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

Abstract

In the present paper, we describe how the band structure and the Fermi surface of the iron-based superconductors vary as the Fe-As-Fe bond angle changes. We discuss how these Fermi surface configurations affect the superconductivity mediated by spin fluctuations, and show that in several situations, frustration in the sign of the gap function arises due to the repulsive pairing interactions that requires sign change of the order parameter. Such a frustration can result in nodes or very small gaps, and generally works destructively against superconductivity. Conversely, we propose that the optimal condition for superconductivity is realized for the Fermi surface configuration that gives the least frustration while maximizing the Fermi surface multiplicity. This is realized when there are three hole Fermi surfaces, 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, the occurrence of such a "sweet spot" situation is limited to a narrow window.

9 figures, submitted to focus issue, SUST

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• The poisoning effect of Mn in LaFe(1-x)Mn(x)AsO(0.89)F(0.11): unveiling a quantum critical point in the phase diagram of iron-based superconductors
• Enhancing the three-dimensional electronic structure in 1111-type iron arsenide superconductors by H-substitution
• Hidden robust presence of a hole Fermi surface in a heavily electron doped iron based superconductor LaFe${}_2$As${}_2$
• NMR Investigation of the iron-based superconductors Ca4(Mg,Ti)3Fe2As2O8-y and Ca5(Sc,Ti)4Fe2As2O11-y
• Understanding the re-entrant superconducting phase diagram of an iron-pnictide Ca${}_4$Al${}_2$O${}_6$Fe${}_2$(As${}_{1-x}$P${}_x$)${}_2$
• Observation of the bands with $d_{xy}$ orbital character near the Fermi level in NdFeAs${}_{1-x}$P${}_x$O${}_{0.9}$F${}_{0.1}$ using angle-resolved photoemission spectroscopy