SPECTROSCOPY OF THE PEIERLS TRANSITION ORDER-PARAMETER IN THE QUASI-ONE-DIMENSIONAL ORGANIC CONDUCTOR (FA)(2)PF6

The broad band spectrum of the optical conductivity of the quasi-one d imensional organic conductor (FA)(2)PF6 (FA = fluoranthene) has been i nvestigated in a wide temperature range including the Peierls transiti on temperature T-p 180 K. A semiconducting gap has been observed both below and above T-p varying between 150 meV at 10 K and 80 meV at 300 K. The (T-p - T)(1/4) temperature dependence for the order parameter h as been deduced from an analysis of the integrated band intensity of p hase phonons thus giving evidence that the Peierls transition is a 2nd order phase transition near the tricritical point.