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Novel Disorder Effects in Multiband Superconductors

Impurities been studied in superconductors for many years because -- depending on the type of impurity and on the symmetry of the superconducting state -- they may break Cooper pairs.  They thus play an important diagnostic role in deducing the type of superconductivity one has in a newly discovered material.  I focus here on the Fe-based superconductors discovered in 2008, which are multiband in character and probably exhibit unconventional pairing states, including so-called "s+/-" states which change sign of the order parameter between bands.  Impurity scattering in such states depends sensitively on the ratio of intra- to interband scattering, giving rise to novel effects of disorder, including possible transitions to conventional "s++" states and lifting of gap nodes. I propose that one can uniquely identify an s+/- state by a sequence of transitions with controlled disorder, observable in bulk quasiparticle transport or NMR.  Finally, I discuss the role of correlations in creating C4 symmetry-broken emergent defect states, which may have important consequences for the ubiquitous electronic nematicity observed in these materials.