Three-Body Collective Excitations in the Superconducting Phase of MgB₂

It is shown that coherent behavior of Cooper's pairs and electrons in the two-band superconductors $MgB_2$ is possible as a result of a strong two-phonon-electron coupling. The spin-$\frac 12$ triples with zero angular momentum are made up of three spin-$\frac 12$ fermions with charge $\emph{e}$. They are gapped Fermi excitations with gap induced by the gaps of the single fermions. Their contribution to the thermodynamics of the $MgB_2$ superconductors is considered.