Low-temperature thermodynamic investigation of the sulphur organic salts (TMTTF)2PF6and (TMTTF)2Br (TMTTF tetramethyltetrathiafulvalene): II. Dynamical aspects

This contribution is in continuation of our preceding publication (Lasjaunias J C, Brison J P, Monceau P, Starēsinić D, Biljaković K, Carcel C and Fabre J M 2002 J. Phys.: Condens. Matter 14 837) in which we have considered general aspects of the low-temperature thermodynamical properties of the quasi-one-dimensional organic salts based on sulphur donors with different ground states: (TMTTF) 2 PF 6 in the spin–Peierls state and (TMTTF) 2 Br (TMTTF = tetramethyltetrathiafulvalene) with commensurate spin modulation. In this part, part II, we present our results on the dynamical aspects related to the non-equilibrium phenomena measured below 1 K. The metastable states excited to slightly higher temperature (by only a few per cent of the starting T 0 ) relax very slowly to the heat bath environment, depending on the duration of the heat pumping. We compare the features observed in the relaxation rates in these two sulphur compounds with those measured for the incommensurate spin-density-wave compound (TMTSF) 2 PF 6 . We discuss the possible nature of the long-living low-energy excitations lying at the origin of this complex dynamical behaviour, in relation to their ground states.