Eliashberg theory

Moreover the effective repulsive Coulomb pseudopotential in the Migdal-Eliashberg theory is expected to decrease (so the effective coupling strength increases) where the interband pairing is dominant. 

Thus, considering results on y(H) only, a possible unconventional pairing in borocarbides cannot be ruled out. Similarly it has been noted by Dobrosavljevic-Grujic and Miranovic (2003) that a sizable anisotropy of the s-wave gap function leads to strong deviations for the specific heat in the superconducting state from the predictions of the one-band isotropic BCS-Eliashberg theory. In particular, at low temperatures gap-node like dependencies may appear. 

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