The new α-Fe(Te,Se) superconductors share the common iron building block and ferminology with the LaFeAsO and BaFe2As2 families of superconductors. In contrast with the predicted commensurate spin-density-wave order at the nesting wave vector (π, 0), a completely different magnetic order with a composition tunable propagation vector (δπ, δπ) was determined for the parent compound Fe1+yTe in this powder and single-crystal neutron diffraction study. The new antiferromagnetic order survives as a short-range one even in the highest TC sample. An alternative to the prevailing nesting Fermi surface mechanism is required to understand the latest family of ferrous superconductors

Bao, Wei

Chang, S

Fang, M

Fitzsimmons, M R

Green, M A

Huang, Q

Mao, Z Q

Pham, H M

Qian, B

Qiu, Y

Spinu, L

Yang, Jinhu

Zajdel, P

Zhernenkov, M

English

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University of New Orleans 
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Physics Faculty Publications Department of Physics 
2009 
Tunable (δπ, δπ)-Type Antiferromagnetic Order in α-Fe(Te,Se) 
Superconductors 
Wei Bao 
Y Qiu 
Q Huang 
M A. Green 
P Zajdel 
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Authors 
Wei Bao, Y Qiu, Q Huang, M A. Green, P Zajdel, M R. Fitzsimmons, M Zhernenkov, S Chang, M Fang, B Qian, 
Jinhu Yang, H M. Pham, L Spinu, and Z Q. Mao 
This article is available at ScholarWorks@UNO: https://scholarworks.uno.edu/phys_facpubs/11 
Tunable (, )-Type Antiferromagnetic Order in -Fe(Te,Se) Superconductors
Wei Bao,1,2,* Y. Qiu,3,4 Q. Huang,3 M.A. Green,3,4 P. Zajdel,3,5 M. R. Fitzsimmons,2 M. Zhernenkov,2 S. Chang,3
Minghu Fang,6,7 B. Qian,6 E. K. Vehstedt,6 Jinhu Yang,7 H.M. Pham,8 L. Spinu,8 and Z.Q. Mao6
1Department of Physics, Renmin University of China, Beijing 100872, China
2Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
3NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA
4Department of Materials Science and Engineering, University of Maryland, College Park, Maryland 20742, USA
5Institute of Physics, University of Silesia, Uniwersytecka 4, 40-007 Katowice, Poland
6Department of Physics, Tulane University, New Orleans, Louisiana 70118, USA
7Department of Physics, Zhejiang University, Hangzhou 310027, China
8Department of Physics, University of New Orleans, New Orleans, Louisiana 70148, USA
(Received 29 September 2008; published 17 June 2009)
The new -Fe(Te,Se) superconductors share the common iron building block and ferminology with the
LaFeAsO and BaFe2As2 families of superconductors. In contrast with the predicted commensurate spin-
density-wave order at the nesting wave vector (, 0), a completely different magnetic order with a
composition tunable propagation vector (, ) was determined for the parent compound Fe1þyTe in
this powder and single-crystal neutron diffraction study. The new antiferromagnetic order survives as a
short-range one even in the highest TC sample. An alternative to the prevailing nesting Fermi surface
mechanism is required to understand the latest family of ferrous superconductors.
DOI: 10.1103/PhysRevLett.102.247001 PACS numbers: 74.25.Ha, 61.05.fm, 74.70.b, 75.30.Fv
The recently discovered ferrous superconductors differ
from phonon-mediated conventional superconductors in an
important way: when the nonmagnetic La in LaFeAs(O,F)
is replaced by magnetic lanthanides, TC increases from
26 K to as high as 55 K [1–4], in contrast to the breaking
of the Cooper pairs by magnetic ions [5]. The La(O,F)
‘‘charge reservoir’’ layer turns out not to be a requirement
for superconductivity and can be replaced by simple metal
layers in ðBa=Sr=Ca;K=NaÞFe2As2 [6–9], or completely
absent as shown more recently in the  phase of Fe(Se,Te)
[10–12]. The common iron layer contributes dominantly to
the electronic states at the Fermi level in these families of
materials [13–17], which thus share similar quasi-two-
dimensional Fermi surfaces with a nesting wave vector
