COMMENSURATE AND INCOMMENSURATE SPIN-DENSITY WAVES AND A MODIFIED PHASE-DIAGRAM OF THE BECHGAARD SALTS

We have performed transport and C-13 NMR measurements on the sulfur-ba sed Bechgaard salt, (TMTTF)(2)Br, at different pressures. At low press ures the ground state antiferromagnetism is commensurate with the latt ice, 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 ind uced by coupling of the electronic states to the anion potential is pr esented.