Bound states of Cooper's pairs and Electrons in the Superconducting Phase of $MgB_2$

Naoum Karchev

Sofia University

The two-band theory of superconductivity of $MgB_2$ with additional six-fermion interaction is considered. It is shown that a bound state of a spin-singlet Cooper pair of fermions from one of the bands and an individual fermion from the other one is possible. The spin-1/2 triples with zero angular momentum are made up of three spin-1/2 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. The most promising way to observe the triples in $MgB_2$ is by tunneling experiments.