Coexistence model of anisotropic electron hole pairing and unconventional superconductivity in Heavy Fermion compounds

Various Heavy Fermion compounds exhibit unconventional superconductivity together with another electronic instability like a spin density wave or possibly a more general type of anisotropic electron-hole pairing, e.g. a spin nematic state. The coexistence behaviour of these order parameters is studied within a simple weak coupling model. It is found that depeding on the symmetry of the order parameters coexistence or phase expulsion may occur. Whereas the former case is possibly realized for theU-based superconductors, CeCu₂Si₂ may be an example of the second case as observed and discussed in the context of elastic constant anomalies.