PHYSICAL REVIEW B 80, 024508 共2009兲

Superconducting state coexisting with a phase-separated static magnetic order in (Ba, K)Fe2As2,(Sr, Na)Fe2As2, and CaFe2As2
T. Goko,1,2,3 A. A. Aczel,3 E. Baggio-Saitovitch,4 S. L. Bud’ko,5 P. C. Canfield,5 J. P. Carlo,2 G. F. Chen,6 Pengcheng Dai,7A. C. Hamann,8 W. Z. Hu,6 H. Kageyama,9 G. M. Luke,3 J. L. Luo,6 B. Nachumi,2 N. Ni,5 D. Reznik,8D. R. Sanchez-Candela,4 A. T. Savici,10 K. J. Sikes,2 N. L. Wang,6 C. R. Wiebe,11 T. J. Williams,3 T. Yamamoto,9W. Yu,3 and Y. J. Uemura2,*
1TRIUMF, 4004 Wesbrook Mall, Vancouver, British Columbia, Canada V6T 2A3
2

Department of Physics, Columbia University, New York, New York 10027, USA

3Department of Physics and Astronomy, McMaster University, Hamilton, Ontario, Canada L8S 4M1
4Centro Brasileiro de Pesquisas Fisicas, Rua Xavier Sigaud 150 Urca, CEP 22290-180 Rio de Janeiro, RJ, Brazil
5
Department of Physics and Astronomy and Ames Laboratory, Iowa State University, Ames, Iowa 50011, USA
6Beijing National Laboratory for Condensed Matter Physics, Institute of Physics,
Chinese Academy of Sciences, Beijing 100080, People’s Republic of China
7
Department of Physics and Astronomy, University of Tennessee, Knoxville, Tennessee 37996, USA
8Forschungszentrum Karlsruhe, Institut für Festkörperphysik, Postfach 3640, D-76021 Karlsruhe, Germany
9Department of Chemistry, Kyoto University, Kyoto 606-8502, Japan10
Department of Physics and Astronomy, Johns Hopkins University, Baltimore, Maryland 21218, USA
11
Department of Physics, Florida State University, Tallahassee, Florida 32310, USA
共Received 20 August 2008; revised manuscript received 10 June 2009; published 14 July 2009兲
By muon spin-relaxation measurements on single-crystal specimens, we show that superconductivity in the
AFe2As2 共A = Ca, Ba, Sr兲 systems, in both the cases of composition and pressure tunings, coexists with a strong
static magnetic order in a partial volume fraction. The superfluid response from the remaining paramagnetic
volume fraction of 共Ba0.5K0.5兲Fe2As2 exhibits a nearly linear variation in T at low temperatures, suggesting ananisotropic energy gap with line nodes and/or multigap effects.
DOI: 10.1103/PhysRevB.80.024508

PACS number共s兲: 74.25.Dw, 74.25.Nf, 75.25.⫹z, 76.75.⫹i

The announcement of superconductivity in La共O,F兲FeAs
共Tc = 26 K兲 共Ref. 1兲 triggered an unprecedented burst of research activities in FeAs-based superconductors and theirparent systems. By now, superconductivity has been reported
in systems with four different crystal structures, including
the “1111” systems RE共O,F兲FeAs with rare earth= La, Nd,Ce, etc.,2 and the “122” systems AFe2As2 共A = Ba, Sr, Ca兲.3–5
Extensive measurements by neutron scattering,6–8 Mössbauer effect,9–11 and muon spin relaxation 共␮SR兲 共Refs.
11–15兲 have revealed collinear antiferromagnetic order in
undoped parent compounds,6,16,17 hyperfine splitting of 57Fe
Mössbauer spectra, and ␮SR frequencies indicative of astatic moment size ranging between 0.3 and 0.8 Bohr magnetons per Fe,9,10,15 and nearly linear scaling between Tc andthe superfluid density12–14 in the 1111 systems following thetrend found in cuprate and other exotic superconductors.18
In studies of magnetic phase diagrams of the 1111 systems, as a function of increasing 共O,F兲 substitution, La共O,F兲FeAs shows19 an abrupt and first-order-like evolution froman antiferromagnetic to superconducting state, Ce共O,F兲FeAs
shows nearly second-order-like evolution,7 and Sm共O,F兲FeAs 共Ref. 20兲 exhibits phase-separated coexistence ofstatic magnetism and superconductivity in a small concentration region around the phase boundary. Despite these differences, superconductivity appears mostly in the region without static magnetic order in the 1111 systems, similar to thecase of the cuprates. In contrast, very little has been reported
on the phase diagrams of the 122 systems. Recent powder
neutron measurements on 共Ba, K兲Fe2As2 共Ref. 8兲 found aphase diagram similar to the one for Sm共O,F兲FeAs with co1098-0121/2009/80共2兲/024508共6兲

