Revealing the intrinsic superconducting gap anisotropy in surface-neutralized BaFe$_2$(As$_{0.7}$P$_{0.3}$)$_2$

TL;DR

This study demonstrates that potassium deposition can neutralize the surface of BaFe$_2$(As$_{0.7}$P$_{0.3}$)$_2$, enabling precise measurement of its intrinsic superconducting gap anisotropy and revealing complex pairing behaviors.

Contribution

The paper introduces a surface neutralization method for 122 superconductors, allowing accurate probing of intrinsic electronic properties and superconducting gap anisotropy.

Findings

Surface neutralization suppresses spectral broadening.

Superconducting gap exhibits anisotropy with deviations from simple models.

Two distinct pairing channels are identified.

Abstract

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 BaFe$_2$(As$_{0.7}$P$_{0.3}$)$_2$, 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$_\pm$ 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.