Neutron Spin Resonance in the Heavily Hole-doped KFe${}_2$As${}_2$ Superconductor

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 ($\sim 1.2$ meV) is considerably larger than the previously reported value ($2\delta$ $\approx 0.32$) at higher energies ($\ge \sim 6$ 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${}_2$.

arXiv commentAccepted for publication in Physical Review Letters

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• Neutron Spin Resonance Near a Lifshitz Transition in Overdoped Ba${}_{0.4}$K${}_{0.6}$Fe${}_2$As${}_2$
• Dispersion kinks from electronic correlations in an unconventional iron-based superconductor
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• Microscopic evidence for Fulde-Ferrel-Larkin-Ovchinnikov state and multiband effects in KFe${}_2$As${}_2$
• Three-dimensional spin susceptibility in Ba${}_{0.75}$K${}_{0.25}$Fe${}_2$As${}_2$: Out-of-plane modulation revealed by neutron spectroscopy and theoretical modeling