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

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