Novel aspects of pairing in multiband systems

The iron-based superconductors discovered in 2008 by Hosono offer a 2nd class of high-Tc materials on which to test ideas about unconventional pairing. In the original subclass of iron pnictide systems, based on layers with Fe and a pnictogen like As, classic spin fluctuation pairing theory works rather well to describe the phenomenology of multiband gap structures observed in experiment. In the more strongly correlated Fe chalcogenides, however, experiments have challenged this paradigm. I'll review what's known about the chalcogenide systems, including a subclass where the electronic structure appears to be quite distinct from the pnictides, and to counter the assumptions of spin fluctuation theory. I'll then consider the effect of two hitherto neglected effects on pairing: a) the presence of incipient bands (bands which approach but do not cross the Fermi level), and b) "orbital selectivity" (variable quasiparticle weights for different Fe d-orbitals), and show how the experimental observations of gap structure in these fascinating materials may be understood. ​