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We report the first extensive study of longitudinal (ρ a ) and transverse (ρ c ) transport properties versus temperature of the organic quasi one dimensional conductor TMTTF2PF6 over the pressure domain extending from ambient up to 26 kbar. The dimensional cross over observed around 14 kbar separates a 1D region dominated by the Mott-Hubbard gap with activated transport at low pressure from a 3D domain where the temperature range of the metallic behavior, extends up to room temperature along the chains and is progressively increased towards high temperatures in the c direction. In the 1D regime, the different activations obtained from ρ a (T) and ρ c (T) data suggest two different modes of conduction in the longitudinal and transverse directions.

The charge ordering observed in the (TMTTF) 2 X family has been studied by X-ray absorption spectroscopy. XANES measurements at the Sulfur K-edge show no evidence of charge disproportionation larger than 0.5 e, and EXAFS at the Phosphorus K-edge indicate no displacements of the PF 6 anion larger than 0.05 A. The difficulty to observe a structural signature of this charge ordering is due to the triclinic symmetry of these salts. By contrast, in the monoclinic charge transfer salt (DMtTTF) 2 ClO 4 , a screw axis symmetry constrains the molecular stacks to be uniform. In this real 1/4-filled system, charge localization is observed at about 150 K. We show that around this temperature an incommensurate modulation of reduced wave vector (0.58,0,-0.275) is stabilized. The transition is strongly hysteretic, but no long range order is established. We suggest that this modulation, which has the 4kF periodicity in the chain direction, stabilizes a local antiferroelectric state similar to the one previously observed in (TMTTF) 2 SCN.

We present a study, of the slow dynamics in the heat relaxation at very low temperature (T < 1 K) in the nominally commensurate SDW compound (TMTTF) 2 Br, that we compare to the incommensurate SDW-(TMTSF) 2 PF 6 . Instead of a broad distribution of relaxation times g(log τ) in the PF 6 salt, the dynamics in the Br salt reveals almost discrete bands in the relaxation spectrum. A saturation towards equilibrium is reached very rapidly (within ∼ 20 mn) at T ∼ 100 mK in comparison with PF 6 (more than 10 hr for the same temperature).

We report the transverse magnetoresistance measurements at ambient pressure in the organic conductor (TMTTF)2Br in the temperature range between 4.2 K and 40 K and in magnetic fields up to 8.5 T. We have found isotropic, negative and temperature dependent magnetoresistance which becomes negligible, close to 40 K. We interpret the observed behaviour within the picture of strongly correlated quasi-one dimensional systems.

Conductivity and permittivity of the quasi-one-dimensional conductor (TMTTF) 2 SbF 6 , have been measured at low frequencies (f= 10 2 ∼ 10 7 Hz) under several pressures (P= 0 ∼ 0.5GPa). The real part of the conductivity has a maximum at Tm due to charge localization of Mott-Hubbard type. Below T m , the real part of the permittivity (e') shows a sharp peak with the magnitude in the order of 10 5 ∼ 10 6 . It has been suggested that the anomaly of e' is due to a charge ordering (CO) phase transition. We found that the CO transition temperature decreases with pressure. Above 0.25GPa, the temperature dependence of e' is strongly dependent on frequency in the frequency range of measurements. We will discuss the pressure effect on the CO phase transition in (TMTTF) 2 X family.

We report magnetotransport measurements in the quasi one dimensional (Q-1-D) organic conductor (TMTTF) 2 Br at pressures up to 26 kbar, down to 0.45 K in magnetic fields up to 19 T along the c * direction. It is found that a superconducting ground state is stabilized under 26 kbar at T c =0.8 K. No magnetic field induced spin density wave (FISDW) transitions are observed below 19T unlike other Q-1-D superconductors pertaining to the selenium series. The computed amplitude of the interchain coupling along transverse directions is unable to explain the missing FISDW instability

Needle axis resistivity measurements on several TMTTF (tetramethyltetrathiafulvalene) salts of the form (TMTTF)2X, with X = Br, ClO4, PF 6, are presented for pressures up to 32 kbar. The pressure-temperature phase diagrams of all three materials are similar with the stabilization under pressure of a metallic state to low temperatures (∼ 20 K), below which there is a well defined metal-insulator transition. For (TMTTF) 2Br the M-I transition is completely suppressed for pressures above some critical pressure Pc ∼ 25 kbar and the compound is metallic to the lowest temperatures considered in this study, ∼ 1 K. Significant differences are found between samples grown from different chemical solvents : in one case the samples have resistance ratios exceeding 400 at 25 kbar with a conductivity greater than 106 (Ωcm) -1 at 4 K, the highest values yet reported for any organic conductor. In the other case the samples show a possible superconducting transition near 3.5 K at 25 kbar, but have smaller resistance ratios ( 35 kbar). Magnetoresistance measurements have been made on samples of (TMTTF)2 Br as a function of pressure, for field applied along the c* axis. The magnetoresistance progressively increases with increasing pressure obtaining substantial values ((ρ(80 kG)-ρ(H = 0))/ρ( H = 0) ∼ 2-4) near Pc. These results establish that under pressure the physical properties of the (TMTTF)2X salts become increasingly to ressemble those of their selenium counterparts, (TMTSeF) 2X.

[(TMTSeF)1-x(TMTTF)x]2ClO4 alloys have been studied under pressure for pressures up to 18 kbar through resistivity measurements. On substitution of TMTSeF in the superconducting compound (TMTSeF)2ClO4 with TMTTF a low temperature metal-insulator (M-I) transition is found at ambient pressure; above some critical pressure, P c, the M-I transition is suppressed giving rise to a metallic state to the lowest temperatures considered in this study. The phase diagrams of the various alloys are of the same form although Pc increases with increasing TMTTF concentration. Superconductivity was not observed in any of these materials in the metallic high pressure regime.

The conducting and magnetic properties of three salts of the DMTTF series are described. Low temperature antiferromagnetic ground states are found for the three compounds. Depending on the anions the crystal organisation is either triclinic (as for the TMTTF salts) or monoclinic. As expected, the position of the magnetic axis is correlated to the crystal symmetry. Finally the origin of the magnetic phase is discussed using the same theoretical framework as for the related TMTTF salts Etude de la conductivite electrique et de la resonance antiferromagnetique de trois sels (ClO 4 , ReO 4 , AsF 6 ): observation d'un etat antiferromagnetique a basse temperature. Etude par diffraction de rayons X: selon l'anion, la maille cristalline est triclinique ou monoclinique. Position des axes magnetiques, fonction de la symetrie cristalline. Discussion de l'origine de la phase magnetique

From the comparison of the physical properties of the (TMTTF)2X salts (mainly X = BF4, ClO4, PF6 and Br), the role of the counterion ordering is discussed and the intrinsic behaviour of the TMTTF chains is evidenced. Then, a parallel is drawn with the properties of the selenium analogs of these salts.