Observation of an isotropic superconducting gap at the Brillouin zone centre of Tl_0.63K_0.37Fe_1.78Se₂

¹ Beijing National Laboratory for Condensed Matter Physics, and Institute of Physics, Chinese Academy of Sciences Beijing 100190, China
² Paul Scherrer Institut, Swiss Light Source - CH-5232 Villigen PSI, Switzerland
³ Centre de Physique Théorique, Ecole Polytechnique, CNRS - F-91128 Palaiseau Cedex, France, EU
⁴ Helmholtz-Zentrum Berlin, BESSY - D-12489 Berlin, Germany, EU
⁵ Department of Physics, Zhejiang University - Hangzhou 310027, China

^(a) p.richard@iphy.ac.cn
^(b) dingh@iphy.ac.cn

Received: 27 August 2012
Accepted: 27 August 2012

Abstract

We performed a high-resolution angle-resolved photoemission spectroscopy study on superconducting (SC) Tl_0.63K_0.37Fe_1.78Se₂ (T_c = 29 K) in the whole Brillouin zone (BZ). In addition to a nearly isotropic ∼8.2 meV 2-dimensional (2D) SC gap (2Δ/k_BT_c ∼ 7) on quasi-2D electron Fermi surfaces (FSs) located around M(π,0,0)-A(π,0,π), we observe a ∼6.2 meV isotropic SC gap (2Δ/k_BT_c ∼ 5) on the Z-centred 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-nearest neighbor exchange interaction in the ferrochalcogenide superconductors.