existing long-range magnetic order and superconductivity
near the phase boundary, without providing information on
the volume fraction of the magnetically ordered region. We
have also reported ␮SR measurements on a single crystal of
共Ba0.55K0.45兲Fe2As2 共Ref. 15兲 which found the coexistence ofphase-separated static magnetic order and superconductivity.
The superfluid density of this crystal was much lower thanthat in the corresponding 1111 systems with comparable Tc’s,
which is suggestive of insufficient carrier doping. In the 122
systems, more definitive studies of magnetic phase diagrams
can be expected due to the availability of large single
crystals,21–23 improvement of the growth method, and applicability of pressure tuning free from randomness due to substitution.
In this paper, we report ␮SR measurements of superconducting single crystals of 共Ba0.5K0.5兲Fe2As2 共Tc ⬃ 37 K兲
and 共Sr0.5Na0.5兲Fe2As2 共Tc ⬃ 35 K兲 in ambient pressure, andof CaFe2As2 in ambient and applied pressure p up to p
= 10 kbar, performed at TRIUMF in Vancouver, Canada.
The former two crystals were prepared at the Institute ofPhysics in Beijing using the FeAs flux method23 andweighed ⬃100 and 40 mg, respectively. As shown in Fig. 1,
sharp superconducting transitions were observed in the magnetic susceptibility of both crystals, which suggests that they
are of good quality. No anomaly due to a structural transition
can be seen in the resistivity results. These crystals were
mounted with their ab planes perpendicular to the muon
beam at the M20 channel. In our ␮SR measurements intransverse external fields applied perpendicular to the abplane, both of these crystals exhibited a superfluid response

024508-1©2009 The American Physical Society

PHYSICAL REVIEW B 80, 024508 共2009兲

GOKO et al.

250

Ba0.5K0.5Fe2As2

50

-2
-4
10

ZFC

χ (a.u.)Resistance (mΩ)

0

5

-4

FC

-8

ZFC

50

100

150

(a)

40
30
20

0

0.8
0.6
0.4
0.2

250

50

100

150

200

Temperature (K)10 20 30 40
T (K)
200

(b)
1.0

0.0
0

-12
0

0
0

30 40
T (K)

Sr0.5Na0.5Fe2As2

10

SrFe2As2
BaFe2As2

10

20

0
15

Frequency (MHz)1000Paramagnetic Fraction150

χ (emu/mol)

Resistivity (µΩcm)

50

200

Sr0.5Na0.5Fe2As2
Ba0.5K0.5Fe2As2

300

Temperature (K)FIG. 1. 共Color online兲 Temperature dependences of the resistivity and the magnetic susceptibility of our specimens of 共a兲
共Ba0.5K0.5兲Fe2As2 共Tc ⬃ 37 K兲 and 共b兲 共Sr0.5Na0.5兲Fe2As2 共Tc⬃ 35 K兲. Due to the irregular shape of the specimen which prevents accurate estimate of the demagnetizing factor, we put theresults of the latter system on a relative 共arbitrary兲 scale. The inset
figure of 共b兲 shows the magnetic susceptibility obtained in the fieldcooling and zero-field-cooling procedures.

corresponding to a muon spin-relaxation rate of ⬃1 ␮s−1 at
T → 0. This indicates strong bulk superconductivity of thesecrystals.
Over 1 g of CaFe2As2 crystals 共in more than 100 pieces兲,
prepared in Ames Laboratory using the Sn flux method,22
were mounted in a pressure cell having a sample space of 7
mm in diameter and 12 mm long. The cell was pressurized atroom temperature before being mounted in the cryostat.
Daphne oil was used as the pressure mediator. This was chosen because it is known not to solidify at room temperature
up to ⬃20 kbar and so generates hydrostatic pressure over a
wider pressure temperature range than Fluorinert, which solidifies above ⬃10 kbar at room temperature. The crystals
were aligned with their ab planes perpendicular to the muon
beam at the M9B channel, where the initial muon spin polarization is tuned to be perpendicular to the beam direction.
␮SR measurements were performed in zero field 共ZF兲 andweak transverse field 共WTF兲 of ⬃30– 50 G to study magnetic ordering, and in transverse field 共TF兲 of 300–500 G to
study superfluid density. A recent study on 共Ca, Sr兲RuO3 and
MnSi in applied pressure24 has demonstrated ␮SR’s uniquecapability of determining volume fractions of regions withand without static magnetic order in systems having phaseseparation. Details of the ␮SR methods can be found in Refs.
24 and 25.

