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In the study of the new iron based superconductors, there are not many global correlations. For example, the early statement that Tc is a maximum (in the 1111 materials) when the As-Fe-As bond angle is the 'regular' tetrahedron value of 109.47 degrees has not been borne out in, e. g., the 111 LiFeAs. However, one correlation pointed out by Bud'ko, Ni and Canfield (Phys. Rev. B 79, 220516 (2009)) based on samples of the 122 iron based superconductors has since (Kim et al., J. Phys.: Conds. Mat. 23, 222201 (2011)) been confirmed in both the 111 and the 11 iron based superconductors. This correlation concerns the discontinuity, deltaC, at the superconducting transition temperature and its dependence on Tc. More recent data, and a comparison of this iron based superconductor behavior with BCS superconductors, will be given. This correlation keeps on being borne out and serves as a.) a metric for sample quality; b.) a test whether a newly discovered iron-containing superconductor is of the same class of superconductor as the other known iron based superconductors and c.) a challenge to theorists to explain this behavior. Host: Joe Thompson |