Superconductivity-induced optical anomaly in an iron arsenide

Area of Science:

• Condensed Matter Physics
• Materials Science
• Solid-State Physics

Background:

• Conventional theories of superconductivity posit that only electronic excitations near the superconducting energy gap are impacted by the transition.
• This principle is widely applied to models of iron-pnictide superconductors.
• Recent discoveries in iron-pnictide superconductors necessitate a re-evaluation of these established theories.

Purpose of the Study:

• To investigate the impact of superconductivity on electronic excitations beyond the superconducting energy gap.
• To challenge the conventional understanding of superconductivity in iron-pnictide materials.
• To explore the role of deep electronic states in the formation of superconducting and spin-density wave states.

Main Methods:

• Comprehensive spectroscopic ellipsometry was employed on a high-quality Ba₀.₆₈K₀.₃₂Fe₂As₂ crystal.
• Density functional calculations were performed to analyze electronic band structures.
• Comparisons were made with parent compounds exhibiting spin-density wave transitions.

Main Results:

• A significant suppression of an absorption band at 2.5 eV, two orders of magnitude above the superconducting gap (2Δ≈20 meV), was observed due to superconductivity.
• This 2.5 eV band was theoretically assigned to transitions involving As-p and Fe-d orbitals at the Fermi level.
• A similar phenomenon was detected at the spin-density wave transition in related iron arsenide compounds.

Conclusions:

• The findings challenge the conventional view that only low-energy electronic excitations are relevant to superconductivity.
• As-p states, located deep below the Fermi level, play a crucial role in the formation of both superconducting and spin-density wave states in iron arsenides.
• This study highlights the complex electronic interactions governing phenomena in iron-based superconductors.