FIG. 2. 共Color online兲 Temperature dependences of 共a兲 the muon
spin precession frequency observed in ZF ␮SR and 共b兲 the paramagnetic volume fraction determined from WTF-␮SR measurements of single-crystal specimens of 共Ba0.5K0.5兲Fe2As2 共Tc⬃ 37 K兲 and 共Sr0.5Na0.5兲Fe2As2 共Tc ⬃ 35 K兲. The results from thepresent work 共solid and open symbols兲 are compared with those of
the undoped parent compounds BaFe2As2 共Ref. 15兲 and SrFe2As2
共Ref. 10兲 共solid and broken lines兲.

Figure 2 shows 共a兲 the muon spin precession frequencies
observed in ZF ␮SR and 共b兲 the paramagnetic volume fraction derived from WTF-␮SR measurements in the 共Ba,K兲
and 共Sr,Na兲 crystals. The solid lines show the reported resultsin the undoped parent compounds BaFe2As2 共Ref. 15兲 and
SrFe2As2.10 In both systems, static magnetism sets in at temperatures well above the superconducting Tc’s, in a large
volume fraction of ⬃90% in the 共Sr,Na兲 system and 50% in
the 共Ba,K兲 system. We observed two different precession frequencies in a given system, presumably coming from two
different muon sites, as was the case in BaFe2As2.15 The
frequencies in the superconducting samples are reduced fromthe values in the undoped compounds only by 20– 30 %,indicating that static magnetic order with a significant Fe
moment size exists in the magnetically ordered regions.
These results demonstrate phase separation between magnetically ordered and paramagnetic volumes.
In TF ␮SR, the precession signal from the paramagnetic
volume fraction exhibits damping below Tc due to an inhomogeneous field distribution in the flux vortex lattice. The
relaxation rate ␴, obtained by fitting the spectra to a Gaussian function exp共−␴2t2 / 2兲, is given by ␴ ⬀ 1 / ␭2 ⬀ ns / mⴱ,
where ␭ is the penetration depth, ns is the superconducting
carrier density, and mⴱ is the effective mass.18,25 An increasein ␴ was observed in both the 共Ba,K兲 and 共Sr,Na兲 crystals
below the superconducting Tc’s. Since the statistical accuracy of the data is much better for the former system with thelarger paramagnetic volume fraction, here we present the results of 共Ba0.5K0.5兲Fe2As2 in TF= 500 G applied parallel to024508-2

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SUPERCONDUCTING STATE COEXISTING WITH A…

-1

Relaxation Rate σ (µsec )1.4(Ba0.5K0.5)Fe2As2(Ba0.55K0.45)Fe2As2
s-BCS
d-YBCO

1.2
1.0
0.8

TF 500 G

0.6
0.4

(a)

0.2
0.0
0

10

20

30

40

50

60

Temperature (K)Transition Temperature Tc (K)

125
100
75

2223

(Ba,K)Fe2As2
Nd(O,F)FeAs
Ce
La
123
Sm
La

(b)

Tl220150 123(Zn)
C60
25
0
0

214
1

2

214Stripe214(Zn)Bi2212
3

4

Relaxation Rate σ(T→ 0) (µsec )-1

FIG. 3. 共Color online兲 共a兲 Temperature dependence of the muon
spin-relaxation rate ␴ observed in a single-crystal specimen of
共Ba0.5K0.5兲Fe2As2 共Tc ⬃ 37 K兲 in TF ␮SR with TF= 500 G 共closed
circles, present work兲, compared with the temperature dependence
expected for the isotropic energy gap of BCS s-wave pairing 共broken line兲, scaled results from YBCO 共Ref. 25兲 共solid line兲, and ourprevious results in a different crystal of Ba0.55K0.45Fe2As2 共Ref. 15兲
共open circles兲. 共b兲 A plot of the relaxation rate ␴共T → 0兲 versus Tc,
including the point for 共Ba0.5K0.5兲Fe2As2 from the present work as
well as those for the FeAs-based 1111 superconductors published inRefs. 12 and 14, various cuprates, and A3C60 superconductors 共Ref.
18兲.

the c axis in Figs. 3共a兲 and 3共b兲. The temperature dependenceof ␴ in 共a兲 is nearly linear with T, as demonstrated by thegood agreement with the scaled data from a YBa2Cu3O6.95
共YBCO兲 system.25 The observed behavior is distinctly different from the case for an isotropic energy gap shown by thebroken line representing a calculation for BCS s-wave coupling. The observed temperature dependence may be attributed to 共1兲 line nodes in an anisotropic energy gap or 共2兲
widely different magnitudes of multiple isotropic gaps as
seen in calculations26,27 based on multiple bands. An
angle resolved photoemission spectroscopy 共ARPES兲
measurement28 on 共Ba, K兲Fe2As2 reported evidence for isotropic multiple gaps, consistent with theories based on an
extended s-wave pairing.29,30
The absolute value of ␴共T → 0兲 is about a factor of 3
larger than that observed in 共Ba0.55K0.45兲Fe2As2 in our previous measurements.15 Given that the Hc2 anisotropy is rela-

