Low-field and anomalous high-field Hall effect in (TMTSF)2ClO 4

We report the study of the Hall effect, at low temperature, in (TMTSF) 2 ClO 4 (slow-cooled state), up to 80 kOe, with the field perpendicular to the (a-b) plane. We show that the linear field dependence of the weak Hall resistance, up to 40 kOe, supports the interpretation of the conducting phase in terms of a quasi planar Fermi surface with a density of carriers corresponding to the stoechiometry of the salt. At higher field, a strong and sharp increase of the Hall constant, is the signature of a phase transition to a semi-metallic state. The transition magnetic field is very temperature dependent. Below 0.7 K, in the semi-metallic state, remarkable steps and plateaux, of the Hall resistance are observed, suggesting either the quantization of the Hall constant of carriers in weakly coupled (a-b) planes or the existence of a sequence of electron instabilities (excitonic phases) Etude de l'effet Hall a basse temperature apres refroidissement lent, jusqu'a un champ de 80 kOe perpendiculaire au plan a-b. Variation lineaire avec le champ de la tres faible resistance de Hall jusqu'a 40 kOe, ce qui indique l'existence d'un conducteur a surface de Fermi quasi plane et une densite de porteurs correspondant a la stoechiometrie. Croissance tres brutale de la constante de Hall en champ eleve, qui correspond a une transition vers un etat semi metallique, avec marches et plateaux de la resistance de Hall au-dessous de 0,7 K. Interpretation soit par la quantification de la constante de Hall des porteurs bidimensionnels a-b, soit par l'existence d'une serie de transitions du type electron-trou (phases excitoniques)