Low temperature thermodynamical properties of the organic chain conductor (TMTSF)(2)AsF6

We report on specific heat measurements of the quasi-one-dimensional organi c salt (TMTSF)(2)AsF6 in its spin density wave state between 75 mK and 7 K. Similarly to (TMTSF)(2)PF6, we find discontinuities in the lattice contrib ution at 1.9 K an d 3.5 K ascribed to sub-spin density wave phases. Time-de pendent effects due to dynamics of low-energy excitations in metastable sta tes occur only below 0.2 K which yields an activation energy for the equili brium energy relaxation process of 0.34 K, 4-5 times smaller than found for (TMTSF)(2)PF6. Finally the reduction of the low-energy excitations contrib ution to the specific heat in comparison to PF6 reveals an intermediate cub ic-like regime between 0.25 and 0.5 K that we tentatively describe as the p hason contribution of the incommensurate spin density wave modulation.