Lower Critical Fields of Superconducting PrFeAsO$_{1-y}$ Single Crystals

0.38 MB·English

by  R. Okazaki

Lower Critical Fields of Superconducting PrFeAsO Single Crystals
1−y
R. Okazaki,1 M. Konczykowski,2 C. J. van der Beek,2 T. Kato,1 K. Hashimoto,1 M. Shimozawa,1 H. Shishido,1
M. Yamashita,1 M. Ishikado,3 H. Kito,4,5 A. Iyo,4,5 H. Eisaki,4,5 S. Shamoto,3,5 T. Shibauchi,1 and Y. Matsuda1,2
1Department of Physics, Kyoto University, Kyoto 606-8502, Japan
2Laboratorie des Solides Irradi´es, CNRS-UMR 7642 & CEA/DSM/IRAMIS, Ecole Polytechnique, 91128, Palaiseau, France
3Quantum Beam Science Directorate, Japan Atomic Energy Agency, Tokai, Naka, Ibaraki 319-1195, Japan
4Nanoelectronics Research Institute (NeRI), National Institute of Advanced Industrial Science and Technology (AIST),
1-1-1 Central 2, Umezono, Tsukuba, Ibaraki 305-8568, Japan and
9 5JST, TRIP, Chiyoda, Tokyo 102-0075, Japan
0
0 We have studied the lower critical fields Hc1 of superconducting iron oxipnictide PrFeAsO1−y
2 singlecrystalsforHparallelandperpendiculartotheab-planes. Measurementsofthelocalmagnetic
induction at positions straddling the sample edge by using a miniature Hall-sensor array clearly
n
a resolve the first flux penetration from the Meissner state. The temperature dependence of Hc1
J for H c is well scaled by the in-plane penetration depth without showing any unusual behavior,
in conktrast to previous reports. The anisotropy of penetration lengths at low temperatures is
4
1 estimated to be 2.5, which is considerably smaller than the anisotropy of the coherence lengths.
≃
This is indicative of multiband superconductivity in this system, in which the active band for
] superconductivity is more anisotropic. We also point out that the local induction measured at
n a position near the center of the crystal, which has been used in a number of reports for the
o determination of Hc1, might seriously overestimate theobtained Hc1-value.
c
-
r PACSnumbers: 74.25.Bt,74.25.Dw,74.25.Op,74.70.-b
p
u
s I. INTRODUCTION ∼ 20 (Ref. 24), which may be partly due to the effects
.
t of strong pinning. In this study, we use an unambiguous
a
method to avoid this difficulty associated with pinning,
m Therecentdiscoveryofhightemperaturesuperconduc-
by determining H as the field H at which first flux
- tivity in Fe-basedcompounds has attracted considerable c1 p
d interest.1 In this new class of compounds with a very penetration occurs from the edge of the crystal. This al-
n low carrier density,2,3,4,5,6,7,8,9 superconductivity occurs lows us to extract the temperature dependent values of
o the lower critical fields parallel to the c-axis (Hc ) and
in proximity to a magnetic instability, and unconven- c1
c theab-plane(Hab),respectivelyaswellastheanisotropy
[ taitoionnasl phaaivreinbgemenecphraonpiossmeds mbyedseiavteerdalbgyromuapgsn.1e0t,1ic1,1fl2uOctnue- parameter Hcc1/cH1ca1b in single crystals of Fe-based super-
2 conductors.
