Neutron spin resonance in a quasi-two-dimensional iron-based superconductor

Abstract

Magnetically mediated Cooper pairing is generally regarded as a key to establish the unified mechanism of unconventional superconductivity. One crucial evidence is the neutron spin resonance arising in the superconducting state, which is commonly interpreted as a spin-exciton from collective particle-hole excitations confined below the superconducting pair-breaking gap ($2\Delta$). Here, on the basis of inelastic neutron scattering measurements on a quasi-two-dimensional iron-based superconductor KCa${}_2$Fe${}_4$As${}_4$F${}_2$, we have discovered a two-dimensional spin resonant mode with downward dispersions, a behavior closely resembling the low branch of the hour-glass-type spin resonance in cuprates. The resonant intensity is predominant by two broad incommensurate peaks near $Q=$(0.5, 0.5) with a sharp energy peak at $E_R=16$ meV. The overall energy dispersion of the mode exceeds the measured maximum total gap ${\Delta }_{\mathrm{tot}}=|{\Delta }_k|+|{\Delta }_{k+Q}|$. These experimental results deeply challenge the conventional understanding of the resonance modes as magnetic excitons regardless of underlining pairing symmetry schemes, and it also points out that when the iron-based superconductivity becomes very quasi-two-dimensional, the electronic behaviors are similar to those in cuprates.

16 pages, 14 figures, including supplementary materials

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Cited by in corpus (6)

• Nodal multigap superconductivity in the anisotropic iron-based compound RbCa2Fe4As4F2
• Preferred Spin Excitations in the Bilayer Iron-Based Superconductor CaK(Fe${}_{0.96}$Ni${}_{0.04}$)${}_4$As${}_4$ with Spin-Vortex Crystal Order
• Anomalous second magnetization peak in 12442-type RbCa${}_2$Fe${}_4$As${}_4$F${}_2$ superconductors
• Second magnetization peak, anomalous field penetration, and Josephson vortices in KCa${}_2$Fe${}_4$As${}_4$F${}_2$ bilayer pnictide superconductor
• Low-energy spin excitations in optimally doped CaFe${}_{0.88}$Co${}_{0.12}$AsF superconductor studied with inelastic neutron scattering
• Spin fluctuations in the 112-type iron-based superconductor Ca${}_{0.82}$La${}_{0.18}$Fe${}_{0.96}$Ni${}_{0.04}$As${}_2$