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

Yi-Ling Zhang, Lei Yang+11 more|arXiv (Cornell University)|Dec 29, 2010

Iron-based superconductors research203 citations

TL;DR

This study uses angle-resolved photoemission spectroscopy (ARPES) to investigate the electronic structure and superconducting gap in heavily electron-doped AxFe2Se2 (A=K, Cs), revealing only electron Fermi pockets and an isotropic s-wave superconducting gap of ~10.3 meV. The absence of hole pockets near the zone center challenges the necessity of inter-pocket nesting or s± pairing symmetry, suggesting a more conventional s-wave pairing mechanism dominates in this highly electron-doped iron-based superconductor.

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.

Motivation & Objective

• To investigate the electronic structure and superconducting gap in AxFe2Se2 (A=K, Cs), the most heavily electron-doped iron-based superconductors discovered to date.
• To determine whether Fermi surface nesting or inter-pocket scattering is essential for unconventional superconductivity in iron-based materials.
• To assess the validity of the s± pairing symmetry in these highly electron-doped systems by measuring the gap anisotropy.
• To evaluate whether the superconducting state in AxFe2Se2 is better described by conventional s-wave or unconventional s± pairing symmetry.
• To examine the role of electron Fermi surfaces in sustaining robust superconductivity in the absence of hole pockets.

Proposed method

• Angle-resolved photoemission spectroscopy (ARPES) was performed using a He-Iα UV lamp (21.2 eV) and a Scienta R4000 electron analyzer with 9–12 meV energy resolution and 0.3° angular resolution.
• In situ cleaving of single crystals was conducted under ultra-high vacuum (~5×10⁻¹¹ torr) to preserve surface quality and avoid contamination.
• Photoemission intensity maps and energy distribution curves (EDCs) were measured at various temperatures (down to 10 K) to probe the superconducting gap and spectral weight evolution.
• Symmetrized EDCs were used to extract the superconducting gap amplitude by identifying the coherent peak position near the Fermi level.
• The momentum-dependent gap structure was mapped around the M point on the δ Fermi pocket and at the Γ point on the κ band to assess gap anisotropy.
• X-ray diffraction and energy dispersive X-ray (EDX) spectroscopy confirmed the nominal and actual stoichiometries of the K₀.₈Fe₂Se₂ and Cs₀.₈Fe₂Se₂ crystals.

Experimental results

Research questions

• RQ1 Does the absence of hole Fermi surfaces in AxFe2Se2 (A=K, Cs) invalidate the role of Fermi surface nesting in driving unconventional superconductivity?
• RQ2 Is the superconducting gap in AxFe2Se2 isotropic or anisotropic, and what does this imply about the pairing symmetry?
• RQ3 Can the s± pairing symmetry, proposed as a leading candidate for iron-based superconductors, be sustained in a system with only electron pockets?
• RQ4 How does the superconducting gap magnitude (~10.3 meV) compare to the BCS prediction, and what does this imply about the pairing mechanism?
• RQ5 To what extent do the observed electronic structures support a strong-coupling, intra-orbital s-wave pairing picture over weak-coupling, inter-orbital s± pairing?

Key findings

• AxFe₂Se₂ (A=K, Cs) exhibits the most heavily electron-doped electronic structure among iron-based superconductors, with no hole Fermi surface near the zone center.
• Only electron Fermi pockets are observed, and the superconducting gap is isotropic with a magnitude of ~10.3 meV, corresponding to ~4kBTc.
• The gap at the Γ point (κ band) is significantly smaller (~4 meV) than around the M point (δ band), violating the expected coskₓcoskᵧ dependence of s± pairing symmetry.
• The absence of hole pockets and the isotropic gap structure rule out inter-pocket scattering or nesting as essential mechanisms for superconductivity in this system.
• The data strongly suggest that the superconducting state is better described by a conventional s-wave pairing symmetry rather than the s± pairing symmetry.
• The robust superconductivity in the absence of hole Fermi surfaces implies that inter-band hopping may not be a dominant pairing mechanism, supporting a strong-coupling, intra-orbital pairing scenario.