The influence of non-magnetic impurities on band ferromagnetism for an electron gas with &function interaction is investigated in the framework of the quantum field theory applied to phase transitions. The Hartree-Fock approximation is used for non-zero temperatures. An equation for the magnetization M and the critical temperature T c is obtained. It is found that the transition temperature is lowered by increasing impurity concentration. There exists a critical concentration which vanishes the ferromagnetism.
A two-band model with short range interactions between electrons is studied in order to find magnetic and excitonic instabilities. Using the vertex part equation method, some general conditions for the appearance of instabilities leading to phase transitions are derived.Due to the existence of different coupling constants and the possibility of various band shapes, many cases appear and are discussed.On a ktudi6 un modhle a deux bandes d'hnergie d'un gaz d'616ctron interagissant par des forces A court rayon d'action. E n utilisant les dquations pour l'opdrateur d'interaction on a obtenu les conditions gdnhrales ndcessaires pour l'apparition des insta bilit6s magn6-tiques e t excitoniques qui conduisent aux transformations de phase. On a ensuite discub5 plusieurs cas particuliers correspondant i divers types de constantes de couplage et de structures de bandes.
Functional intcgral techniques are utilized in the study of the superconductivity in a system of electrons containing both, a large band and a narrow-one with large cffcctivc mass and an attractive coupling. After deducing the free energy functional, the equations for the order parameter and transition temperature arc obtained and discussed. Finally the electronic specific heat and its jump at T, arc calculated.On Btudie par la technique d'integration fonctionelle la superconduetivith d'une systhrne d'blcctrons a dcux bandes d'kncrgie: une assez kpaisse, I'autre ktroitc a trks grandc masse effective 616~-tronique. On dBduit la fonctionelle de 1'6nergic libre, les Bquations pour la tempkraturc critique ct le paramctre de l'ordrc. Finallcment on calcule la chaleur spbcifique ClCctronique c t son saut a T,.
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