Common Fermi-surface topology and nodeless superconducting gap of K0.68Fe1.79Se2 and (Tl0.45K0.34)Fe1.84Se2 superconductors revealed via angle-resolved photoemission

We carried out high-resolution angle-resolved photoemission measurements on the electronic structure and superconducting gap of K0.68Fe1.79Se2 (T-c = 32 K) and (Tl0.45K0.34)Fe1.84Se2 (T-c = 28 K) superconductors. In addition to the electron-like Fermi surface near M(pi, pi), two electron-like Fermi pockets are revealed around the zone center Gamma (0,0) in K0.68Fe1.79Se2. This observation makes the Fermi surface topology of K0.68Fe1.79Se2 consistent with that of (Tl, Rb)(x)Fe2-ySe2 and (Tl, K)(x)Fe2-ySe2 compounds. A nearly isotropic superconducting gap Delta is observed along the electron-like Fermi pocket near the M point in K0.68Fe1.79Se2 (Delta similar to 9 meV) and (Tl0.45K0.34)Fe1.84Se2 (Delta similar to 8 meV). The establishment of a universal picture on the Fermi surface topology and superconducting gap in the A(x)Fe(2-y)Se(2) (A = K, Tl, Cs, Rb, etc.) superconductors will provide important information for understanding the superconductivity mechanism of iron-based superconductors.