tively low near Tc, varying between 3.5 and 2.5 forH ⬍ 14 T 共Ref. 21兲 and decreasing for higher fields,31 weplot the present results in the ␴共T → 0兲 versus Tc plot in
Fig. 3共b兲 without single crystal to polycrystalline conversion
corrections.32 The resulting point from the present 共Ba,K兲
crystal 共red solid square symbol兲 indicates that the present
system has a superfluid density close to those in the 1111
systems with comparable Tc’s and that a sufficiently doped
122 FeAs system follows the nearly linear relationship between Tc and ns / mⴱ found in the cuprates and 1111 systems.
The superconducting state can also be obtained by applying pressure 共using a liquid pressure medium兲 to the undoped
parent compounds of the 122 systems.4,5 In particular, theCaFe2As2 system shows superconductivity below T ⬃ 10 K
at relatively low pressures p of 3–8 kbar, which are attainable using the available ␮SR piston-cylinder pressure cell.
We studied static magnetic order of CaFe2As2 at ambientpressure and at p = 3.9, 6.2, and 9.9 kbar by performing WTF
␮SR with WTF= 50 G. Solid symbols in Fig. 4共a兲 show theparamagnetic volume fraction, obtained after subtracting thecontribution from the pressure cell in which the singlecrystal specimens were placed with their c axis oriented parallel to the beam direction. Open circle symbols represent
additional results in ambient pressure obtained without thepressure cell. Figure 4共a兲 demonstrates that static magnetic
order sets in at temperatures well above the superconducting
Tc in a partial volume fraction both at p = 3.9 and 6.2 kbar.
The static magnetism disappears at p = 9.9 kbar, where thesuperconducting state no longer exists. Figure 4共b兲 shows the
low-temperature 共T → 0兲 values of the volume fraction of themagnetically ordered region 共from WTF data兲 as well as themuon spin precession frequency in ZF ␮SR, which is proportional to the size of the ordered Fe moment. We present
the resulting pressure-temperature phase diagram of
CaFe2As2 in Fig. 4共c兲. The superconducting phase boundary
in this figure is based on the reported resistivity results.4
Figure 4 indicates that a rather strong magnetism exists in a
substantial volume fraction below T = 50– 100 K, which iswell above the superconducting Tc, as in the cases of the
共Ba,K兲 and 共Sr,Na兲 crystals at ambient pressure.
Resistivity4 and neutron33 measurements in applied pressure, the former 共the latter兲 using Fluorinert 共He gas兲 as thepressure mediator, have been reported on CaFe2As2 single
crystals prepared by an identical method to that used for thepresent specimens.22 In the resistivity studies, a sharp jump
was observed at T = 170 K at ambient pressure, corresponding to the first-order tetragonal-to-orthorhombic structuralphase transition below which magnetic order was detectedboth by neutrons and muons. With increasing pressure thisfeature broadens and ordering moves toward lower temperatures, which is qualitatively consistent with the present results in Fig. 4共c兲. The resistivity anomaly disappears abovep ⬃ 4 kbar, and the neutron magnetic Bragg-peak intensity
at T = 50 K becomes nearly equal to the background level at
p = 6.3 kbar 关Fig. 1共c兲 in Ref. 33兴, while ␮SR detected magnetic order continuing to exist at p = 6.2 kbar, albeit in apartial volume fraction.
Quite recently, additional neutron measurements underpressure, using Fluorinert 共and not He gas兲 as the pressure
mediator, were performed34 to examine CaFe2As2 specimens

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Paramagnetic Fraction

CaFe2As2

(a)

1.0
0.8
TF 50 G

0.6
0.4

9.9 kbar
6.2
3.9
0.0

0.2
0.0
0

50

100

150

200

250

Temperature (K)FIG. 4. 共Color online兲 共a兲 The volume fraction of regions without static magnetic order in CaFe2As2, as a function of temperatureand pressure, determined by WTF-␮SR measurements with WTF⬃50 G. The points with closed 共open兲 symbols were obtained inmeasurements with 共without兲 a pressure cell. 共b兲 Pressure dependence of the volume fraction of the magnetically ordered region
共purple closed circles; left axis兲 from WTF ␮SR and the muon spin
precession frequency 共blue triangles; right axis兲 from ZF ␮SR at
T ⬃ 2 K in CaFe2As2. 共c兲 The phase diagram as a function of pressure and temperature in CaFe2As2. The Tc values are taken from thereported resistivity results 共Ref. 4兲. Upper, middle, and lower temperatures attached to the closed circle symbols for TN represent
temperatures at which the volume fraction with static magnetic order becomes 30%, 50%, and 70% of the value at T → 0,
respectively.

