Interplay between electronic and structural degrees of freedom in quarter-filled low dimensional conductors

We review the basic aspects of the charge density wave (CDW) and bond order wave (BOW) instabilities observed in one dimension (1D) organic conductors at either the 2kF and/or 4kF critical wave vectors. We start by recalling the main features of the coupled structural/electronic Peierls instabilities observed in donor–acceptor (D–A) charge transfer (CT) salts. Then we consider the specific case of 2:1 salts D2X where X is a monovalent anion. We show that the incipient CDW/BOW instabilities of the Bechgaard and Fabre salts are those of the parent quarter-filled CT salts TMTSF-DMTCNQ and TMTTF-DMTCNQ respectively. We also consider more specifically the influence of specific features of D2X salts such as the stack dimerization, the Fermi surface warping and the coupling to the anions. Then we discuss more generally the role of the anions in the Bechgaard and Fabre salts by pointing out the influence of polarization and charge displacement induced by the anion shift. Finally we show that some of these features are also relevant to understand the subtle interplay between structural and electronic degrees of freedom in 2D quarter-filled organic salts such as the (BEDT-TTF)2X series.