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

The infrared response of the organic conductor (TMTSF)₂PF₆ and the Mott insulator (TMTTF)₂PF₆ 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 (TMTTF)₂PF₆, 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 (TMTTF)₂PF₆ exhibits a crossover from a quasi-one-dimensional to a higher-dimensional metal upon reducing the temperature. For (TMTSF)₂PF₆ 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.