Effects of the electron-phonon interaction of the charge ordering in quasi-one-dimensional charge-transfer salts

Abstract

The Bechgaard salts and their sulfur analogs are considered to be in quasi-one-dimensional correlated electron systems with a quarter-filled π band and exhibit various types of charge and/or spin ordering. A variety of mechanisms have been proposed in an attempt to interpret the variety of instabilities, featuring dimensionality, which can be turned by pressure as well as chemically, intrachain dimerization due to the anion columns, and competing on-site and even long-range Coulomb repulsions. Here we investigate quantum and thermal phase transitions in the Bechgaard salts and their sulfur analogs with particular emphasis on lattice degree of freedom. We consider the one-dimensional extended Peierls-Hubbard model with long-range Coulomb and electron-lattice interactions. Due to the intrinsic dimerization accompanied by the anion columns, we introduce alternation to transfer integrals, nearest-neighbor Coulomb interactions and spring constants. We fully demonstrate the interplay between charge ordering and lattice instability within the Hartree approximation. Besides the purely electronic charge and spin density waves which are well established both experimentally and theoretically, we reveal a lattice-tetramerized phase stabilized essentially in Peierls-Hubbard model. Our results inspected and compared with the results of experiments.