arXiv:0807.3932v1 [cond-mat.supr-con] 24 Jul 2008
Resonant Spin Excitation in the High Temperature Superconductor Ba0.6K0.4Fe2As2
A. D. Christianson,1E. A. Goremychkin,2,3R. Osborn,2S. Rosenkranz,2M. D. Lumsden,1C. D.
Malliakas,2,4l. S. Todorov,2H. Claus,2D. Y. Chung,2M. G. Kanatzidis,2,4R. I. Bewley,3and T. Guidi3
1Neutron Scattering Science Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA
2Materials Science Division, Argonne National Laboratory, Argonne, IL 60439-4845, USA
3ISIS Pulsed Neutron and Muon Facility, Rutherford Appleton Laboratory, Chilton, Didcot OX11 0QX, United Kingdom
4Department of Chemistry, Northwestern University, Evanston, IL 60208-3113
(Dated: July 24, 2008)
The recent observations of superconductivity at temperatures up to 55K in compounds containing
layers of iron arsenide [1, 2, 3, 4] have revealed a new class of high temperature superconductors
that show striking similarities to the more familiar cuprates. In both series of compounds, the
onset of superconductivity is associated with the suppression of magnetic order by doping holes
and/or electrons into the band  leading to theories in which magnetic fluctuations are either
responsible for or strongly coupled to the superconducting order parameter . In the cuprates,
theories of magnetic pairing have been invoked to explain the observation of a resonant magnetic
excitation that scales in energy with the superconducting energy gap and is suppressed above the
superconducting transition temperature, Tc. Such resonant excitations have been shown by inelastic
neutron scattering to be a universal feature of the cuprate superconductors , and have even been
observed in heavy fermion superconductors with much lower transition temperatures [8, 9, 10]. In
this paper, we show neutron scattering evidence of a resonant excitation in Ba0.6K0.4Fe2As2, which
is a superconductor below 38K , at the momentum transfer associated with magnetic order in
the undoped compound, BaFe2As2, and at an energy transfer that is consistent with scaling in
other strongly correlated electron superconductors. As in the cuprates, the peak disappears at
Tc providing the first experimental confirmation of a strong coupling of the magnetic fluctuation
spectrum to the superconducting order parameter in the new iron arsenide superconductors.
Unconventional superconductivity has been the sub-
ject of considerable theoretical and experimental interest
since the discovery of superconductivity in CeCu2Si2and
other heavy fermion compounds , an interest that was
only intensified by the discovery of cuprate superconduc-
tors with transition temperatures in excess of 100K .
Although significant progress has been made, the ori-
gin of unconventional superconductivity is still not un-
derstood. The observation of a magnetic resonance in
the spin excitation spectrum which appears concurrently
with the onset of superconductivity in both the high Tc
cuprates [12, 13, 14, 15, 16] and the heavy fermion su-
perconductors [8, 9, 10] offers the tantalizing possibility
of a unifying theme for unconventional superconductivity
that spans a diverse range of superconducting materials.
Recently, a new family of superconductors containing lay-
ers of Fe2As2has been discovered with Tcs in excess of
50K stimulating considerable experimental and theoreti-
cal activity [1, 2, 3]. Although there is mounting evidence
that the superconductivity in this new family is also un-
conventional , there is as yet no consensus concerning
the mechanism giving rise to superconductivity or even
the superconducting pairing symmetry.
we describe neutron scattering data that confirm for the
first time the existence of a resonant spin excitation be-
low Tcin the iron arsenide materials at an energy that
shows similar scaling to other heavy fermion and high-Tc
In this letter,
Although the first iron arsenide superconductors were
based on doped variants of RFeAsO, where R is a rare
earth element, there has been considerable interest in a
new series of tetragonal compounds based on AFe2As2
(A = Ba, Sr, Ca), in which superconductivity is in-
duced either by doping the A-site with potassium or
FIG. 1: The crystal structure of Ba0.6K0.4Fe2As2 (Fe: blue
spheres, As: yellow spheres, Ba/K: red spheres). The unit
cell contains two layers of Fe2As2 tetrahedra separated by
planes of barium atoms. The blue arrows show the ordering
of the iron spins observed in the undoped parent compound
BaFe2As2 . The atomic distance of 2.77˚ A that charac-
terizes both the antiferromagnetic modulation and the newly
observed resonant excitation is shown by the red arrows.
