Revealing the intrinsic superconducting gap anisotropy in surface-neutralized BaFe2(As0.7P0.3)2

Alkaline-earth iron arsenide (122) is one of the most studied families of iron-based superconductors, especially for angle-resolved photoemission spectroscopy. While extensive photoemission results have been obtained, the surface complexity of 122 caused by its charge-non-neutral surface is rarely considered. Here, we show that the surface of 122 can be neutralized by potassium deposition. In potassium-coated BaFe2(As0.7P0.3)2, the surface-induced spectral broadening is strongly suppressed, and hence the coherent spectra that reflect the intrinsic bulk electronic state recover. This enables the measuring of superconducting gap with unpreceded precision. The result shows the existence of two pairing channels. While the gap anisotropy on the outer hole/electron pockets can be well fitted using an s± gap function, the gap anisotropy on the inner hole/electron shows a clear deviation. Our results provide quantitative constraints for refining theoretical models and also demonstrate an experimental method for revealing the intrinsic electronic properties of 122 in future studies.

Revealing the intrinsic superconducting gap anisotropy in surface-neutralized BaFe2(As0.7P0.3)2

Alkaline-earth iron arsenide (122) is one of the most studied families of iron-based superconductors, especially for angle-resolved photoemission spectroscopy. Extensive results have been obtained including band structure, gap anisotropy, etc. However, the complicacy of 122 caused by its charge-non-neutral cleavage surface is rarely considered. Here, we show that the surface of 122 can be neutralized by potassium deposition. In potassium-coated BaFe2(As0.7P0.3)2, the surface-induced spectral broadening is strongly suppressed, while the coherent spectra that reflects the intrinsic bulk electronic state recovers. This raises the accuracy of the gap measurement and gap fitting to an unpreceded level. The results clearly distinguish two pairing channels originated respectively from the inner and outer Fermi pockets. While the gap anisotropy on the outer hole/electron pockets can be well fitted using an s± gap function, the gap magnitude on the inner hole/electron pockets show a clear deviation. Our results provide quantitative constraints for refining theoretical models and demonstrate an experimental method for revealing the intrinsic electronic properties of 122 in future studies.

Multiple intertwined pairing states and temperature-sensitive gap anisotropy for superconductivity at a nematic quantum-critical point

The proximity of many strongly correlated superconductors to density-wave or nematic order has led to an extensive search for fingerprints of pairing mediated by dynamical quantum-critical (QC) fluctuations of the corresponding order parameter. Here we study anisotropic s-wave superconductivity induced by anisotropic QC dynamical nematic fluctuations. We solve the non-linear gap equation for the pairing gap $$\Delta (\theta ,{\omega }_{m})$$ Δ ( θ , ω m ) and show that its angular dependence strongly varies below $${T}_{{\rm{c}}}$$ T c . We show that this variation is a signature of QC pairing and comes about because there are multiple s-wave pairing instabilities with closely spaced transition temperatures $${T}_{{\rm{c}},n}$$ T c , n . Taken alone, each instability would produce a gap $$\Delta (\theta ,{\omega }_{m})$$ Δ ( θ , ω m ) that changes sign $$8n$$ 8 n times along the Fermi surface. We show that the equilibrium gap $$\Delta (\theta ,{\omega }_{m})$$ Δ ( θ , ω m ) is a superposition of multiple components that are nonlinearly induced below the actual $${T}_{{\rm{c}}}={T}_{{\rm{c}},0}$$ T c = T c , 0 , and get resonantly enhanced at $$T={T}_{{\rm{c}},n}\ <\ {T}_{{\rm{c}}}$$ T = T c , n < T c . This gives rise to strong temperature variation of the angular dependence of $$\Delta (\theta ,{\omega }_{m})$$ Δ ( θ , ω m ) . This variation progressively disappears away from a QC point.