Strong nodeless pairing on separate electron Fermi surface sheets in (Tl, K)Fe_1.78Se₂ probed by ARPES

¹ Beijing National Laboratory for Condensed Matter Physics, and Institute of Physics, Chinese Academy of Sciences Beijing 100190, China
² Department of Physics, Zhejiang University - Hangzhou 310027, China

^a dingh@iphy.ac.cn

Received: 27 January 2011
Accepted: 5 February 2011

Abstract

We performed a high-resolution angle-resolved photoemission spectroscopy study of the Tl_0.63K_0.37Fe_1.78Se₂ superconductor (T_c=29 K). We show the existence of two electron-like bands at the M(π, 0)-point which cross the Fermi level at similar Fermi wave vectors to form nearly circular electron-like Fermi surface pockets. We observe a nearly isotropic ∼8.5 meV superconducting gap (2Δ/k_BT_c∼7) on these Fermi surfaces. Our analysis of the band structure around the Brillouin zone centre reveals two additional electron-like Fermi surfaces: a very small one and a larger one with k_F comparable to the Fermi surfaces at M. Interestingly, a superconducting gap with a magnitude of ∼8 meV also develops along the latter Fermi surface. Our observations are consistent with the s-wave strong-coupling scenario.