FIG. 2: Inelastic neutron scattering from Ba0.6K0.4Fe2As2measured using incident neutron energies of 60meV (a,b) and 15meV
(c,d), at temperatures of 7K (a,c) and 50K (b,d). Above Tc (b,d), the magnetic response consists of a column of excitations
centred at 1.15˚ A−1and a dispersive ferromagnetic excitation. Below Tc, low-energy intensity in the column is transferred to
the resonant excitation at 15meV. The strong scattering at low energy transfers in each plot arises from the tail of strong
elastic nuclear scattering, while the scattering increases strongly at higher Q due to inelastic phonon scattering. The colour
scale is in units of mbarns/sr/meV/mol.
sodium [4, 19] or by applying pressure . These con-
tain the same tetrahedrally-coordinated Fe2As2planes as
the LaFeAsO compounds (Fig. 1), separated by planes
of the doped A-site, which acts as a charge reservoir. So
far, the maximum Tcis 38K  seen in Ba0.6K0.4Fe2As2,
which is the compound we are investigating in this letter.
The antiferromagnetic structure of the undoped parent
compound, BaFe2As2is illustrated in Figure 1 .
Polycrystalline samples of Ba0.6K0.4Fe2As2 were pre-
pared by solid state synthesis techniques described in the
supplemental information. From XRD and SEM/EDS
measurements, the estimated phase purity of the sam-
ples is ∼90% and a sharp superconducting transition
was observed by magnetic susceptibility at the previ-
ously reported temperature of 38K .
neutron scattering experiments were performed on the
recently commissioned time-of-flight MERLIN spectrom-
eter at the ISIS Pulsed Neutron and Muon Facility ,
using incident energies of 15, 30, 60, and 100meV. The
data were placed on an absolute intensity scale by nor-
malization to a vanadium standard.
Figure 2 shows colour plots of the measured inelastic
neutron scattering intensity as a function of momentum
transfer, Q, and energy transfer, ω, at two incident neu-
tron energies, 15 and 60meV, below (T = 7K) and above
(T = 50K) the superconducting transition temperature.
The range of data is limited by the kinematics of the
scattering process at these low scattering angles so it is
necessary to combine data from multiple incident ener-
gies in order to access data over a reasonable range of
energy transfers at such low Qs.
The most striking difference between the scattering
FIG. 3: Inelastic neutron scattering from Ba0.6K0.4Fe2As2
integrated over a Q-range of 1.0 to 1.3˚ A−1(a) at 7K mea-
sured using incident neutron energies of 15meV (yellow cir-
cles), 30meV (blue circles) and 60meV (green circles), and
(b) at 7K (green circles) and 50K (red circles) using an in-
cident neutron energy of 60meV. The error bars are derived
from the square root of the raw detector counts. The data
show a clear resonant peak at 7K and the transfer of spectral
weight from this peak to lower energies at 50K, i.e., above
the superconducting transition.
above and below Tc is seen at Q∼1.15˚ A−1and an en-
ergy transfer of ∼14meV. At 7K, there is clearly a peak
that is well-defined in both Q and ω which is not present
at 50K (Fig. 2a and 2b). Conversely, measurements
at lower incident energy show that above Tc, there is
a column of scattering intensity as a function of energy
transfer, centred at Q = 1.15˚ A−1(Fig. 2d), which is no
longer visible below Tc(Fig. 2c). The Q characterizing
these contributions to the magnetic response corresponds
to the periodicity of the antiferromagnetic order within
each plane of iron spins observed in the undoped par-
ent compound, BaFe2As2  (see also Fig. 1), so this
scattering is consistent with the persistence of antiferro-
magnetic fluctuations in the absence of long-range spin
order. Our measurements indicate that these antiferro-
magnetic fluctuations condense into a sharp resonant ex-
citation, localized in both Q and ω, as previously seen in
the cuprate and heavy fermion superconductors.
Before discussing this resonant excitation in more de-
tail, we should mention that there is also clear evidence
for ferromagnetic fluctuations shown by the additional
band of excitations in Fig. 2c and 2d that disperses in Q
and ω. The (Q,ω)-dependence is consistent with the dis-
persion of ferromagnetic spin waves emerging from Q =
0, flattening at the ferromagnetic Brillouin zone bound-
ary (which is equivalent to the antiferromagnetic zone
centre) at ω ∼ 6meV. The existence of ferromagnetic
fluctuations in the undoped parent compound has been
predicted theoretically , although it was argued that
they would be suppressed by doping before the onset of
superconductivity. We have also observed this mode in
BaFe2As2but it is clear that they are still present in the
superconducting phase although there is some redistri-
bution of intensity on crossing Tc. The strong intensity
of this mode, coupled with the intensity changes at Tc,
indicate that it is intrinsic to the bulk superconducting
phase. We will discuss this aspect of our data further in
a future publication.