grown by the same group22 as those discussed in the present
work. These neutron measurements revealed the coexistenceof two structural phases 共antiferromagnetic orthorhombic
and nonmagnetic collapsed tetragonal phases兲 at low temperatures, which is also consistent with our ␮SR results. Thisbehavior probably arises from the applied pressure not actually being truly hydrostatic, as CaFe2As2 seems to bestrongly affected by any slight deviation from hydrostaticity.
More specifically, superconductivity was not observed in resistivity and susceptibility measurements35 of CaFe2As2 under He gas pressure, which is capable of providing true hydrostatic pressure over a much wider pressure temperature
range than either Fluorinert or Daphne oil. The crystals investigated by Yu et al. were also grown by the same group22
as those studied in the present work. Superconductivity, thecoexistence of magnetically ordered and nonmagnetic fractions and the coexistence of the orthorhombic and collapsed
tetragonal structures appear only under pressure using a liquid pressure medium. The magnetic order in a partial volume
fraction may play an important role in the emergence of superconductivity in CaFe2As2.
The present ␮SR results do not provide direct evidence
for distinguishing whether superconductivity lives uniformly
over the entire volume or exclusively in the paramagnetic
volume fraction. Evidence for the latter case has been reported in ARPES studies on a lightly doped single crystal of
共Ba, K兲Fe2As2,36 which found an intensity ratio of an ungapped magnetic response to a gapped superconducting response comparable to the ratio of volumes with and without
static magnetic order found by ␮SR. This situation is similar
to the case of 共La, Eu, Sr兲2CuO4 where the static volume
fraction and superfluid density in ␮SR exhibit a tradeoff.37
The present study does not provide an estimate of the lengthscale of the phase-separated regions. In La2CuO4.11,38 weestimated this length to be a few nanometers.
Phase separation at the border of magnetic and superconducting states has been found also in the organic
共BEDT-TTF兲2-X superconductors39 and CeCu2Si2.40 Firstorder phase transitions, similar to those in the La1111 FeAs
system,19 have been found in the A3C60,41 Ce共Co, Cd兲In5,
and CeRhIn5 共Ref. 42兲 systems. Phase separation was also
discovered at the border of a collinear antiferromagnetic state
and a nonmagnetic spin-gap state in an insulating J1-J2 spin
system Cu共Cl, Br兲La共Nb, Ta兲2O7,43 and at the border of anitinerant heli/ferromagnetic state and paramagnetic state in
MnSi and 共Sr, Ca兲RuO3.24 Further exploration of these behaviors will lead to a better understanding of superconductivity and magnetism in correlated-electron systems.
Regarding the pairing symmetry, available experimental
results on the 1111 and 122 systems are divided between
those favoring an isotropic nodeless gap44 and those supporting line nodes and/or multigap features.26,27,45 The
共Ba0.5K0.5兲Fe2As2 result in Fig. 3共a兲 has established at least
one definite case which does not agree with a single isotropic
energy gap. The different temperature dependences of thesuperfluid density between the two 共Ba, K兲Fe2As2 crystals in
Fig. 3共a兲 may be due to a doping dependence of multiple gap
magnitudes, as proposed in Ref. 30. The nearly linear relationship between Tc and the superfluid density 关Fig. 3共b兲兴
followed by cuprates, 1111 FeAs, 122 FeAs, and A3C60 sys-

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We acknowledge financial support from NSF under Grant
Nos. DMR-05-02706 and 08-06846 共Material World Network兲 at Columbia, NSF under Grant No. DMR-07-56568 at

UT Knoxville, NSF under Grant No. DMR-08-04173 andFlorida state at FSU, DOE under Contract No. DE-AC0207CH11358 at Ames, NSERC and CIFAR 共Canada兲 at McMaster, CNPq on MWN-CIAM program at CBPF 共Brazil兲,
NSFC, CAS, and 973 project of MOST 共China兲 at IOP,
Beijing, and JSPS U.S.-Japan cooperative program at Kyoto
University 共Japan兲.

*Author to whom correspondence should be addressed;