of the remarkable features, which is in sharp contrast to
v
the high-T cuprates, appears to be the multiband na-
9 c
tureofsuperconductivity,inelectronandholepockets.13
6
6 Recently, a multiband effect on superconductivity has
3 beenreportedinseveralcompounds.14,15,16 Inparticular, We directly determine H by measuring the magnetic
p
1. the two-gap superconductivity in MgB2 manifests itself induction just inside and outside the edge of the single
1 in the unusual temperature- and magnetic field depen- crystals, by using a miniature Hall-sensor array. First,
8 dence of the anisotropyparametersin the superconduct- weshowthatlocalmagnetizationmeasurementsatapo-
0 ing state.17,18,19 However, the crucial difference is that sition near the center of the crystal, which have been
v: theinterbandcouplingisveryweakinMgB2,whileinFe- used by several groups for the determination of Hc1,
i basedcompoundsnestingbetweenthehole-andelectron seriously overestimate H in systems with strong pin-
X c1
bands was suggested to be important for the occurrence ning. Second, we find that the temperature dependence
ar of high temperature superconductivity.10,11,12,20,21,22,23 ofHc1 determinedattheedgedoesnotshowanyunusual
In this context, a detailed clarification of the multiband behavior28 and is well scaled by the penetration depth
natureofsuperconductivityintheFe-basedoxypnictides results measured on the crystal in the same batch.29
is indispensable for the elucidation of the superconduct- Finally, we find that the anisotropy of the penetration
ing properties,andespeciallyforthe pairingmechanism. depths γ ≡ λ /λ ≃ Hc /Hab, where λ and λ
λ c ab c1 c1 c ab
An accurate determination of the lower critical field areout-of-planeandin-planepenetrationdepths,respec-
H is an importantmeans to clarify not only the super- tively, is much smaller than the anisotropy of the coher-
c1
conductinggapsymmetry,butalsothemultibandnature ence lengths γ ≡ ξ /ξ = Hab/Hc , where ξ and ξ
ξ ab c c2 c2 ab c
ofsuperconductivity. However,thereliablemeasurement are in- and out-of- plane coherence lengths, respectively,
of the lower critical field is a difficult task, in particular andHab andHc aretheuppercriticalfieldsparalleland
c2 c2
whenstrongvortexpinningispresent. Wealsopointout perpendicular to the ab-plane, respectively. This result
thatto date the reportedvalues ofanisotropyparameter providesstrongevidence for the multiband nature of the
strongly vary24,25,26,27 spanning from 1.2 (Ref. 27) up to superconductivity.
2
20
10 32 K
30 K
T) 28 K
m 26 K
M ( edge 0 2242 KK
20 K
µ0 18 K
-10 16 K
14 K
PrFeAsO #1
1-y
H || c
-20
-30 -20 -10 0 10 20 30
µH (mT)
0 a
FIG.1: (coloronline). (a)Differentialmagneto-opticsimages
of PrFeAsO1−y #1 with field modulation δB = 0.2 mT in
FIG. 2: (color online). Local magnetization loops for H c,
zero field. The Meissner screening occurs completely within k
measured by the miniature Hall sensor located at 10 µm
narrow temperature range. (b) MO images at T = 7.1 K. ≤
from the edge of thecrystal.
Magneticfluxpenetratesfromtheedgeofthecrystalandthe
field distribution shows theBean critical state.
of neighboring sensors is 20 µm. The local induction at
II. EXPERIMENTAL theedgeofthecrystalwasdetectedbytheminiatureHall
sensor located at ≤10 µm from the edge. The magnetic
field H is applied for Hk c and Hk ab-plane by using
a
Experiments have been performed on high-quality
a low-inductance 2.4 T superconducting magnet with a
PrFeAsO1−y single crystals, grown by a high-pressure negligibly small remanent field.
synthesis method using a belt-type anvil apparatus
The in-plane resistivity is measured by the standard
(Riken CAP-07). Powders of PrAs, Fe, Fe O were used
2 3 four-probemethodundermagneticfieldsupto10T.The
as the starting materials. PrAs was obtained by react-
electricalcontacts were attached by using the W deposi-
◦
ing Pr chips and As pieces at 500 C for 10 hours, fol-
tion technique in a Focused-Ion-Beam system.
