Heteronuclear Cooper Pairs in An Ultracold Atomic Gas
  In an ultracold mixture of two different Fermi species of atoms, Cooper pairs can be formed between two different atoms. The masses of one atom and its partner in this kind of Cooper pairs may differ by order of magnitude. In this system, each species of atoms are in the same atomic spin state and two species have the same atomic densities. The pairing gap diminishes if two species have different densities and vanishes when the density imbalance reaches a critical value. 
Yongle Yu