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

TL;DR

This study reveals an unusual isotropic superconducting gap in Ba(Fe$_{0.65}$Ru$_{0.35}$)$_2$As$_2$, with a unique $k_z$ dependence influenced by spin-orbit coupling, challenging previous understanding of gap behaviors in iron-based superconductors.

Contribution

It demonstrates the impact of spin-orbit coupling on the superconducting gap structure, showing both hole Fermi surfaces have similar $k_z$ dependence, unlike other Fe-based superconductors.

Findings

Superconducting gap is isotropic on both electron and hole Fermi surfaces.

The gap magnitude decreases near the Z point, showing a unique $k_z$ dependence.

Spin-orbit coupling influences the mixing of $d_{z^2}$ orbital character into hole Fermi surfaces.

Abstract

We have investigated the superconducting gap of optimally doped Ba(Fe$_{0.65}$Ru$_{0.35}$)$_2$As$_2$ by angle-resolved photoemission spectroscopy (APRES) using bulk-sensitive 7 eV laser and synchrotron radiation. It was found that the gap is isotropic in the $k_x$-$k_y$ plane both on the electron and hole Fermi surfaces (FSs). The gap magnitudes of two resolved hole FSs show similar $k_z$ dependences and decrease as $k_z$ approaches $\sim$ 2$\pi$/$c$ (i.e., around the Z point) unlike the other Fe-based superconductors reported so far, where the superconducting gap of only one hole FS shows a strong $k_z$ dependence. This unique gap structure can be understood in the scenario that the $d_{z^2}$ orbital character is mixed into both hole FSs due to the finite spin-orbit coupling between almost degenerate FSs and is reproduced by calculations within the random phase approximation including the spin-orbit coupling.