◦
lowed by a treatment at 850 C for 5 hours in an evac-
uated quartz tube. The starting materials were mixed
at nominal compositions of PrFeAsO and ground in
0.6 III. RESULTS AND DISCUSSION
an agate mortar in a glove box filled with dry nitro-
gen gas. The mixed powders were pressed into pel-
lets. The samples were then grown by heating the pel- In Fig. 2 we show the field dependence of the “local
lets in BN crucibles under a pressure of about 2 GPa magnetization”, Medge ≡ µ−01Bedge − Ha, at the edge
at 1300◦C for 2 hours. Platelet-like single crystals of of the crystal, for Hk c, measured after zero field cool-
dimensions up 150×150×30 µm3 were mechanicallyse- ing. After the initial negativeslope correspondingto the
lected from the polycrystalline pellets. The single crys- Meissner state, vorticesenter the sample and Medge(Ha)
talline nature of the samples was checkedby Laue X-ray showsalargehysteresis. Theshapeofthemagnetization
diffraction.30 Our crystals, whose T (≈ 34 K) is lower loops (almost symmetric about the horizontal axis) in-
c
than the optimum Tc ≈ 51 K of PrFeAsO1−y,31 are in dicates that the hysteresis mainly arises from bulk flux
the underdoped regime (y ∼0.1),32 which is close to the pinning rather than from the (Bean-Livingston) surface
spin-density-wave order.33 The sample homogeneity was barrier.36
checked by magneto-optical (MO) imaging. MO images As shown in Fig. 2, the initial slope of the mag-
of PrFeAsO1−y sample #1 (∼ 135×63×18 µm3) are netization exhibits a nearly perfect linear dependence,
shown in Fig. 1(a). The crystal exhibits a nearly perfect M = −αH . Since the Hall sensor is placed on the
edge a
Meissner state ∼ 2 K below T ; no weak links are ob- top surface, with a small but non-vanishing distance be-
c
served, indicating a good homogeneity. At low tempera- tweenthesensorandthecrystal,themagneticfieldleaks
tures,themagneticfielddistributioniswelldescribedby around the sample edge with the result that the slope
the Bean critical state model as shown in Fig. 1(b).34 α is slightly smaller than unity. Figure 3 shows typical
The local induction near the surface of the platelet curves of B1/2 ≡µ1/2(M+αH )1/2 at the edge (circles)
0 a
crystal has been measured by placing the sample on and at the center (squares) of the crystal, plotted as a
top of a miniature Hall-sensor array tailored in a function of H ; the external field orientation Hk c and
a
GaAs/AlGaAs heterostructure.35 Each Hall sensor has T =22K.TheαH -termisobtainedbyaleastsquaresfit
a
anactiveareaof3×3µm2;the center-to-centerdistance ofthelow-fieldmagnetization. Thefirstpenetrationfield
3
15x10-3 3 10 50
1/21/2B (T) 1500 PTHH r =F|p| e 2c(A2e ecds KdeOgnge1te)-eyr #1 (ac)enter edcgreyosuttaslide 210 edgeΔ(J) (10 A/m)1/231/2 µH (mT)p0 111864420 ceedngteer µ∆H (mT)p0 864200 10 T 2(0K) µµ3000∆MHrepm 404321000000rem (mT)Mµ
(b)
2 PrFeAsO #1
Hp (center) μ0Ha H || c 1-y
-5 -1 0
0 2 4 6 8 10 0 10 20 30 40
μH (mT) T (K)
0 a
FIG.3: (coloronline). Typicalcurvesof√B(leftaxis)atthe FIG. 4: (color online). The temperature dependence of the
edge (circles) and at the center (squares) of the crystal and flux penetration fields Hp at the edge and the center of the
p∆jedge (right axis) plotted as a function of Ha for H cat crystal. The inset shows the temperature dependence of the
k
T =22K,inwhichHa isincreasedafterZFC.Theinsetsare differencebetweenHpinthecenterandattheedge(leftaxis),
schematicillustrationsoftheexperimentalsetupforHkc(a) as well as theremanent magnetization Mrem (right axis).
and H ab-plane(b).
