  This talk presents a study of massless relativistic Dirac fields in three
Euclidean dimensions, at finite temperature and density, in the presence of a
uniform electromagnetic background. Apart from explaining the behavior of
Hall's conductivity for graphene, our results show a direct relationship
between the selection of a phase for the Dirac determinant and the generation
(or lack thereof) of Berry's phases and Chern-Simons terms.
