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

A detailed thermodynamical investigation of the quasi-one-dimensional sulphur-based organic salts (TMTTF) 2 PF 6 and (TMTTF) 2 Br is presented in the temperature range from 30 mK to 7 K. In this part (part I), we consider the general aspects of the low-temperature specific heat of these materials in relationship with their ground states and we compare them with those previously measured for selenium members of the same family. All these compounds exhibit very similar thermodynamical behaviour, despite a variety of electronic ground states: spin-Peierls for (TMTTF) 2 PF 6 , commensurate antiferromagnetic spin modulation for (TMTTF) 2 Br, incommensurate spin-density wave for (TMTSF) 2 PF 6 (TMTSF≡tetramethyltetraselenafulvalene). We show that the low-energy excitations which are predominant below 1 K have a similar character to that observed in glassy systems, at least on short timescales of measurements. The dynamical aspects of the non-equilibrium thermodynamics will be presented separately in the following part (part II).