2014 Nous avons effectué des mesures de la chaleur spécifique du supraconducteur organique (TMTSF)2ClO4 entre 0,4 et 2 K. La forte anomalie de la contribution électronique à la chaleur spécifique observée à 1,22 K a été attribuée à l'établissement de l'ordre supraconducteur tridimensionnel. En outre la très forte dépendance de la contribution électronique en champ magnétique au-dessus de 1,22 K n'est pas caractéristique d'un conducteur usuel dans un état normal. Un champ magnétique de 63 kOe appliqué suivant l'axe c* des monocristaux stabilise un état fondamental du système présentant une très faible valeur de la densité d'états au niveau de Fermi (un état soit semiconducteur à petit « gap » soit semimétallique). On observe sous champ une transition isolantconducteur vers 1,4 K. Nos résultats expérimentaux sont bien en accord avec l'existence d'un large domaine fluctuatif supraconducteur unidimensionnel. Abstract. 2014 The specific heat of the organic superconductor (TMTSF)2ClO4 has been measured between 0.4 and 2 K. A large anomaly of the electronic contribution at 1.22 K has been attributed to the onset of three-dimensional superconducting order. There is a large magnetic field dependence of the electronic contribution above 1.22 K which is not characteristic of a normal conducting state. The application of a magnetic field of 63 kOe along the c*-axis of the single crystals restores a ground state exhibiting a very low density of states at the Fermi level (a narrow gap semiconducting or a semimetallic state). A phase transition towards a conducting state with a finite density of states at Fermi level is visible around 1.4 K. Our specific heat results support the existence of a wide 1-D superconducting fluctuating regime.

We report resistivity measurements along the high conductivity axis of (TMTSF)2AsF6 for various hydrostatic pressures up to 12 kbar, and for transverse magnetic fields of up to 70 kOe, in the temperature range 0.1 K-300 K. We derive a phase diagram which shows a « triple » point near Tc = 1.22 K and Pc = 10.5 kbar, and a reentrant behaviour of the superconducting state inside the semiconducting state. Moreover we observe a striking anisotropy in the transverse critical fields H c2 and large changes in these fields near the « triple » point. These features suggest we are dealing, in these compounds, with non usual superconducting properties. The transverse magnetoresistance also shows very interesting features including a large anisotropy and a very similar behaviour in the conducting and semiconducting phases. These peculiarities might be rationalized if one assumes (on account of the low dimensionality of the electron system) that first the superconducting instability can grow from below about 30 K and secondly that it can coexist to some extent with the competing SDW instability responsible for the semiconducting phase due to the existence of significant electron-electron couplings