k
creases steeply with decreasing temperature. Also plot-
H corresponds to the field H (edge), above which B1/2 ted in the inset of Fig. 4 is the remanent magnetization
inpcreases almost linearly, is clpearly resolved. In Fig. 3, Mrem (i.e. the Ha = 0 value of Medge on the decreas-
we show the equivalent curve, measured at the center of ing field branch), measured at near the crystal center.
the crystal. At the center, B1/2 also increases linearly, Thisisproportionaltothecriticalcurrentdensityjc aris-
starting from a larger field, H (center). ing from flux pinning. The temperature dependence of
p
∆H is very similar to that of j , which indicates that
We have measured the positional dependence of H p c
p H (center) is strongly influenced by pinning. Hence, the
and observed that it increases with increasing distance p
present results demonstrate that the lower critical field
from the edge. To examine whether H (edge), i.e. H
p p value determined by local magnetization measurements
measuredat≤10µmfromtheedge,trulycorrespondsto
carriedoutatpositionsclosetothecrystalcenter,suchas
the field of first flux penetration at the boundary of the
reported by several groups,is affected by vortex pinning
crystal, we have determined the local screening current
density j = µ−1(B −B )/∆x at the crystal effects and might be seriously overestimated.28,37
edge 0 edge outside The absolute value of H is evaluated by taking into
boundary. Here B is the local magnetic induction c1
edge account the demagnetizing effect. For a platelet sample,
measuredbythe sensorjustinside the edge,andB
outside H is given by
is the induction measuredby the neighboringsensorjust c1
outside the edge. For fields less than the first penetra-
H =H /tanh 0.36b/a (1)
tion field, j ≃ βH is the Meissner current, which is c1 p
edge a p
simply proportional to the applied field (β is a constant
where a and b are the width and the thickness of the
determined by geometry). At Hp, the screening current crystal, respectively.38 In the situation where Hkc, a=
starts to deviate from linearity. Figure 3 shows the de-
63 µm and b = 18 µm, while a= 18 µm and b= 63 µm
viation ∆jedge ≡ jedge −βHa as a function of Ha. As for Hk ab-plane. These values yield Hc = 3.22H and
c1 p
depictedinFig.3, ∆jedge againincreaseslinearlywith Hca1b = 1.24Hp, respectively. In Fig. 5, we plot Hc1 as
Ha above Hp(edge)p. This indicates that the Hp(edge) is a function of temperature both for H k c and H k ab-
very close to the true field of first flux penetration. plane. The solid line in Fig. 5 indicates the temperature
In Fig. 4, we compare the temperature dependence dependence of the superfluid density normalized by the
of H (edge) and H (center). In the whole tempera- value at T = 0 K, which is obtained from ab-plane pen-
p p
turerange,H (center)wellexceedsH (edge). Moreover, etration depth measurements of a sample from the same
p p
H (center) increases with decreasing T without any ten- batch.29 Hc (T) is well scaled by the superfluid density,
p c1
dency towards saturation. In sharp contrast, H (edge) which is consistent with fully gapped superconductivity;
p
saturatesat low temperatures. The insetof Fig.4 shows itdoesnotshowtheunusualbehaviorreportedinRef.28.
