Least momentum space frustration as a condition for 'high $T_c$ sweet spot' in the iron-based superconductors

摘要

In the present paper, we describe how the band structure and the Fermisurface of the iron-based superconductors vary as the Fe-As-Fe bond anglechanges. We discuss how these Fermi surface configurations affect thesuperconductivity mediated by spin fluctuations, and show that in severalsituations, frustration in the sign of the gap function arises due to therepulsive pairing interactions that requires sign change of the orderparameter. Such a frustration can result in nodes or very small gaps, andgenerally works destructively against superconductivity. Conversely, we proposethat the optimal condition for superconductivity is realized for the Fermisurface configuration that gives the least frustration while maximizing theFermi surface multiplicity. This is realized when there are three hole Fermisurfaces, where two of them have $d_{XZ/YZ}$ orbital character and one has$d_{X^2-Y^2}$ {\it for all $k_z$} in the three dimensional Brillouin zone.Looking at the band structures of various iron-based superconductors, theoccurrence of such a "sweet spot" situation is limited to a narrow window.