Quantum criticality of a Fermi gas with a spherical dispersion minimum.

We describe the quantum phase transition of a Fermi gas occurring when the quasiparticle excitation energy has a minimum in momentum space which crosses zero on a sphere of radius k(0) not equal 0. The quasiparticles have a universal interaction which controls the physical properties in the vicinity of the quantum-critical point. We discuss possible applications to fermionic superfluids formed by pairing two fermion species, near the point where the densities of the two species become unequal.