To elucidate the evolution of the magnetic response, we
combine measurements below Tcat three incident ener-
gies using, in each case, a low-energy cutoff that excludes
the tail of strong elastic nuclear scattering. The resulting
data are shown in Fig. 3a where the resonant excitation is
seen to peak sharply at ω0≈ 14meV. We cannot rule out
that there could be a small phononic contribution to the
scattering within this energy window, but the strong tem-
perature dependence indicates that it is predominantly
magnetic. A comparison of the data above and below Tc
measured with an incident energy of 60meV shows that
the spectral weight of the column of scattering evident
in Fig. 2d has transferred into the resonant peak in the
We performed a series of shorter measurements in or-
der to determine the temperature dependence of this res-
onant excitation. Fig. 4 shows data integrated over the
(Q,ω)-region of maximum intensity in the resonant exci-
tation. As also observed in the cuprates, the intensity of
the resonance falls to zero at Tc confirming the strong
coupling of this excitation to the superconducting order
The existence of similar resonant excitations in other
FIG. 4: Inelastic neutron scattering from Ba0.6K0.4Fe2As2 as
a function of temperature integrated over a Q range of 1.0
to 1.3˚ A−1and an energy transfer range of 12.5 to 17.5meV.
The integration range corresponds to the region of maximum
intensity of the resonant excitation observed below Tc (see
Fig. 2). The error bars are derived from the square root of
the raw detector counts. The dashed line is a guide to the eye
below Tc and shows the average value of the integrals above
strongly correlated superconductors, such as the high-Tc
cuprates and the heavy fermion superconductors, is com-
monly taken as evidence of an unconventional symmetry
of the superconducting order parameter . The dy-
namic magnetic susceptibility is predicted to be enhanced
at certain values of Q in the superconducting phase by a
coherence factor provided that the energy gap symmetry
has the following form:
where k and k + Q are wavevectors on different parts
of the Fermi surface.
In the cuprates and heavy fermion superconductors,
this is realized by dx2−y2-symmetry, which has nodes in
the energy gap within a single Fermi surface. In these
cases, Q spans sections of the same Fermi surface that
are gapped with opposite phase, such as Q = (π,π) in
the cuprates. However, such a scenario seems to be ruled
out by photoemission results on Ba0.6K0.4Fe2As2 that
show no evidence of any anisotropy of the energy gap
. According to band structure calculation, the Fermi
surfaces of the iron arsenide superconductors are predom-
inantly derived from the iron d-electrons, and comprise
two small hole pockets centred at the centre of the Bril-
louin zone and two small electron pockets at the zone
boundary [25, 26]. ARPES sees isotropic gaps around
each of the measured surfaces apparently ruling out a
d-wave gap symmetry .
A resolution of this apparent discrepancy has been pro-
vided by theoretical predictions that the gap symmetry
is not d-wave, but rather extended s±-wave . Mazin
et al have postulated that the gaps in each hole and elec-
tron pocket are isotropic, but that the gaps on the hole
and electron pockets have opposite phase. This means
that magnetic fluctuations are amplified by the coherence
factor at values of Q that couple the hole and electron
pockets, as has been confirmed by explicit calculations of
the neutron scattering intensities . This is precisely
where we have observed the resonant excitation, so our
measurements, combined with the ARPES data, provide
strong experimental support for the validity of extended
s±-wave gap models.
In conclusion, we have presented the first experimental
evidence in the new class of iron arsenide superconduc-
tors for the existence of a resonant excitation in the dy-
namic magnetic susceptibility that disappears above the
superconducting transition temperature. The energy of
this resonant excitation is at ω0∼ 14meV, or 4.3Tc, just
under the canonical value of 5Tcseen in the cuprate su-
perconductors . However, Stock et al have argued that
it is more appropriate to scale ω0/2∆0, where ∆0is the
maximum value of the gap, and they estimate that this
ratio ranges from 0.62 to 0.74 in a wide range of materials
. From ARPES data on Ba0.6K0.4Fe2, ∆0 ∼ 12meV
, giving a ratio of ω0/2∆0 ∼ 0.58. It is remarkable
that materials with such a divergent range of Tcs (over
two orders of magnitude) could be unified by such a sim-
ple scaling relation.
We acknowledge helpful scientific discussions with
Christopher Stock. This work was supported by the Divi-
sion of Materials Sciences and Engineering Division and
the Scientific User Facilities Division of the Office of Ba-
sic Energy Sciences, U.S. Department of Energy Office of
Science, under Contract Nos. DE-AC02-06CH11357 and
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