the difference between H measured in the center and To roughly estimate the in-plane penetration depth at
p
at the edge, ∆H = H (center) − H (edge). ∆H in- low temperatures, we use the approximate single-band
p p p p
4
12 1.2
µH =
10 PrFeAsO1-y #1 1.0 1.0 PrFeAsO1-y sample #3 0 7 T
6 T
5 T
µH (mT)c01 86 H || c 00..86 abab(0)/(Tλλ22 ρρT ()/ (40 K) 000...864 432110 ..TTTT58 TT
4 0.4 ) 0.6 T
0.4 T
0.2 0.2 T
2 H || ab 0.2 (a) H || c 00 .T1 T
0.0
0 0.0 12
0.0 0.2 0.4 T / T0.6 0.8 1.0 1.0 10 HH |||| cab 90 % µ0H = 10 T
c T)8 50 % 9 T
FIG. 5: (color online). Lower critical fields as a function 40 K) 0.8 µH (c0264 876 TTT
oTfhetemsopliedraltinuere(rinighPtrFaexAiss)Op1r−eysensitnsgltehecrsyusptaelrsflu(ildeftdeanxsiist)y. ρT)/ ( 0.6 2 10 % 54 TT
λ2ab(0)/λ2ab(T) determined by surface impedance measure- ρ ( 0.4 028 30 32 34 36 38 32 TT
mentson crystals from thesame batch.29 T (K) 1 T
0.2 0.5 T
(b) H || ab 0 T
0.0
London formula, 25 30 35 40
T (K)
Φ λ
µ Hc = 0 ln ab +0.5 (2)
0 c1 4πλ2ab (cid:20) ξab (cid:21) FIG. 6: (color online). Temperature dependence of the in-
plane resistivity in PrFeAsO1−y single crystals for H c (a)
whereΦ0 isthefluxquantum. Usinglnλab/ξab+0.5∼5, and H ab-plane (b). Inset shows the temperature kdepen-
k
we obtain λab ∼ 280 nm. This value is in close corre- dence of the upper critical fields Hc2 determined by several
spondence with the µSR results in slightly underdoped criteria that the resistivity reaches 10%, 50%, and 90% of
LaFeAs(O,F).39 the normal-state resistivity. The experimental configuration
Figures 6(a) and (b) depict the temperature depen- is also sketched.
dence of the in-plane resistivity for Hk c and Hk ab-
plane, respectively. In the inset of Fig. 6(b), we dis-
play the fields at which the resistivity is equal to 10%,
whereγ is determined bythe lociof10%,50%and90%
ξ
50%, and 90% of the normal-state resistivity. For suf-
ofthenormal-stateresistivity(seetheinsetofFig.6(b)).
ficiently high magnetic field, these resistance loci are
SinceH increasesrapidlyandwellexceeds10Tjustbe-
c2
roughly proportional to the upper critical field. In zero low T for Hk ab, plotting γ is restricted to a narrow
c ξ
field,theresistivetransitionexhibitsarathersharptran-
temperature interval. In Fig. 7, we also plot the H -
c2
sitionwiththe transitionwidth ∆T ≈ 2 K.By applying
c anisotropy data measured on NdFeAsO0.82F0.18 by the
a magnetic field along the c-axis, the transition shifts
authors of Ref. 42. These indicate that the temperature
to slightly lower temperatures and becomes broadened.
dependence of γ is markedly different from that of γ .
λ ξ
The resistive transition curves broaden less for Hk ab-
According to the anisotropic Ginzburg-Landau (GL)
plane. These results indicate that the anisotropy of the
equationinsingle-bandsuperconductors,γ shouldcoin-
λ
upper critical fields in the present system is rather large
cide with γ over the whole temperature range. There-
ξ
and that fluctuation effects play an important role for
fore, the large difference between these anisotropies pro-
the transition in magnetic fields,40 similar to high-T
c videsstrongevidenceformultibandsuperconductivityin
cuprates.41
the present system. We discuss the anisotropy parame-
Finally,Fig.7showstheanisotropyofthelowercritical
tersfor the multibandsuperconductivity below. Accord-
fields, γ obtainedfrom the results in Fig. 5. Here, since
λ ing to GL theory, γ and γ at T are given as
λ ξ c
the penetration lengths are much larger than the coher-
ence lengths for both Hk ab and Hk c, the logarithmic hΩ2v2i
γ2(T )=γ2(T )= a , (3)
terminEq.(2)doesnotstronglydependonthe direction ξ c λ c hΩ2v2i
of magnetic field. We thus assumed Hc /Hab ≃ λ /λ . c
c1 c1 c ab
The anisotropy γ ≈ 2.5 at very low temperature, and whereh···idenotestheaverageovertheFermisurface,v
λ a
increases gradually with temperature. In Fig. 7, the and v are the Fermi velocities parallel and perpendicu-
c
anisotropy of the upper critical fields γ is also plotted, lartotheab-plane,respectively.43,44 Ωrepresentsthegap
ξ
5
10 isotropicscattering,13 whichcorrespondstoγ ∼4. This
λ
PrFeAsO value is close to the observed value. The fact that γξ
1-y
8 #1 Hc1 well exceeds γλ indicates that the active band for super-
#2 H conductivity is more anisotropic than the passive band.
