High temperature superconductivity from fine tuning of Fermi surface singularities in iron oxypnictides

In the family of the iron based superconductors, the REFeAsO type compounds with RE being a rare earth metal exhibit the highest bulk superconducting transition temperatures Tc up to 55 amp; 8201;K and thus hold the key to the elusive pairing mechanism. Recently, it has been demonstrated that the intrinsic electronic structure of SmFe0.92Co0.08AsO Tc amp; 8201; amp; 8201;18 amp; 8201;K is highly nontrivial and consists of multiple band edge singularities in close proximity to the Fermi level. However, it remains unclear whether these singularities are generic to the REFeAsO type materials and if so, whether their exact topology is responsible for the aforementioned record Tc. In this work, we use angle resolved photoemission spectroscopy ARPES to investigate the inherent electronic structure of the NdFeAsO0.6F0.4 compound with a twice higher Tc amp; 8201; amp; 8201;38 amp; 8201;K. We find a similarly singular Fermi surface and further demonstrate that the dramatic enhancement of superconductivity in this compound correlates closely with the fine tuning of one of the band edge singularities to within a fraction of the superconducting energy gap amp; 916; below the Fermi level. Our results provide compelling evidence that the band structure singularities near the Fermi level in the iron based superconductors must be explicitly accounted for in any attempt to understand the mechanism of superconducting pairing in these material