Collective modes in singlet superconductors
  A brief summary of collective mode excitations that can exist in singlet superconductors with irreducible representation $L$ is given. Such excitations may be classified as the coupled excitations of the charge density $\rho$ and the phase $\phi $ of the order parameter, or of the amplitude $\Delta$ of order parameter. Each of these classes may be further characterized in the long wavelength limit by the irreducible representation $\ell$ of the excitation, which may or may not be the same as the ground state $L$. 
Yafis Barlas and C. M. Varma
cond-mat.supr-con