Nodal gap structure in Fe-based superconductors due to the competition between orbital and spin fluctuations

Abstract

To understand the origin of the nodal gap structure realized in BaFe${}_2$(As,P)${}_2$, we study the three-dimensional gap structure based on the three-dimensional ten-orbital Hubbard model with quadrupole interaction. In this model, strong spin and orbital fluctuations develop by using the random-phase-approximation. By solving the Eliashberg gap equation, we obtain the fully-gapped s-wave state with (without) sign reversal between hole-like and electron-like Fermi surfaces due to strong spin (orbital) fluctuations, so called the $s_{\pm }$-wave ($s_{++}$-wave) state. When both spin and orbital fluctuations strongly develop, which will be realized near the orthorhombic phase, we obtain the nodal s-wave state in the crossover region between $s_{++}$-wave and $s_{\pm }$-wave states. The obtained nodal s-wave state possesses the loop-shape nodes on electron-like Fermi surfaces, due to the competition between attractive and repulsive interactions in k-space. In contrast, the SC gaps on the hole-like Fermi surfaces are fully-gapped due to orbital fluctuations. The present study explains the main characters of the anisotropic gap structure in BaFe${}_2$(As,P)${}_2$ observed experimentally.

12 pages, 13 figures, accepted for publication in PRB. An appendix has been added

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• Vortex Structure and Anisotropic Superconducting Gaps in Ba[Fe(Ni)]${}_2$As${}_2$
• Structure of the pairing gap from orbital nematic fluctuations
• Strongly three-dimensional electronic structure and Fermi Surfaces of SrFe${}_2$(As${}_{0.65}$P${}_{0.35}$)${}_2$: Comparison with BaFe${}_2$(As${}_{1-x}$P${}_x$)${}_2$
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• Pairing mechanism for nodal $s$-wave superconductivity in BaFe${}_2$(As,P)${}_2$: Analysis beyond Migdal-Eliashberg formalism
• Nodal superconductivity coexists with low-moment static magnetism in single-crystalline tetragonal FeS: A muon spin relaxation and rotation study
• Non-unitary multiorbital superconductivity from competing interactions in Dirac materials
• Anomalous Superconducting-Gap Structure of Slightly Overdoped Ba(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$
• Unusual nodal behaviors of the superconducting gap in the iron-based superconductor Ba(Fe${}_{0.65}$Ru${}_{0.35}$)${}_2$As${}_2$: Effects of spin-orbit coupling
• Electronic Structure of ${\mathrm{Sr}}_{1-y}{\mathrm{Ca}}_y\mathrm{F}{\mathrm{e}}_2({\mathrm{As}}_{1-x}{\mathrm{P}}_x{)}_2$ ($x$ = 0.25, $y$ = 0.08) Revealed by Angle-Resolved Photoemission Spectroscopy
• Band-dependent superconducting gap in SrFe${}_2$(As${}_{0.65}$P${}_{0.35}$)${}_2$ studied by angle-resolved photoemission spectroscopy