SUPERCONDUCTIVITY AND SUPERFLUIDITY IN FERMI SYSTEMS WITH REPULSIVE INTERACTIONS

We study the possibility of a pairing instability in Fermi systems with a purely repulsive interaction potential. The effective interaction between particles at the Fermi surface is shown to contain a contribution from the fermionic background. This contribution is attractive and in many cases it overshadows the interactive potential in the pairing channels with nonzero orbital angular momenta. In particular, we show that 3D Fermi gas with repulsive interaction is unstable against p-pairing. Both the value of T(c) and its field dispersion are consistent with the experimental data for pure He-3. It is also shown that in a magnetic field and in the two-band model there is the possibility of a significant increase of T(c). We also consider the situation in two dimensions, where the effect of the fermionic background is much weaker than in the 3D case. Nevertheless, we show that the positive-U Hubbard model may have an intrinsic instability at arbitrarily small carrier densities. In particular, the Hubbard model with hopping between nearest neighbors is unstable against d-wave pairing with d(xy) symmetry of the gap wave function. The superconducting states we study correspond to the actual minima of the Free energy.