c1
#3 Hc2 (10 %) According to band structure calculations, there are five
6 Hc2 (50 %) relevant bands in LaFeAsO1−xFx. Among them, one of
, γλξ Hc2 (90 %) the three hole bands near the Γ point and the electron
γ bands near the M point are two-dimensional and cylin-
4
drical. The other two hole bands near the Γ point have
moredispersionalongthecaxis,13 althoughtheshapeof
theseFermisurfacesissensitiveto the positionofthe As
2
atomwithrespecttotheFeplane,whichinturndepends
on the rareearth.50 Our results implying that the active
0 band is more anisotropic is in good correspondence with
0.0 0.2 0.4 0.6 0.8 1.0 1.2
the view that the nesting between the cylindrical hole
T / T
c and electron Fermi surfaces is essential for superconduc-
tivity. This is expected to make these two-dimensional
FIG. 7: (color online). Normalized temperature depen- bands the active ones, with a large gap, and the other
dence of the anisotropies of Hc1 (γλ, closed circles) and Hc2 more three-dimensional bands passive ones with smaller
(γξ, closed squares) in PrFeAsO1−y single crystals. The gaps.
anisotropy of Hc2 in NdFeAsO0.82F0.18 (γξ, open squares)
measured by Y. Jia et al.42 is also plotted. The dashed line
is a guide to theeye.
IV. SUMMARY
In summary, we have measured the lower critical field
anisotropy(hΩ2i=1),whichisrelatedtothepairpoten-
tialV(v,v′)=V Ω(v)Ω(v′). AtT =0K,theanisotropy Hc1 in PrFeAsO1−y single crystals for Hkc and Hkab-
0 planebyutilizinganarrayofminiatureHallsensor. Con-
of the penetration depths is
ventionalmethodsusingasinglemicro-Hallprobeplaced
on the center of the crystal might overestimate H due
hv2i c1
γλ2(0)= hva2i. (4) to strong flux pinning. Hc1 measured by the sensor lo-
c cated very near to the edge of the crystal shows satu-
rating behavior at low temperatures, which is consistent
The gap anisotropy does not enter γ (0), while γ at
λ ξ with the previousreportsonthe penetrationdepth mea-
T = 0 K is mainly determined by the gap anisotropy of
surements. TheanisotropyofH slightlydecreaseswith
the active band responsible for superconductivity. Thus c1
decreasingtemperatureandisindicativeofmultibandsu-
the gradualreductionofγ with decreasingtemperature
can be accounted for by cλonsidering that the contribu- perconductivityinPrFeAsO1−y,inwhichtheactiveband
for superconductivity is more anisotropic.
tion of the gap anisotropy diminished at low tempera-
tures. This alsoimpliesthatthesuperfluiddensityalong
the c-axis λ2(0)/λ2(T) has steeper temperature depen-
c c ACKNOWLEDGEMENTS
dence than that in the plane λ2 (0)/λ2 (T). A pro-
ab ab
nounced discrepancy between γ and γ provides strong
ξ λ
evidenceforthemultibandnatureofsuperconductivityin We thank A. E. Koshelev for useful discussion and
PrFeAsO1−y,withdifferentgapvaluesindifferentbands. T. Terashima for technical assistance. This work was
We note that similar differences between γ (T) and supported by KAKENHI (No. 20224008) from JSPS,
ξ
γ (T), as well as λ2(0)/λ2(T) and λ2 (0)/λ2 (T), have by Grant-in-Aid for the Global COE program “The
λ c c ab ab
beenreportedinthetwo-gapsuperconductorMgB .18,19 Next Generationof Physics,Spun from Universalityand
2
We also note that angle-resolved photoemission spec- Emergence”andbyGrant-in-AidforSpeciallyPromoted
troscopy (ARPES),45 Andreev reflection,46 and pene- Research(No. 17001001)from MEXT, Japan. R.O. and
tration depth47 measurements on (K1−xBax)Fe2As2 and H.S. were supported by the JSPS Research Fellowship
NMR48 andpenetrationdepth49 studiesofLnFeAs(O,F) for Young Scientists.
