Commensurate and Incommensurate Spin-Density Waves and a Modified Phase Diagram of the Bechgaard Salts

Abstract

We have performed transport and ${}_{}{}^{13}{}_{}{}^{}C$ NMR measurements on the sulfur-based Bechgaard salt, (TMTTF${)}_2$Br, at different pressures. At low pressures the ground state antiferromagnetism is commensurate with the lattice, while at high pressures many signatures of incommensurate density waves are found. Evidence that the transition, or crossover, between the two regimes is coincident with the suppression of a charge gap induced by coupling of the electronic states to the anion potential is presented.