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Contribution to the study of quasi-one dimensional conductors under magnetic field

Contribution à l'étude des conducteurs quasi-unidimensionnels sous champ magnétique

Abstract : This work deals with the magnetic field effects on the phase diagram of quasi-one dimensional conductors. Its aim is to understand how a magnetic field can destroy a metallic phase and induce a Spin Density wave (SDW) phase, as is experimentally observed in organic conductors such as the Bechgaard salts. It is divided in two main parts : - The magnetic field Zeeman coupling to the one-dimensional(lD)electron gas freezes certain electronic nteraction processes and thus alters the character of ID fluctuations. The evolution of the various instabilities of the electron gas (Charge or Spin Density wave, singlet or triplet superconductivity) is described when the field, as well as the chemical potential, are varied. The latter has been shown to have the same effect on the charge degrees of freedom in the vicinity of half--band filling as the magnetic field on the spin degrees of freedom. -- In the case of the quasi-one dimensional electron gas, SDW condensation is shown to depend both on the Fermi surface geometry (nesting properties) and on a new mechanism due to the magnetic field orbital coupling : the quantization of electronic motion. This new aspect, baptized quantized nesting effect, has the following consequences : . A series of first order transitions between SDW sub--phases appears as the magnetic field increases . Each subphase is labelled by an integer quantum number n. It is characterized by n completely filled Landau levels. . At zero temperature, one expects a Hall effect quantization. This theory allows to understand practically all experimental results obtained so far in field induced SDN phases in the Bechgaard salts.