Deconfinement transition and dimensional crossover in the Bechgaard-Fabre salts: Pressure- and temperature-dependent optical investigations

Abstract

The infrared response of the organic conductor ${(\text{TMTSF})}_{2}{\text{PF}}_{6}$ and the Mott insulator ${(\text{TMTTF})}_{2}{\text{PF}}_{6}$ are investigated as a function of temperature and pressure and for the electric field polarized parallel and perpendicular to the molecular stacks. By applying external pressure on ${(\text{TMTTF})}_{2}{\text{PF}}_{6}$, the Mott gap rapidly diminishes until the deconfinement transition occurs when the gap energy is approximately twice the interchain transfer integral. In its deconfined state ${(\text{TMTTF})}_{2}{\text{PF}}_{6}$ exhibits a crossover from a quasi-one-dimensional to a higher-dimensional metal upon reducing the temperature. For ${(\text{TMTSF})}_{2}{\text{PF}}_{6}$ this dimensional crossover is observed either with increase in external pressure or with decrease in temperature. We quantitatively determine the dimensional crossover line in the pressure-temperature diagram based on the degree of coherence in the optical response perpendicular to the molecular stacks.