One-dimensional (1D) conductors such as Bechgaard and Fabre salts are a prototypal example of correlated systems where the phase diagram is controlled by sizable electron-electron repulsions. In deuterated ${\left(\mathrm{TMTTF}\right)}_2\mathrm{P}{\mathrm{F}}_6$, where this interaction achieves charge localization at ambient pressure on donor stacks, magnetostructural coupling plays a decisive role to stabilize a spin-Peierls (SPs) ground state at $T_{\mathrm{SP}}=13\mathrm{K}$. In this paper, we present the first inelastic neutron scattering investigation of SP magnetic excitations in organics. Our paper reveals the presence above $T_{\mathrm{SP}}$ of sizable critical fluctuations leading to the formation of a pseudogap in the 1D antiferromagnetic (AF) $S=1/2$ magnetic excitation spectrum of the donor stack, concomitant with the local formation of singlet of paired spins into dimers below $T_{\mathrm{SP}}^{\mathrm{MF}}\approx 40\mathrm{K}$. In addition, the inelastic neutron scattering investigation allows us also to probe the SP critical lattice dynamics and to show that at ambient pressure these dynamics are of relaxation or order-disorder type. Below $T_{\mathrm{SP}}$, our paper reveals the emergence of a two gap SP magnetic excitation spectrum towards a well-defined $S=1$ magnon mode and a continuum of two excitations, as theoretically predicted. Our measurements allow us to locate the ambient pressure SP phase of ${\left(\mathrm{TMTTF}\right)}_2\mathrm{P}{\mathrm{F}}_6$ in the classical (adiabatic) limit close to the classical/quantum crossover line. Then we provide arguments suggesting that pressurized ${\left(\mathrm{TMTTF}\right)}_2\mathrm{P}{\mathrm{F}}_6$ shifts to the quantum (antiadiabatic) SP gapped phase, which ends in a quantum critical point allowing the stabilization of an AF phase that competes with superconductivity at higher pressure. Finally, we propose that the magnetostructural coupling mechanism in the Fabre salts is caused by dimer charge/spin fluctuations driven by the coupling of donors with anions.

• Received 9 May 2017
• Revised 20 June 2017

DOI:https://doi.org/10.1103/PhysRevB.96.035127