Specific heat measurements of the lattice contribution and spin-density-wave transition in (TMTSF)(2)X (X = PF6 and AsF6) and (TMTTF)(2)Br salts

The specific heats of the quasi-one-dimensional organic compounds (TMTSF)(2 )X (X = PF6, AsF6 and ClO4) and (TMTTF)(2)Br were measured over the tempera ture range from 1.8 to 40 K. The lattice contribution follows a T-3-law onl y in a small temperature range below 3 K, and shows a typical bump in C/T-3 ,at about 7.0 K for every compound (4 K for (TMTSF)(2)ClO4). We interpret t his behaviour as a phonon dimensionality crossover from a low-T tridimensio nal behaviour, towards a lower-dimensionality T-alpha-behaviour with (alpha = 2.3 to 2.5) above 3 K, plus two additional Einstein modes in agreement w ith the frequencies of soft lattice modes detected by far-infrared spectros copy. The spin-density-wave (SDW) transition temperature was found to take the values 12.2, 12.4 and 11.7 K for the compounds (TMTSF)(2)PF6, (TMTSF)(2 )AsF6 and (TMTTE)(2)Br, respectively. The magnitude of the SDW transition a nomaly is 4.5%, 2.5% and less than 1% of the total specific heat for each s ample, respectively. The magnitude of the specific heat anomaly for (TMTSF) (2)PF6 is too large to be explained by the electron spin contribution alone , which implies that the lattice is also involved in the transition. From o ur accurate measurements, we confirm that the SDW transition is a second-or der transition without any characteristics of first order, like hysteresis and latent heat. The specific heat was found to be time dependent over the whole measurement temperature range as its value is quite sensitive to the kinetics process for all three compounds.