Simultaneous Evidence for Pauli Paramagnetic Effects and Multiband Superconductivity in KFe$_2$As$_2$ by Small-Angle Neutron Scattering Studies of the Vortex Lattice

TL;DR

This study uses small-angle neutron scattering to reveal that KFe$_2$As$_2$ exhibits multiband superconductivity and Pauli paramagnetic effects, with anisotropy strongly dependent on magnetic field strength.

Contribution

It provides direct experimental evidence of multiband superconductivity and Pauli limiting effects in KFe$_2$As$_2$ through vortex lattice imaging.

Findings

Anisotropy is strongly field dependent, indicating multiband behavior.

High field anisotropy exceeds upper critical field anisotropy, supporting Pauli limiting.

Direct evidence of Pauli paramagnetism affecting vortex core quasiparticles.

Abstract

We study the intrinsic anisotropy of the superconducting state in KFe$_2$As$_2$, using small-angle neutron scattering to image the vortex lattice as the applied magnetic field is rotated towards the FeAs crystalline planes. The anisotropy is found to be strongly field dependent, indicating multiband superconductivity. Furthermore, the high field anisotropy significantly exceeds that of the upper critical field, providing further support for Pauli limiting in KFe$_2$As$_2$ for field applied in the basal plane. The effect of Pauli paramagnetism on the unpaired quasiparticles in the vortex cores is directly evident from the ratio of scattered intensities due to the longitudinal and transverse vortex lattice field modulation.