(, 0) in the reciprocal Fe square lattice. The antiferro-
magnetic order observed in the parent compounds of both
the LaFeAsO [18] and BaFe2As2 [19] families of materi-
als, Fig. 1(c), has been predicted by the nesting spin-
density-wave (SDW) mechanism [20]. In view of insuffi-
cient electron-phonon coupling [21–23], spin excitations
from the only known mode at (, 0) have been proposed as
the bosonic ‘‘glue’’ mediating high TC superconductivity
in these ferrous materials [13–17,20].
However, the weak-coupling SDWmechanism critically
depends on the matching electron and hole Fermi surfaces
in the parent compounds [14]. The nesting condition is lost
when adding electrons or holes to the systems [24]. This
expectation is confirmed in systematic doping [25–27] and
pressure studies [28], which show the destruction of the
SDW order well before the optimal superconducting state
is established. Moreover, despite the same (, 0) SDW
order being predicted for -FeTe in first-principles theory
[17], we observed a completely different antiferromagnetic
order with the in-plane propagation vector (, ) along
the diagonal direction of the Fe ‘‘square’’ lattice, Fig. 1(b).
The  is tunable from an incommensurate 0.38 to the
commensurate 0.5. Therefore, experimental results re-
ported here call for a better understanding of the mecha-
nism of magnetism and its role in superconductivity for the
ferrous superconductors.
The single-phase FeðTe; SeÞz material in the tetragonal
PbO structure exists in a composition range near z ¼ 1
[29]. In this  phase [10–12] (called  phase in [29]), iron-
chalcogen forms with the same edge-sharing antifluorite
layers as found in the FeAs superconductors. The -FeSe
with the nominal composition FeSe0:88 was recently re-
ported to superconduct at TC  8 K [10], which increases
to 27 K at 1.48 GPa [30]. The isovalent series
FeðTe1xSexÞz in the  phase with nominal z ¼ 0:82 has
been synthesized, and the TC is enhanced to 14 K at x ¼
0:4 at ambient pressure [11]. Similar results have also been
reported for the nominal z ¼ 1 series [12].
The end member -FeTez is not superconducting, and
bulk measurements indicate a phase transition at TS 
60–75 K [11,12]. As a function of z, there exist two distinct
types of transport behaviors in the low temperature phase:
for z  0:90 the samples change from a semiconductor to a
metal, while for z < 0:90 the samples remain semiconduct-
ing [11]. Therefore, we selected a typical composition
from each range of z for this study, FeTe0:82 and
FeTe0:90. For superconducting -FeðTe; SeÞz, we chose
the highest TC  14 K compound FeðTe0:6Se0:4Þ0:82. The
high-resolution powder diffraction spectra of polycrystal-
line samples, weighing 15–16 g, were measured with
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neutrons of wavelength 2.0785 A˚ using BT1 at NIST
(Fig. 2). See Ref. [31] for a table listing the refined pa-
rameters. The high-temperature phase of these samples
indeed has the tetragonal P4=nmm structure [10]; however,
the chalcogen and Fe(1) sites of the PbO structure are fully
occupied, and the excess Fe partially occupies the inter-
stitial site Fe(2), see [31] and Fig. 1(a). Thus, the more
appropriate formula for nominal -FeðTe1xSexÞz is
Fe1þyðTe1xSexÞ.
While Fe1:080Te0:67Se0:33 remains tetragonal in the
superconducting state at 4 K [[31], Table I(c)], the parent
compounds Fe1:141Te and Fe1:076Te experience a first-order
magnetostructural transition, see Fig. 3, similar to that in
BaFe2As2 [19]. The semiconducting Fe1:141Te distorts to
an orthorhombic Pmmn structure below TS  63 K, with
the a axis expanding and the b axis contracting, Fig. 3(c)
and [31], Table I(a). This results in the splitting of the (h0k)
Bragg peaks of the high-temperature structure, Fig. 2(b).
The orthorhombic distortion here, however, does not
double the unit cell, different from that observed in either
the LaFeAsO [32] or BaFe2As2 [19]. The metallic
Fe1:076Te has a monoclinic P21=m structure below TS 
