A mixed approach to the study of the magnetic and pairing correlations in the Bechgaard salts under pressure

In this work we investigate the role of local interaction in magnetic and superconducting instabilities of Bechgaard salts. A modified Holstein-Hubbard model is introduced for the description of local hole-hole and hole-phonon interactions along the dimerized chain. The hole-phonon coupling constant ?, the phonon frequency ? and the Coulomb repulsion U are determined using optical data. The values of these constants fall in the regime of intermediate adiabaticity and coupling. Our method allows an accurate analysis of the ground state of the model in this range of parameters. This method consists in a mixed approach: a variational state is introduced for the phonon subsystem while the electron subsystem is treated exactly. We stress the need to use an accurate calculation in order to achieve a reliable scenario of the Bechgaard salts properties. In particular, our approach is able to recover both the pairing and the magnetic correlations. Moreover, the effect of pressure is considered and the results are qualitatively related to the behaviour of these compounds under high pressure.