Strong nodeless pairing on separate electron Fermi surface sheets in (Tl, K)Fe$_{1.78}$Se$_2$ probed by ARPES

TL;DR

This study uses ARPES to reveal nodeless, isotropic superconducting gaps on separate electron Fermi surface sheets in Tl/K-doped FeSe, supporting an s-wave strong coupling pairing mechanism.

Contribution

It provides detailed ARPES measurements showing multiple electron Fermi surfaces with superconducting gaps in Tl/K-doped FeSe, highlighting nodeless pairing on separate sheets.

Findings

Nearly isotropic 8.5 meV superconducting gap on electron Fermi surfaces

Existence of multiple electronlike Fermi surfaces with gaps

Results support an s-wave strong coupling pairing scenario

Abstract

We performed a high-resolution angle-resolved photoemission spectroscopy study of the Tl$_{0.63}$K$_{0.37}$Fe$_{1.78}$Se$_2$ superconductor ($T_c=29$ K). We show the existence of two electronlike bands at the M$(\pi, 0)$ point which cross the Fermi level at similar Fermi wave vectors to form nearly circular electronlike Fermi surface pockets. We observe a nearly isotropic $\sim$ 8.5 meV superconducting gap ($\Delta/k_BT_c\sim 7$) on these Fermi surfaces. Our analysis of the band structure around the Brillouin zone centre reveals two additional electronlike Fermi surfaces: a very small one and a larger one with $k_F$ comparable to the FS pockets at M. Interestingly, a SC gap with a magnitude of $\sim$ 8 meV also develops along the latter FS. Our observations are consistent with the s-wave strong coupling scenario.