Superconducting gap symmetry of Ba_0.6K_0.4Fe₂As₂ studied by angle-resolved photoemission spectroscopy

¹  Department of Physics, Tohoku University - Sendai 980-8578, Japan
²  TRIP, Japan Science and Technology Agency (JST) - Kawaguchi 332-0012, Japan
³  WPI Research Center, Advanced Institute for Materials Research, Tohoku University - Sendai 980-8577, Japan
⁴  Department of Physics, Boston College - Chestnut Hill, MA 02467, USA
⁵  Beijing National Laboratory for Condensed Matter Physics, and Institute of Physics, Chinese Academy of Sciences Beijing 100080, China

Corresponding author: k.nakayama@arpes.phys.tohoku.ac.jp

Received: 3 December 2008
Accepted: 21 February 2009

Abstract

We have performed high-resolution angle-resolved photoemission spectroscopy on the optimally doped Ba_0.6K_0.4Fe₂As₂ compound and determined the accurate momentum dependence of the superconducting (SC) gap in four Fermi-surface sheets including a newly discovered outer electron pocket at the M-point. The SC gap on this pocket is nearly isotropic and its magnitude is comparable (Δ ~ 11 meV) to that of the inner electron and hole pockets (~ 12 meV), although it is substantially larger than that of the outer hole pocket (~ 6 meV). The Fermi-surface dependence of the SC gap value is basically consistent with the formula expected for the extended s-wave symmetry. The observed finite deviation from the simple formula suggests the importance of multi-orbital effects.