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

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$(\ddot{I}€,0)$ 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 ($\hat{I}”/k_B{T}_c\sim 7$) 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.

5 pages, 4 figures