Sound Velocity and Meissner Effect in Light-heavy Fermion Pairing   Systems
  In the frame of a four fermion interaction theory, we investigated the collective excitation in light-heavy fermion pairing systems. When the two species of fermions posses different masses and chemical potentials but keep the same Fermi surface, we found that the sound velocity in superfluids and the Meissner mass or inverse penetration depth in superconductors have the same mass ratio dependence as the ratio of the transition temperature to the zero temperature gap. 
Lianyi He, Meng Jin, and Pengfei Zhuang