(Ln = Pr, Sm) have suggested multiband superconduc-
tivity with two gap values in Fe-based oxypnictides. Note added: Recently, H measurements on Nd-
c1
Band structure calculations for LaFeAsO1−xFx yield FeAs(O,F) by using Hall probes are reported,51 which
an anisotropy of the resistivity of approximately 15 for show similar temperature dependence of H .
c1
6
1 Y.Kamihara,T.Watanabe,M.Hirano,andH.Hosono,J. arXiv:0809.4223 (unpublished).
Am. Chem. Soc. 130, 3296 (2008). 27 D. Kubota, T. Ishida, M. Ishikado, S. Shamoto, H. Kito,
2 H.Takahashi,K.Igawa,K.Arii,Y.Kamihara,M.Hirano, A. Iyo,and H. Eisaki, arXiv:0810.5623 (unpublished).
and H.Hosono, Nature(London) 453, 376 (2008). 28 C. Ren, Z.-S. Wang, H. Yang, X. Zhu, L. Fang, G. Mu,
3 G. F. Chen, Z. Li, D. Wu, G. Li, W. Z. Hu, J. Dong, P. L. Shan,and H.-H.Wen,arXiv:0804.1726 (unpublished).
Zheng, J. L. Luo, and N. L. Wang, Phys. Rev. Lett. 100, 29 K. Hashimoto, T. Shibauchi, T. Kato, K. Ikada,
247002 (2008). R. Okazaki, H. Shishido, M. Ishikado, H. Kito, A. Iyo,
4 Z.-A.Ren,J.Yang,W.Lu,W.Yi,G.-C.Che,X.-L.Dong, H. Eisaki, S. Shamoto, and Y. Matsuda, Phys. Rev. Lett.
L.-L. Sung, and Z.-X. Zhao, Mater. Res. Innov. 12, 106 102, 017002 (2009).
(2008). 30 K. Hashimoto, T. Shibauchi, T. Kato, K. Ikada,
5 H. Kito, H. Eisaki, and A. Iyo, J. Phys. Soc. Jpn. 77, R. Okazaki, H. Shishido, M. Ishikado, H. Kito, A. Iyo,
063707 (2008). H.Eisaki,S.Shamoto,andY.Matsuda,J.Phys.Soc.Jpn.
6 Z.-A. Ren, J. Yang, W. Lu, W. Yi, X.-L. Shen, Z.-C. Li, 77, Suppl.C, 145 (2008).
G.-C. Che, X.-L. Dong, L.-L. Sung, F. Zhou, and Z.-X. 31 Z.-A.Ren,G.-C.Che,X.-L.Dong,J.Yang,W.Lu,W.Yi,
Zhao, Europhys.Lett. 82, 57002 (2008). X.-L. Shen, Z.-C. Li, L.-L. Sun, F. Zhou, Z.-X. Zhao, Eu-
7 X.H.Chen,T.Wu,G.Wu,R.H.Liu,H.Chen,andD.F. rophys.Lett. 83, 17002 (2008).
Fang, Nature(London) 453, 761 (2008). 32 C.-H. Lee, A. Iyo, H. Eisaki, H. Kito, M. T. Fernandez-
