Piers Coleman: Questions on the theory of iron-based superconductors.

This post is linked as a comment to the various excellent talks [1, 2, 3, 4] on the theory of iron-based superconductors presented at the ICTP2010 PDSCES conference. Almost all of the proposed theories are set in momentum space. Many of these theories are based on the s+- gap scenario as proposed by Mazin et al and Kuroki et al. This gap structure  can take unique advantage of the nested electron-hole pockets to develop a robust paired state with gap-nodes positioned between the pockets.  Nevertheless questions remain - the state appears to be far more robust to disorder than current theories can account for. Furthermore, there are clear links between the pairing and the local structure that are not naturally accounted for by the momentum-space based spin-fluctuation theories.

My key question:


what is the relation of iron-based superconductivity  to the local structure and chemistry of the material?  

One might argue that since these systems are of intermediate coupling strength, and that consequently, momentum space is the right venue for theories of  these systems. However, there are three reasons to question such answers:

• The observation by Lee et al (JPSJ, 77, 083704 (2008), that the superconducting transition temperature is sensitive to the local tetrahedral environment surrounding the iron atoms, peaking quite sharply at the point where the structure attains perfect tetragonal structure. To my knowledge, this remarkable observation can not be accounted for by any existing theory. Some have suggested it is connected with the one-body band-structure - but I know of no mechanism by which the sharp selection of the tetrahedral angle is produced by band theory.  In a spin-fluctuation picture, magnetic interactions would also be sensitive to these bond angles, but as yet, there is also no convincing accounting of the tetrahedral maximum.  This leads open the possibility of a many body explanation - could there be a multiparticle (eg pair, or composite pair) bound-state that is symmetry-selected favored by the tetrahedral environment? 

• From a purely pragmatic point of view, if we are to generalize these systems, with for example the goal of raising the transition temperature,  then we need to understand the relationship between the pairing and the local chemistry. 

• Since these systems lie between strong coupling and weak coupling, their physics should be equally accessible from a strong coupling perspective, and one might learn a lot from the exercise of starting from a strong coupling perspective, as suggested by Si and Abrahams.   There is perhaps here, a good analogy with intermediate valence f-electron systems, which can be treated starting with a finite U Anderson model, or as the large Kondo coupling limit of a Kondo model.  Hund's coupling will play an essential role in such starting points.