Observation of Fermi-surface–dependent nodeless superconducting gaps in Ba _0.6 K _0.4 Fe ₂ As ₂

TLDR

We performed high‑resolution angle‑resolved photoelectron spectroscopy on Ba₀.₆K₀.₄Fe₂As₂ (Tc = 37 K). ARPES measurements revealed two nodeless, isotropic superconducting gaps in Ba₀.₆K₀.₄Fe₂As₂: a large ~12 meV gap on the small hole‑ and electron‑like Fermi‑surface sheets and a smaller ~6 meV gap on the large hole‑like sheet, both closing at Tc and indicating orbital‑dependent pairing that likely originates from inter‑band interactions between the nested small sheets.

Abstract

We have performed a high-resolution angle-resolved photoelectron spectroscopy study on the newly discovered superconductor Ba0.6K0.4Fe2As2 (Tc=37 K). We have observed two superconducting gaps with different values: a large gap (Δ∼12 meV) on the two small hole-like and electron-like Fermi surface (FS) sheets, and a small gap (∼6 meV) on the large hole-like FS. Both gaps, closing simultaneously at the bulk transition temperature (Tc), are nodeless and nearly isotropic around their respective FS sheets. The isotropic pairing interactions are strongly orbital dependent, as the ratio 2Δ/kBTc switches from weak to strong coupling on different bands. The same and surprisingly large superconducting gap due to strong pairing on the two small FSs, which are connected by the (π, 0) spin-density-wave vector in the parent compound, strongly suggests that the pairing mechanism originates from the inter-band interactions between these two nested FS sheets.