 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. (paper)