Heavily electron-doped electronic structure and isotropic superconducting gap in AxFe2Se2 (A=K,Cs)

TL;DR

This study uses angle-resolved photoemission spectroscopy to analyze the electronic structure of heavily electron-doped AxFe2Se2 superconductors, revealing isotropic gaps and challenging existing pairing symmetry models.

Contribution

It provides new insights into the electronic structure and pairing symmetry of AxFe2Se2, showing that conventional s-wave pairing may be more appropriate than the sign-changing s± model.

Findings

Only electron pockets observed with isotropic superconducting gap

No hole Fermi surface near the zone center

Sign-changing s± pairing symmetry is unlikely

Abstract

The low energy band structure and Fermi surface of the newly discovered superconductor, AxFe2Se2 (A=K,Cs), have been studied by angle-resolved photoemission spectroscopy. Compared with iron pnictide superconductors, AxFe2Se2 (A=K,Cs) is the most heavily electron-doped with Tc~30 K. Only electron pockets are observed with an almost isotropic superconducting gap of ~10.3 meV, while there is no hole Fermi surface near the zone center, which indicates the inter-pocket hopping or Fermi surface nesting is not a necessary ingredient for the unconventional superconductivity in iron-based superconductors. Thus, the sign changed s$_\pm$ pairing symmetry, a leading candidate proposed for iron-based superconductors, becomes conceptually irrelevant in describing the superconducting state here. A more conventional s-wave pairing is a better description.