75 K, [31], Table I(b). In addition to the differentiation
of the a and b axis [Fig. 3(d)], the c axis rotates towards the
a axis to   89:2. Thus, the monoclinic distortion not
only splits the (200) but also the (112) Bragg peak,
Fig. 2(d). In the weaker first-order transition of Fe1:141Te,
a mixed phase exists in the pink-shaded region in Fig. 3(c).
At 55 K upon warming, 85% of the sample is orthorhombic
and 15% tetragonal. See Ref. [31] for the temperature
dependence of distances and angles between various
atoms.
The additional magnetic Bragg reflections of the mono-
clinic metal in Fig. 2(d) can be indexed by a commensurate
magnetic wave vector q ¼ ð12 0 12Þ, as previously reported
[33]. However, magnetic Bragg reflections of the ortho-
rhombic semiconductor in Fig. 2(b) cannot be indexed by
multiples of the nuclear unit cell. By performing single-
FIG. 2 (color online). Neutron powder diffraction spectra of Fe1:141Te and Fe1:076Te above and below the phase transition.
FIG. 1 (color online). (a) Crystal structure of -Fe(Te,Se). Magnetic structures of (b) -FeTe and (c) BaFe2As2 are shown in the
primitive Fe square lattice for comparison. Note that the basal square lattice of the PbO unit cell in (a) is a
ffiffiffi
2
p  ffiffiffi2p superlattice of that
in (b).
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crystal neutron diffraction using the triple-axis spectrome-
ter BT9 at NIST, we determine the incommensurate mag-
netic wave vector q ¼ ð0 12Þ, where   0:380, for
Fe1:141Te. The q determines that magnetic moments in
each row along the b axis are parallel to each other. The
rows of moments in an Fe plane modulate with the prop-
agating vector 2=a, which is 45 away from that of
previous FeAs materials, see Figs. 1(b) and 1(c). From one
plane to the next along the c axis, magnetic moments
simply alternate direction. In the same magnetostriction
pattern as previously observed in the magnetic state of
NdFeAsO [34] and BaFe2As2 [19], the lattice contracts
in the b axis, along which the magnetic moments are
parallel to each other, and it expands in the a and c axis,
which are the directions of the antiferromagnetic align-
ment. Once again, the unusual coupling between the lattice
and magnetic degrees of freedom is what expected from
multiple d-orbital magnetism [15].
The observed magnetic powder spectra can be refined by
a collinear sinusoid model,
M lðRÞ ¼ Mlb^ cosðq RþRÞ; (1)
where R is the position of the Fe, Ml the staggered mag-
netic moment, R the additional phase at the Fe site. The
unit vector b^ fixes the moment along the b-axis, and q is
the observed magnetic wave vector. Refined magnetic
parameters at low temperature are listed in Ref. [31]
Table I(a) and (b). However, for an incommensurate q, a
spiral model with the moment rotating in the ac plane,
M sðRÞ ¼ Ms½a^ cosðq RþRÞ þ c^ sinðq RþRÞ	;
(2)
offers an equivalent description of the unpolarized neutron
diffraction results, with the relation between the respective
neutron diffraction cross sections
2lðQÞ=hMli2 
 sðQÞ=hMsi2: (3)
Thus, any linear combination of Eqs. (1) and (2) is also an
equivalent description. On the other hand, l and s are
partial cross sections for different channels of polarized
neutron scattering [35]. Therefore, they can be readily
determined using polarized neutrons.
We measured a Fe1:141Te single-crystal sample, aligned
in the (h0l) horizontal scattering plane, using polarized
neutron spectrometer Asterix at the Lujan Center of
LANL. The neutron spin is controlled to align either
perpendicular to the (h0l) plane (VF) or parallel to the
momentum transfer (HF). All four channels (þþ, þ ,
þ ,  ) in both the VF and HF configurations were
measured for the (001) and (0 12 ) Bragg peaks. The flip-
ping ratio of the instrument is 10.3 as measured at the
nuclear (001) peak. The (0 12 ) is proved magnetic by the
spin-flip scattering in HF. The normalized intensity of
(0 12 ) in VF is 8.24(28) in the non-spin-flip (NSF) chan-
