Low-temperature thermodynamical properties of Bechgaard salts:: (TMTSF)2PF6 versus (TMTSF)2AsF6

We report on specific heat measurements of (TMTSF)(2)AsF6 in the temperature range 75 mK to 7 K and compare them with the results obtained for (TMTSF)(2)PF6. In addition to the generic features of density waves (DW) heat capacities, there are similar discontinuities in the lattice contribution in both systems at 1.9 K and 3.5 K, which we ascribe to transitions between sub-SDW phases. The low-energy excitations (ZEE) contribution dominant at T<0.5K, is strongly reduced in the case of AsF6 as well as the time dependent effects in the specific hear. We discuss the origin of the hyperfine T-2 term and give the arguments that it can only be provided by the metastable LEEs. Finally, we relate similarities but also differences, of the specific properties of mixed CDW/SDW states which has been recently found in structural investigations of both compounds.