Neutron Spin Resonance in the Heavily Hole-Doped <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi>KFe</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mi>As</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math> Superconductor

Abstract

We report high-resolution neutron scattering measurements of the low energy spin fluctuations of ${\mathrm{KFe}}_{2}{\mathrm{As}}_{2}$, the end member of the hole-doped ${\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{K}}_{x}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ family with only hole pockets, above and below its superconducting transition temperature ${T}_{c}$ ($\ensuremath{\sim}3.5\text{ }\text{ }\mathrm{K}$). Our data reveal clear spin fluctuations at the incommensurate wave vector ($0.5\ifmmode\pm\else\textpm\fi{}\ensuremath{\delta}$, 0, $L$), ($\ensuremath{\delta}=0.2$) (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 ($2\ensuremath{\delta}=0.4$) of the resonance mode ($\ensuremath{\sim}1.2\text{ }\text{ }\mathrm{meV}$) is considerably larger than the previously reported value ($2\ensuremath{\delta}\ensuremath{\approx}0.32$) at higher energies ($\ensuremath{\ge}\ensuremath{\sim}6\text{ }\text{ }\mathrm{meV}$). 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 ${\mathrm{KFe}}_{2}{\mathrm{As}}_{2}$.

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