Nodes in the Order Parameter of Superconducting Iron Pnictides Observed by Infrared Spectroscopy

TL;DR

This study uses infrared spectroscopy to investigate the superconducting gap structure in electron-doped BaFe2As2, revealing evidence for gap nodes or very small gaps, with different behaviors on hole and electron pockets.

Contribution

It provides new insights into the momentum-dependent gap structure and possible nodal behavior in iron pnictide superconductors through optical measurements.

Findings

Evidence for gap nodes or very small gaps in the crossover region.

The hole pocket has an isotropic s-wave gap.

The electron pocket exhibits an anisotropic larger gap.

Abstract

The temperature and frequency dependences of the conductivity are derived from optical reflection and transmission measurements of electron doped BaFe$_2$As$_2$ crystals and films. The data is consistent with gap nodes or possibly a very small gap in the crossover region between these two possibilities. This can arise when one of the several pockets known to exist in these systems has extended s-wave gap symmetry with an anisotropic piece canceling or nearly so the isotropic part in some momentum direction. Alternatively, a node can be lifted by impurity scattering which reduces anisotropy. We find that the smaller gap on the hole pocket at the $\Gamma$ point in the Brillouin zone is isotropic s-wave while the electron pocket at the $M$ point has a larger gap which is anisotropic and falls in the crossover region.