Superfluid phases of 3He in a magnetic field

We derive from symmetry considerations the free energy as a function of the order parameter in a Ginzburg–Landau approximation. The minima of the free energy with respect to the order parameter are calculated by solving the gap equations in a magnetic field. Using the strong coupling corrections on the basis of a model of a Fermi gas with only short-range repulsions, it is shown that only the A1, ABM and extended BW states can lead to an absolute minimum.