14 J. P. Carlo, Y. J. Uemura, T. Goko, G. J. MacDougall, J. A.

tomo@lorentz.phys.columbia.edu
1 Y. Kamihara, T. Watanabe, M. Hirano, and H. Hosono, J. Am.
Chem. Soc. 130, 3296 共2008兲.
2 X. H. Chen, T. Wu, G. Wu, R. H. Liu, H. Chen, and D. F. Fang,
Nature 共London兲 453, 761 共2008兲; G. F. Chen, Z. Li, D. Wu, G.
Li, W. Z. Hu, J. Dong, P. Zheng, J. L. Luo, and N. L. Wang,
Phys. Rev. Lett. 100, 247002 共2008兲; Z.-A. Ren, Guang-Can
Che, Xiao-Li Dong, Jie Yang, Wei Lu, Wei Yi, Xiao-Li Shen,
Zheng-Cai Li, Li-Ling Sun, Fang Zhou, and Zhong-Xian Zhao,
EPL 83, 17002 共2008兲.
3 M. Rotter, M. Tegel, and D. Johrendt, Phys. Rev. Lett. 101,
107006 共2008兲.
4 M. S. Torikachvili, S. L. Bud’ko, N. Ni, and P. C. Canfield, Phys.
Rev. Lett. 101, 057006 共2008兲.
5 P. L. Alireza, Y. T. Chris Ko, J. Gillett, C. M. Petrone, J. M.
Cole, S. E. Sebastian, and G. G. Lonzarich, J. Phys.: Condens.
Matter 21, 012208 共2009兲.
6 C. de la Cruz, Q. Huang, J. W. Lynn, Jiying Li, W. Ratcliff II, J.
L. Zarestky, H. A. Mook, G. F. Chen, J. L. Luo, N. L. Wang, andPengcheng Dai, Nature 共London兲 453, 899 共2008兲.
7 J. Zhao, Q. Huang, Clarina de la Cruz, Shiliang Li, J. W. Lynn,
Y. Chen, M. A. Green, G. F. Chen, G. Li, Z. Li, J. L. Luo, N. L.
Wang, and Pengcheng Dai, Nature Mater. 7, 953 共2008兲.
8 H. Chen, Y. Ren, Y. Qiu, Wei Bao, R. H. Liu, G. Wu, T. Wu, Y.
L. Xie, X. F. Wang, Q. Huang, and X. H. Chen, EPL 85, 17006
共2009兲.
9 M. Rotter, M. Tegel, and D. Johrendt, I. Schellenberg, W. Hermes, R. Pottgen, Phys. Rev. B 78, 020503共R兲 共2008兲.
10
A. Jesche, N. Caroca-Canales, H. Rosner, H. Borrmann, A. Ormeci, D. Kasinathan, H. H. Klauss, H. Luetkens, R. Khasanov,A. Amato, A. Hoser, K. Kaneko, C. Krellner, and C. Geibel,
Phys. Rev. B 78, 180504共R兲 共2008兲.
11 H.-H. Klauss, H. Luetkens, R. Klingeler, C. Hess, F. J. Litterst,
M. Kraken, M. M. Korshunov, I. Eremin, S.-L. Drechsler, R.
Khasanov, A. Amato, J. Hamann-Borrero, N. Leps, A. Kondrat,
G. Behr, J. Werner, and B. Büchner, Phys. Rev. Lett. 101,
077005 共2008兲.
12
H. Luetkens, H.-H. Klauss, R. Khasanov, A. Amato, R. Klingeler, I. Hellmann, N. Leps, A. Kondrat, C. Hess, A. Köhler, G.
Behr, J. Werner, and B. Büchner, Phys. Rev. Lett. 101, 097009
共2008兲; R. Khasanov, H. Luetkens, A. Amato, H.-H. Klauss,
Z.-A. Ren, J. Yang, W. Lu, and Z.-X. Zhao, Phys. Rev. B 78,
092506 共2008兲.
13
A. J. Drew, F. L. Pratt, T. Lancaster, S. J. Blundell, P. J. Baker,
R. H. Liu, G. Wu, X. H. Chen, I. Watanabe, V. K. Malik, A.
Dubroka, K. W. Kim, M. Rössle, and C. Bernhard, Phys. Rev.
Lett. 101, 097010 共2008兲.

