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Energy and Dielectric Relaxation in Bechgaard-Fabre Salts

We present an overview of energy and dielectric relaxation measurements on Bechgaard-Fabre salts, which enlarge the already very rich phase diagram. It is shown that a now generic property of the whole family of (TMTTF)2X Fabre salts is charge ordering, which occurs in the temperature range of 40-200 K. This new electronic charge superstructure at 4kF, a generalization of the clas&not ; ; sical Wigner crystal, manifests itself by a ferroelectric character. Specific heat measurements in different controlled kinetic conditions in the vicinity of the sub-phase at 3.5 K in the spin-density wave state of (TMTSF)2PF6 show features characteristic of freezing in supercooled liquids, indicating a glass transition. Thermodymimical measurements below 1 K on both (TMTTF)2X and (TMTSF)2X salts reveal extremely long dynamics of energy relaxation and nonequilibrium phenomena (aging). For incommensurate Bechgaard salts, the relaxation time distribution is Gaussian while for commensurate ground states in Fabre salts, it shows a bimodal character.

(TMTTF)2X, (TMTSF)2X, spin density waves, ferroelectric order, glass transition, energy relaxation, dielectric relaxation

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