Nature of magnetic excitations in superconducting BaFe1.9Ni0.1As2

Journal name:
Nature Physics
Year published:
Published online

Since the discovery of the metallic antiferromagnetic (AF) ground state near superconductivity in iron pnictide superconductors1, 2, 3, a central question has been whether magnetism in these materials arises from weakly correlated electrons4, 5, as in the case of spin density wave in pure chromium6, requires strong electron correlations7, or can even be described in terms of localized electrons8, 9 such as the AF insulating state of copper oxides10. Here we use inelastic neutron scattering to determine the absolute intensity of the magnetic excitations throughout the Brillouin zone in electron-doped superconducting BaFe1.9Ni0.1As2 (Tc=20K), which allows us to obtain the size of the fluctuating magnetic moment left fencem2right fence, and its energy distribution11, 12. We find that superconducting BaFe1.9Ni0.1As2 and AF BaFe2As2 (ref. 13) both have fluctuating magnetic moments left fencem2right fence3.2 μB2 per Fe(Ni), which are similar to those found in the AF insulating copper oxides14, 15. The common theme in both classes of high-temperature superconductors is that magnetic excitations have partly localized character, thus showing the importance of strong correlations for high-temperature superconductivity16.

At a glance


  1. Summary of neutron scattering and calculation results.
    Figure 1: Summary of neutron scattering and calculation results.

    Our experiments were carried out on the MERLIN time-of-flight chopper spectrometer at the Rutherford-Appleton Laboratory, UK (ref. 33). We co-aligned 28g of single crystals of BaFe1.9Ni0.1As2 (with in-plane mosaic of 2.5°and out-of-plane mosaic of 4°). The incident beam energies were Ei=20,25,30,80,250,450,600meV, and mostly with Ei parallel to the c axis. To facilitate easy comparison with spin waves in BaFe2As2 (ref. 13), we defined the wave vector Q at (qx,qy,qz) as (H,K,L)=(qxa/2π,qyb/2π,qzc/2π) reciprocal lattice units (r.l.u.) using the orthorhombic unit cell, where a=b=5.564Å, and c=12.77Å. The data are normalized to absolute units using a vanadium standard13, which may have a systematic error up to 20% owing to differences in neutron illumination of the vanadium and sample, and time-of-flight instruments. a, AF spin structure of BaFe2As2 with Fe spin ordering. The effective magnetic exchange couplings along different directions are shown. b, RPA and LDA+DMFT calculations of χ′′(ω) in absolute units for BaFe2As2 and BaFe1.9Ni0.1As2. c, The solid lines show the spin wave dispersions of BaFe2As2 for , along the [1,K] and [H,0] directions obtained in ref. 13. The filled circles and triangles are the spin excitation dispersions of BaFe1.9Ni0.1As2 at 5K and 150K, respectively. d, The solid line shows the low-energy spin waves of BaFe2As2. The horizontal bars show the full-width at half-maximum of spin excitations in BaFe1.9Ni0.1As2. e, Energy dependence of χ′′(ω) for BaFe2As2 (filled blue circles) and BaFe1.9Ni0.1As2 below (filled red circles) and above (open red circles) Tc. The solid and dashed lines are guides to the eye. The vertical error bars indicate statistical errors of one standard deviation. The horizontal error bars in e indicate the energy integration range.

  2. Constant-energy slices through the magnetic excitations of BaFe1.9Ni0.1As2 at different energies in several Brillouin zones.
    Figure 2: Constant-energy slices through the magnetic excitations of BaFe1.9Ni0.1As2 at different energies in several Brillouin zones.

    Two-dimensional images of spin excitations at the energies indicated. The images were obtained after subtracting the background integrated from 1.8<H<2.2 and −0.2<K<0.2. The colour bars represent the vanadium-normalized absolute spin excitation intensity in the units of mbsr−1meV−1f.u.−1 (mb is millibarn) and the dashed boxes indicate the AF zone boundaries for a single FeAs layer.

  3. Constant-energy cuts of the spin excitation dispersion as a function of increasing energy along the
[1,K] and
[H,0] directions for BaFe1.9Ni0.1As2.
    Figure 3: Constant-energy cuts of the spin excitation dispersion as a function of increasing energy along the [1,K] and [H,0] directions for BaFe1.9Ni0.1As2.

    The solid lines show identical cuts for spin waves of BaFe2As2 in absolute units. a-f, Constant-energy cut along the [1,K] direction at the energies indicated. g-i Constant-energy cut along the [H,0] direction at the energies indicated. The error bars indicate statistical errors of one standard deviation.

  4. Constant-energy/wave-vector (Q) dependence of the spin excitations and dynamic spin-spin correlation lengths for BaFe1.9Ni0.1As2 and BaFe2As2.
    Figure 4: Constant-energy/wave-vector (Q) dependence of the spin excitations and dynamic spin–spin correlation lengths for BaFe1.9Ni0.1As2 and BaFe2As2.

    ad, Constant- Q cuts at Q=(1,0.05),(1,0.2),(1,0.35), and (1,0.5), respectively, at T=5 (solid red circles) and 150K (yellow filled circles), with the background at Q=(2,0) subtracted. The negative scattering in the data is due to oversubtraction of the phonon background. The solid lines are identical cuts from spin waves in BaFe2As2. For excitations below 100meV, the intensity of the scattering of BaFe1.9Ni0.1As2 is suppressed compared with that of BaFe2As2. For energies above 100meV, the magnetic scattering is virtually identical between the parent and superconductor. Inset indicates the Q-cuts shown in ad. e, Constant-energy cuts at the neutron spin resonance energy of E=9±1meV (ref. 20) below and above Tc. The solid lines are Gaussian fits on linear backgrounds. f, Temperature dependence of spin excitations at E=90±5meV. g, Energy dependence of the dynamic spin–spin correlation lengths (ξ) at 5K obtained from a Fourier transform of constant-energy cuts similar to those in Fig. 3a–f and Fig. 4e,f. For all the excitation energies probed (10≤E≤200meV), the dynamic spin–spin correlation lengths are independent of energy. The solid line shows the energy dependence of ξ for BaFe2As2. The error bars indicate statistical errors of one standard deviation.


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Author information


  1. Department of Physics and Astronomy, The University of Tennessee, Knoxville, Tennessee 37996-1200, USA

    • Mengshu Liu,
    • Leland W. Harriger,
    • Meng Wang &
    • Pengcheng Dai
  2. Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China

    • Huiqian Luo,
    • Meng Wang &
    • Pengcheng Dai
  3. ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, UK

    • R. A. Ewings &
    • T. Guidi
  4. Department of Physics, Rutgers University, Piscataway, New Jersey 08854, USA

    • Hyowon Park,
    • Kristjan Haule &
    • Gabriel Kotliar
  5. H. H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, UK

    • S. M. Hayden


P.D. and M.L. planned neutron scattering experiments. M.L., L.W.H., H.L., R.A.E., T.G. and P.D. carried out neutron scattering experiments. Data analysis was done by M.L. with help from L.W.H., R.A.E., T.G. and S.M.H. The samples were grown by H.L. and co-aligned by M.L. and M.W. The DFT and DMFT calculations were done by H.P., K.H. and G.K. The paper was written by P.D., K.H. and G.K. with input from S.M.H. and M.L. All coauthors provided comments on the paper.

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