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 \neq 0. The quasiparticles have a universal interaction which controls the physical properties in vicinity of the quantum critical point. We discuss applications to fermionic superfluids formed by pairing two fermion species, near the point where the densities of the two species become unequal. 
Kun Yang and Subir Sachdev