Possibility of Horizontal Line Node in KFe$_2$As$_2$ Probed by Muon Spin Rotation

TL;DR

This study investigates the superconducting gap structure of KFe$_2$As$_2$ using muon spin rotation, revealing evidence for a horizontal line node in the gap function through anisotropic penetration depth measurements.

Contribution

It provides experimental evidence for a horizontal line node in the superconducting gap of KFe$_2$As$_2$, advancing understanding of its pairing symmetry.

Findings

Triangular flux-line lattice when field is parallel to c-axis

Different temperature dependence of penetration depth along different axes

Evidence supporting a horizontal line node in the gap function

Abstract

The anisotropy and temperature dependence of the magnetic field penetration depth in the iron-based superconductor KFe$_2$As$_2$ have been measured down to 20 mK by means of muon spin rotation. We have observed that the flux-line lattice forms triangular symmetry when we applied the field parallel to c-axis, suggesting the interaction between vortices should be isotropic. The temperature dependence of the penetration depth observd with an applied field parallel and perpendicular to c-axis are different. These results can be accounted for by a superconducting gap function with a horizontal line node in the basal plane.