Evidence from $\mathsf{^1}$H-NMR for a Crossover from “Local-Moment” Antiferromagnetism to Spin-Density Wave in (TMTTF)$\mathsf{_2}$Br with Application of Pressure

We have made measurements of the proton spin-lattice relaxation in the low-temperature antiferromagnetic ground state of (TMTTF)$_2$Br at different pressures. At ambient pressure, the relaxation rate $1/T_1$ is peaked at the transition temperature with isotropic critical fluctuations apparent over a wide temperature range. $1/T_1$ declines monotonically as the temperature is lowered. Application of hydrostatic pressure results in additional relaxation, very likely from collective excitations, in the ordered phase. The results taken together imply the system is driven from commensurability at a very low pressure ($< 4$ kbar). The behavior is summarized in a $P-T$ phase diagram