Neutron Spin Resonance in the Heavily Hole-doped KFe$_{2}$As$_{2}$ Superconductor

TL;DR

This study uses neutron scattering to investigate low-energy spin fluctuations in KFe$_{2}$As$_{2}$, revealing incommensurate spin resonance modes that support an $s$-wave pairing with sign reversal between hole pockets.

Contribution

First high-resolution measurement of low-energy spin fluctuations in KFe$_{2}$As$_{2}$, linking spin resonance features to its superconducting pairing symmetry.

Findings

Incommensurate spin fluctuations at wave vector ($0.5 extpm0.2$, 0, $L$)

Spin resonance mode appears below $T_c$ with an energy of about 1.2 meV

Resonance mode's incommensurability is larger than previously reported

Abstract

We report high-resolution neutron scattering measurements of the low energy spin fluctuations of KFe$_{2}$As$_{2}$, the end member of the hole-doped Ba$_{1-x}$K$_x$Fe$_2$As$_2$ family with only hole pockets, above and below its superconducting transition temperature $T_c$ ($\sim$ 3.5 K). Our data reveals clear spin fluctuations at the incommensurate wave vector ($0.5\pm\delta$, 0, $L$), ($\delta$ = 0.2)(1-Fe unit cell), which exhibit $L$-modulation peaking at $L=0.5$. Upon cooling to the superconducting state, the incommensurate spin fluctuations gradually open a spin-gap and form a sharp spin resonance mode. The incommensurability ($2\delta$ = 0.4) of the resonance mode ($\sim1.2$ meV) is considerably larger than the previously reported value ($2\delta$ $\approx0.32$) at higher energies ($\ge\sim6$ meV). The determination of the momentum structure of spin fluctuation in the low energy limit allows a direct comparison with the realistic Fermi surface and superconducting gap structure. Our results point to an $s$-wave pairing with a reversed sign between the hole pockets near the zone center in KFe$_{2}$As$_{2}$.