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We study the pairing mechanism in iron pnictide superconductors based on thefive-orbital Hubbard-Holstein model. Due to Fe-ion oscillations, the s-wavesuperconducting (SC) state without sign reversal (s_{++}-wave state) is inducedby orbital fluctuations by using realistic model parameters. The virtue of thepresent theory is that the famous empirical relation between Tc and theAs-Fe-As bond angle is automatically explained, since the electron-phonon(e-ph) coupling that creates the orbital fluctuations is the strongest when theAs$_4$-tetrahedron is regular. The negative iron isotope effect is alsoreproduced. In addition, the magnitude of the SC gap on hole-pockets ispredicted to be rather insensitive to the corresponding d-orbital (xz/yz- orz^2-orbital), which is consistent with the recent bulk-sensitive angle-resolvedphotoemission spectroscopy (ARPES) measurement for (Ba,K)Fe$_2$As$_2$ andBaFe$_2$(As,P)$_2$. These obtained results indicate that theorbital-fluctuation-mediated s_{++}-wave state is a plausible candidate foriron pnictides.

physical review. b, condensed matter and materials physics

Orbital Fluctuation Theory in Iron Pnictides: Effects of As-Fe-As Bond  Angle, Isotope Substitution, and $Z^2$-Orbital Pocket on the  Superconductivity