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- Activity Report 2017 -

H. Luo, S. Li, and T. Masuda

Neutron Spin Resonance in the 112-Type Iron-Based Superconductor

Fig. 1. (a) Crystal and magnetic structure of Ca1−xLaxFeAs2 system. (b) Linear scaling of spin resonance energy and critical temperature in Fe-based superconductors. (c) Spin resonance in superconducting 112 compound measured at HRC (BL-12). (d) Polarized analysis of spin resonance at E = 11 meV.

In unconventional superconductors, the neutron spin resonance is a crucial evidence for spin fluctuation mediated superconductivity in the proximity of an antiferromagnetic (AF) instability [1]. Although the spin resonance mode is theoretically believed to be a spin-1 exciton from the singlet-triplet excitations of the electron Cooper pairs in copper oxides and heavy fermions [2], solid evidences are still not well established yet in the iron-based superconductors (FeSC). The proximity to the AF order and spin-orbital coupling give complexity on the energy and momentum distribution of spin resonance, and the common features of the resonance mode need to be testified in various materials [3].

By using inelastic neutron scattering, we have comprehensively studied the low-energy spin excitations of the new discovered 112-type iron pnictide Ca0.82La0.18Fe0.96Ni0.04As2 with bulk superconductivity below Tc =22 K(Fig. 1(a)) [4]. A two-dimensional spin resonance mode is found around E =11 meV by using high-resolution chopper (HRC) spectrometer (Fig. 1(c)), where the resonance energy is almost temperature independent and linearly scales with Tc along with other iron-based superconductors (Fig. 1(b)). Polarized neutron analysis reveals the resonance is nearly isotropic in spin space without any L modulations (Fig. 1(d)). Because of the unique monoclinic structure with additional zigzag arsenic chains, the As 4p orbitals contribute to a three dimensional hole pocket around the Γ point and an extra electron pocket at the X point. Our results suggest that the energy and momentum distribution of the spin resonance does not directly respond to the kz dependence of the fermiology, and the spin resonance intrinsically is a spin-1 mode from singlet-triplet excitations of the Cooper pairs in the case of weak spin-orbital coupling. This work is published on Physical Review Letters [5].


  • [1] P. Dai, Rev. Mod. Phys. 87, 855 (2015).
  • [2] M. Eschrig, Adv. Phys. 55, 47 (2006).
  • [3] P. D. Johnson, G. Xu, and W.-G. Yin, Iron-Based Superconductivity (Springer, New York, 2015).
  • [4] N. Katayama et al., J. Phys. Soc. Jpn. 82, 123702 (2013).
  • [5] Tao Xie et al., Phys. Rev. Lett. 120, 137001 (2018).


  • T. Xiea,b, D. Gonga,b, H. Ghoshc,d, A. Ghoshc,d, M. Soda, T. Masuda, S. Itohe, F. Bourdarotf, L.-P. Regnaultg, S. Danilkinh, S. Lia,b,i, and H. Luoa
  • aChinese Academy of Sciences
  • bUniversity of Chinese Academy of Sciences
  • cHomi Bhabha National Institute
  • dRaja Ramanna Centre for Advanced Technology
  • eHigh Energy Accelerator Research Organization
  • fUniversité Grenoble Alpes
  • gIntitut Laue Langevin
  • hAustralian Nuclear Science and Technology Organization
  • iCollaborative Innovation Center of Quantum Matter