Tunneling into multiband superconductors: The case of magnesium diboride and iron-based superconductors

Despite a long history, the two-band/two-gap theory of superconductivity (S) had no important implications in the last century. The first specific but representative case was MgB₂, a comparatively simple compound with an unexpectedly high normal-to-superconductor transition temperature T_c = 39 K. Superconductivity in MgB₂ resides in two distinct groups of bands. Whereas the intraband impurity scattering may vary in large limits, the interband scattering remains weak. Recently, multiband superconductivity was found in iron-based pnictides and chalcogenides with strong differences comparing to MgB₂: (i) in new materials superconductivity occurs by chemical doping of antiferromagnetic parents, thus, spin dynamics can play a central role and the Cooper pairing may be mediated by antiferromagnetic (AF) spin fluctuations (in sharp contrast to the conventional phonon mechanism in MgB2); (ii) an extended s-wave gap scenario, with opposite signs on hole and electron Fermi-surface sets, has been experimentally supported (instead of a trivial equal-sign but different-value s-wave gaps in MgB₂); (iii) the parent compounds are “bad" metals in the normal (N) state, this fact quite directly implies strong electron correlations in the Fe-based materials which, most probably, are at the boundary of electronic localization and itinerancy; (iv) the latter statement means that a strong-coupling approach to the electronic states is more appropriate for them, this fact invokes nesting of hole and electron Fermi surface pockets to drive both the spin fluctuations and superconductivity; (v) the family of iron pnictides is characterized by multi (more than two) Fermi surface sets, and an importance of multiband structure is obviously exhibited in recent measurements of the Fe isotope effect.