Rodriguez, W. Yu, G. M. Luke, Pengcheng Dai, N. Shannon, S.
Miyasaka, S. Suzuki, S. Tajima, G. F. Chen, W. Z. Hu, J. L. Luo,and N. L. Wang, Phys. Rev. Lett. 102, 087001 共2009兲.
15 A. A. Aczel, E. Baggio-Saitovitch, S. L. Budko, P. C. Canfield, J.
P. Carlo, G. F. Chen, Pengcheng Dai, T. Goko, W. Z. Hu, G. M.
Luke, J. L. Luo, N. Ni, D. R. Sanchez-Candela, F. F. Tafti, N. L.
Wang, T. J. Williams, W. Yu, and Y. J. Uemura, Phys. Rev. B 78,
214503 共2008兲.
16 Q. Huang, Y. Qiu, W. Bao, M. A. Green, J. W. Lynn, Y. C.
Gasparovic, T. Wu, G. Wu, and X. H. Chen, Phys. Rev. Lett.
101, 257003 共2008兲.
17
A. I. Goldman, D. N. Argyriou, B. Ouladdiaf, T. Chatterji, A.
Kreyssig, S. Nandi, N. Ni, S. L. Bud’ko, P. C. Canfield, and R.
J. McQueeney, Phys. Rev. B 78, 100506共R兲 共2008兲.
18 Y. J. Uemura, G. M. Luke, B. J. Sternlieb, J. H. Brewer, J. F.
Carolan, W. N. Hardy, R. Kadono, J. R. Kempton, R. F. Kiefl, S.
R. Kreitzman, P. Mulhern, T. M. Riseman, D. Ll. Williams, B.
X. Yang, S. Uchida, H. Takagi, J. Gopalakrishnan, A. W. Sleight,
M. A. Subramanian, C. L. Chien, M. Z. Cieplak, Gang Xiao, V.
Y. Lee, B. W. Statt, C. E. Stronach, W. J. Kossler, and X. H. Yu,
Phys. Rev. Lett. 62, 2317 共1989兲; Y. J. Uemura, L. P. Le, G. M.
Luke, B. J. Sternlieb, W. D. Wu, J. H. Brewer, T. M. Riseman,
C. L. Seaman, M. B. Maple, M. Ishikawa, D. G. Hinks, J. D.
Jorgensen, G. Saito, and H. Yamochi, ibid. 66, 2665 共1991兲; Y.
J. Uemura, Physica B 374-375, 1 共2006兲.
19 H. Luetkens, H.-H. Klauss, M. Kraken, F. J. Litterst, T. Dellmann, R. Klingeler, C. Hess, R. Khasanov, A. Amato, C. Baines,
M. Kosmala, O. J. Schumann, M. Braden, J. Hamann-Borrero,
N. Leps, A. Kondrat, G. Behr, J. Werner, and B. Büchner, Nature Mater. 8, 305 共2009兲.
20 A. J. Drew, Ch. Niedermayer, P. J. Baker, F. L. Pratt, S. J. Blundell, T. Lancaster, R. H. Liu, G. Wu, X. H. Chen, I. Watanabe, V.
K. Malik, A. Dubroka, M. Rössle, K. W. Kim, C. Baines, and C.
Bernhard, Nature Mater. 8, 310 共2009兲.
21 N. Ni, S. L. Bud’ko, A. Kreyssig, S. Nandi, G. E. Rustan, A. I.
Goldman, S. Gupta, J. D. Corbett, A. Kracher, and P. C. Canfield, Phys. Rev. B 78, 014507 共2008兲.
22
N. Ni, S. Nandi, A. Kreyssig, A. I. Goldman, E. D. Mun, S. L.
Bud’ko, and P. C. Canfield, Phys. Rev. B 78, 014523 共2008兲.
23
G. F. Chen, Z. Li, J. Dong, G. Li, W. Z. Hu, X. D. Zhang, X. H.
Song, P. Zheng, N. L. Wang, and J. L. Luo, Phys. Rev. B 78,
224512 共2008兲.
24 Y. J. Uemura, T. Goko, I. M. Gat-Malureanu, J. P. Carlo, P. L.
Russo, A. T. Savici, A. Aczel, G. J. MacDougall, J. A. Rodriguez, G. M. Luke, S. R. Dunsiger, A. McCollam, J. Arai, Ch.
Pfleiderer, P. Böni, K. Yoshimura, E. Baggio-Saitovitch, M. B.
Fontes, J. Larrea, Y. V. Sushko, and J. Sereni, Nat. Phys. 3, 29

tems suggests the existence of an underlying generic principle common to the condensation mechanisms of all these
exotic superconductors.18,46

024508-5

PHYSICAL REVIEW B 80, 024508 共2009兲

GOKO et al.

Canfield, and G. M. Luke, Phys. Rev. B 79, 020511共R兲 共2009兲.

