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We performed a high-resolution angle-resolved photoemission spectroscopystudy 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$(\pi, 0)$ pointwhich cross the Fermi level at similar Fermi wave vectors to form nearlycircular electronlike Fermi surface pockets. We observe a nearly isotropic$\sim$ 8.5 meV superconducting gap ($\Delta/k_BT_c\sim 7$) on these Fermisurfaces. Our analysis of the band structure around the Brillouin zone centrereveals two additional electronlike Fermi surfaces: a very small one and alarger one with $k_F$ comparable to the FS pockets at M. Interestingly, a SCgap with a magnitude of $\sim$ 8 meV also develops along the latter FS. Ourobservations are consistent with the s-wave strong coupling scenario.

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