nels, and 4.13(20) in the spin-flip (SF) channels. After
correcting for the finite flipping ratio of the instrument,
we obtained l=s 
 INSF=ISF ¼ 7:91ð27Þ=3:37ð16Þ.
Therefore, the incommensurate magnetic structure for
Fe1:141Te is
M ðRÞ ¼Ml þMs
¼ M½wb^ cosðq RþR þ c Þ þ a^ cosðq R
þRÞ þ c^ sinðq RþRÞ	;
(4)
where w 
 ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2l=sp ¼ 2:17ð6Þ, M ¼ Ml=
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2þ w2
p
¼
0:76ð2ÞB=Fe and c is an arbitrary phase between the
spiral and the sinusoidal components; see Fig. 1(b).
To understand whether the incommensurate magnetic
structure in the orthorhombic semiconducting phase is
locked or tunable, we examined another sample
FIG. 3 (color online). (a),(b) The magnetic Bragg peak (, 0,
1=2) (blue symbols) and the splitting of the structural peak (200)
or (112) of the tetragonal phase (red symbols) show the thermal
hysteresis in the first-order transition. (c),(d) The lattice parame-
ters through the transition.
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Fe1:165ð3ÞTe by powder diffraction at BT1, [31], Table I(d).
The incommensurability is greatly affected and measures
at  ¼ 0:346, despite no appreciable differences in either
the moment or the phase  from Fe1:141Te in [31],
Table I(a). Thus,  can be tuned by varying the excess Fe
in the orthorhombic phase, and it reaches a commensurate
value 12 for the composition Fe1:076Te in the metallic mono-
clinic phase with less excess iron, see inset (a) of Fig. 4.
Having unveiled a tunable (, )-type of anti-
ferromagnetic order in the parent compound Fe1þyTe,
it is natural to ask whether the new magnetic order sur-
vives in the optimal TC superconducting sample
Fe1:080Te0:67Se0:33. While there is neither long-range mag-
netic order nor structural transition, we observed pro-
nounced short-range quasielastic magnetic scattering at
the incommensurate wave vector (0.438, 0, 12 ), Fig. 4, using
SPINS at NIST. The temperature insensitive half-width-at-
the-half-maximum 0:25 A1 indicates a short magnetic
correlation length of 4 A˚, approximately only two
nearest-neighbor Fe spacings. The concave shape of the
peak intensity as a function of temperature in inset (b)
indicates the expected diffusive nature of the short-range
magnetic correlations. This is very different from the case
of the (, 0) SDW which is completely suppressed in the
optimal TC FeAs samples [18,22,25,27].
To summarize, the -Fe(Te,Se) shares a common elec-
tronic structure with the previously reported FeAs-based
superconductor systems. Though the same (, 0) SDW
order has been predicted [17], we show the presence of a
fundamentally different (, ) antiferromagnetic order
which propagates along the diagonal direction. The incom-
mensurability  in the orthorhombic semiconducting phase
is easily tunable with excess Fe and locks into a commen-
surate 12 in the monoclinic metallic phase. This magnetic
order, which survives as short-range order even in the
optimal superconducting state, cannot be the result of
Fermi surface nesting, which is along the (, 0) direction
and delicately depends on electronic band filling for its
existence.
Work at LANL was supported by the DOE-OS-BES; at
Tulane by the NSF grant DMR-0645305, the DOE DE-
FG02-07ER46358 and the Research Corp.; at ZU by
NBRP of China (No. 2006CB01003, 2009CB929104)
and the PCSIRT of the MOE of China (IRT0754); at
UNO by DARPA Grant No. HR0011-07-1-0031. SPINS
is in part supported by NSF under Agreement DMR-
0454672.
*wbao@ruc.edu.cn
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FIG. 4 (color online). Short-range magnetic order in super-
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Tunable (δπ, δπ)-Type Antiferromagnetic Order in α-Fe(Te,Se) Superconductors

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Tunable (δπ, δπ)-Type Antiferromagnetic Order in α-Fe(Te,Se) Superconductors

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