共2007兲.
25 J. E. Sonier, J. H. Brewer, and R. F. Kiefl, Rev. Mod. Phys. 72,

769 共2000兲.
26 L. Benfatto, M. Capone, S. Caprara, C. Castellani, and C. Di

Castro, Phys. Rev. B 78, 140502共R兲 共2008兲.
C. Ren, Z.-S. Wang, H.-Q. Luo, H. Yang, L. Shan, and H.-H.
Wen, Phys. Rev. Lett. 101, 257006 共2008兲.
28 H. Ding, P. Richard, K. Nakayama, K. Sugawara, T. Arakane, Y.
Sekiba, A. Takayama, S. Souma, T. Sato, T. Takahashi, Z. Wang,
X. Dai, Z. Fang, G. F. Chen, J. L. Luo, and N. L. Wang, EPL 83,
47001 共2008兲.
29
I. I. Mazin, D. J. Singh, M. D. Johannes, and M. H. Du, Phys.
Rev. Lett. 101, 057003 共2008兲; K. Kuroki, S. Onari, R. Arita, H.
Usui, Y. Tanaka, H. Kontani, and H. Aoki, ibid. 101, 087004
共2008兲.
30
M. M. Parish, J. Hu, and B. A. Bernevig, Phys. Rev. B 78,
144514 共2008兲.
31
M. M. Altarawneh, K. Collar, C. H. Mielke, N. Ni, S. L. Bud’ko,
and P. C. Canfield, Phys. Rev. B 78, 220505共R兲 共2008兲; H. Q.
Yuan, J. Singleton, F. F. Balakirev, S. A. Baily, G. F. Chen, J. L.
Luo, and N. L. Wang, Nature 共London兲 457, 565 共2009兲.
32 P. L. Russo,, C. R. Wiebe, Y. J. Uemura, A. T. Savici, G. J.
MacDougall, J. Rodriguez, G. M. Luke, N. Kaneko, H. Eisaki,
M. Greven, O. P. Vajk, S. Ono, Yoichi Ando, K. Fujita, K. M.
Kojima, and S. Uchida, Phys. Rev. B 75, 054511 共2007兲.
33 A. Kreyssig, M. A. Green, Y. Lee, G. D. Samolyuk, P. Zajdel, J.
W. Lynn, S. L. Bud’ko, M. S. Torikachvili, N. Ni, S. Nandi, J. B.
Leão, S. J. Poulton, D. N. Argyriou, B. N. Harmon, R. J. McQueeney, P. C. Canfield, and A. I. Goldman, Phys. Rev. B 78,
184517 共2008兲.
34
A. I. Goldman, A. Kreyssig, K. Prokeš, D. K. Pratt, D. N. Argyriou, J. W. Lynn, S. Nandi, S. A. J. Kimber, Y. Chen, Y. B. Lee,
G. Samolyuk, J. B. Leão, S. J. Poulton, S. L. Bud’ko, N. Ni, P.
C. Canfield, B. N. Harmon, and R. J. McQueeney, Phys. Rev. B
79, 024513 共2009兲.
35 W. Yu, A. A. Aczel, T. J. Williams, S. L. Bud’ko, N. Ni, P. C.
27

36 D. V. Evtushinsky, D. S. Inosov, V. B. Zabolotnyy, A. Koitzsch,

M. Knupfer, B. Büchner, M. S. Viazovska, G. L. Sun, V.
Hinkov, A. V. Boris, C. T. Lin, B. Keimer, A. Varykhalov, A. A.
Kordyuk, and S. V. Borisenko, Phys. Rev. B 79, 054517 共2009兲.
37 K. M. Kojima, S. Uchida, Y. Fudamoto, I. M. Gat, M. I. Larkin,
Y. J. Uemura, and G. M. Luke, Physica B 326, 316 共2003兲.
38
A. T. Savici, Y. Fudamoto, I. M. Gat, T. Ito, M. I. Larkin, Y. J.
Uemura, G. M. Luke, K. M. Kojima, Y. S. Lee, M. A. Kastner,
R. J. Birgeneau, and K. Yamada, Phys. Rev. B 66, 014524
共2002兲.
39 T. Sasaki, N. Yoneyama, and N. Kobayashi, Phys. Rev. B 77,054505 共2008兲.
40
G. M. Luke, A. Keren, K. Kojima, L. P. Le, B. J. Sternlieb, W.
D. Wu, Y. J. Uemura, Y. Ōnuki, and T. Komatsubara, Phys. Rev.
Lett. 73, 1853 共1994兲.
41
J. Arvanitidis, K. Papagelis, Y. Takabayashi, T. Takenobu, Y.
Iwasa, M. J. Rosseinsky, and K. Prassides, J. Phys.: Condens.
Matter 19, 386235 共2007兲.
42 T. Park, F. Ronning, H. Q. Yuan, M. B. Salamon, R. Movshovich, J. L. Sarrao, and J. D. Thompson, Nature 共London兲 440, 65
共2006兲.
43 Y. J. Uemura, A. Aczel, Y. Ajiro, J. Carlo, T. Goko, D. Goldfeld,
A. Kitada, G. Luke, G. Macdougall, I. Mihailescu, J. Rodriguez,
P. Russo, Y. Tsujimoto, C. Wiebe, T. Williams, T. Yamamoto, K.
Yoshimura, and H. Kageyama, arXiv:0806.2021 共unpublished兲.
44 K. Hashimoto, T. Shibauchi, T. Kato, K. Ikada, R. Okazaki, H.
Shishido, M. Ishikado, H. Kito, A. Iyo, H. Eisaki, S. Shamoto,and Y. Matsuda, Phys. Rev. Lett. 102, 017002 共2009兲.
45
L. Shan, Y. Wang, X. Zhu, G. Mu, L. Fang, C. Ren, and H.-H.
Wen, EPL 83, 57004 共2008兲; H.-J. Grafe, D. Paar, G. Lang, N.
J. Curro, G. Behr, J. Werner, J. Hamann-Borrero, C. Hess, N.
Leps, R. Klingeler, and B. Büchner, Phys. Rev. Lett. 101,
047003 共2008兲.
46 Y. J. Uemura, arXiv:0811.1546 共unpublished兲.

024508-6