Using Gap Symmetry and Structure to Reveal the Pairing Mechanism in Fe-based Superconductors

摘要

I review theoretical ideas and implications of experiments for the gapstructure and symmetry of the Fe-based superconductors. Unlike any other classof unconventional superconductors, one has in these systems the possibility totune the interactions by small changes in pressure, doping or disorder. Thus,measurements of order parameter evolution with these parameters should enable adeeper understanding of the underlying interactions. I briefly review the"standard paradigm" for $s$-wave pairing in these systems, and then focus ondevelopments in the past several years which have challenged this picture. Idiscuss the reasons for the apparent close competition between pairing in s-and d-wave channels, particularly in those systems where one type of Fermisurface pocket -- hole or electron -- is missing. Observation of a transitionbetween $s$- and $d$-wave symmetry, possibly via a time reversal symmetrybreaking "$s+id$" state, would provide an importantconfirmation of these ideas.Several proposals for detecting these novel phases are discussed, including theappearance of order parameter collective modes in Raman and opticalconductivities. Transitions between two different types of $s$-wave states,involving various combinations of signs on Fermi surface pockets, can alsoproceed through a ${\cal T}$-breaking "$s+is$" state. I discuss recent workthat suggests pairing may take place away from the Fermi level over asurprisingly large energy range, as well as the effect of glide plane symmetryof the Fe-based systems on the superconductivity, including various exotic,time and translational invariance breaking pair states that have been proposed.Finally, I address disorder issues, and the various ways systematicintroduction of disorder can (and cannot) be used to extract information on gapsymmetry and structure.