From specific heat measurements (ΔC/C(T C) one knows that the Cooper pairs in the superconducting state are also formed by the heavy quasiparticles. In contrast to normal superconductors it cannot be described by BCS-Theory. Due to the large effective mass, the Fermi velocity is reduced and comparable to the inverse Debye frequency. This leads to the failing of the picture of electrons polarizing the lattice as an attractive force.