  We performed a high-resolution angle-resolved photoemission spectroscopy
study on superconducting (SC) Tl$_{0.63}$K$_{0.37}$Fe$_{1.78}$Se$_2$ ($T_c=29$
K) in the whole Brillouin zone (BZ). In addition to a nearly isotropic $\sim$
8.2 meV 2-dimensional (2D) SC gap ($2\Delta/k_BT_c\sim7$) on quasi-2D electron
Fermi surfaces (FSs) located around M$(\pi,0,0)$-A$(\pi,0,\pi)$, we observe a
$\sim 6.2$ meV isotropic SC gap ($2\Delta/k_BT_c\sim5$) on the Z-centered
electron FS that rules out any d-wave pairing symmetry and rather favors an
s-wave symmetry. All isotropic SC gap amplitudes can be fit by a single gap
function derived from a local strong coupling approach suggesting an
enhancement of the next-next neighbor exchange interaction in the
ferrochalcogenide superconductors.