8 J.Yang,Z.-C.Li,W.Lu,W.Yi,X.-L.Shen,Z.-A.Ren,G.- Diaz, T.Ito,K.Kihou,H.Matsuhata, M.Braden, andK.
C. Che, X.-L. Dong, L.-L. Sun, F. Zhou, and Z.-X. Zhao, Yamada, J. Phys. Soc. Jpn. 77, 083704 (2008).
Supercond.Sci. Technol. 21, 082001 (2008). 33 J. Zhao, Q. Huang, C. de la Cruz, S. Li, J. W. Lynn,
9 C.Wang,L.Li,S.Chi,Z.Zhu,Z.Ren,Y.Li,Y.Wang,X. Y.Chen,M.A.Green,G.F.Chen,G.Li,Z.Li,J.L.Luo,
Lin,Y.Luo,S.Jiang,X.Xu,G.Cao,andZ.Xu,Europhys. N. L.Wang, and P.Dai, NatureMater. 7, 953 (2008).
Lett. 83, 67006 (2008). 34 E.Zeldov,J.R.Clem,M.McElfresh,andM.Darwin,Phys.
10 I. I. Mazin, D. J. Singh, M. D. Johannes, and M. H. Du, Rev.B 49, 9802 (1994).
Phys. Rev.Lett. 101, 057003 (2008) 35 T. Shibauchi, M. Konczykowski, C. J. van der Beek,
11 K.Kuroki,S.Onari,R.Arita,H.Usui,Y.Tanaka,H.Kon- R.Okazaki,Y.Matsuda,J.Yamaura,Y.Nagao,andZ.Hi-
tani, and H.Aoki, Phys. Rev.Lett. 101, 087004 (2008). roi, Phys.Rev.Lett. 99, 257001 (2007).
12 H. Aoki, arXiv:0811.1656 (unpublished). 36 M. Konczykowski, L. I. Burlachkov, Y. Yeshurun, and
13 D. J. Singh and M.-H. Du, Phys. Rev. Lett. 100, 237003 F. Holtzberg, Phys.Rev.B 43, 13707 (1991).
(2008). 37 C. Ren, Z.-s. Wang, H.-q. Luo, H. Yang, L. Shan, and
14 G. Seyfarth, J. P. Brison, M.-A. M´easson, J. Flouquet, H.-H.Wen,Phys. Rev.Lett. 101, 257006 (2008).
K. Izawa, Y. Matsuda, H. Sugawara, and H. Sato, Phys. 38 E. H. Brandt, Phys.Rev.B 60, 11939 (1999).
Rev.Lett. 95, 107004 (2005). 39 H. Luetkens, H.-H. Klauss, M. Kraken, F. J. Litterst, T.
15 Y. Kasahara, T. Iwasawa, H. Shishido, T. Shibauchi, Dellmann,R.Klingeler,C.Hess,R.Khasanov,A.Amato,
K.Behnia,Y.Haga,T.D.Matsuda,Y.Onuki,M.Sigrist, C. Baines, J. Hamann-Borrero, N. Leps, A. Kondrat, G.
and Y.Matsuda, Phys.Rev.Lett. 99, 116402 (2007). Behr, J. Werner, and B. Buechner, arXiv:0806.3533 (un-
16 Y. Nakajima, T. Nakagawa, T. Tamegai, and H. Harima, published).
Phys. Rev.Lett. 100, 157001 (2008). 40 R. Okazaki, Y. Kasahara, H. Shishido, M. Konczykowski,
17 F.Bouquet,Y.Wang,I.Sheikin,T.Plackowski,A.Junod, K. Behnia, Y. Haga, T. D. Matsuda, Y. Onuki,
S.Lee,andS.Tajima, Phys.Rev.Lett.89,257001 (2002) T. Shibauchi, and Y. Matsuda, Phys. Rev. Lett. 100,
18 L. Lyard, P. Szabo, T. Klein, J. Marcus, C. Marcenat, 037004 (2008).
K. H. Kim, B. W. Kang, H. S. Lee, and S. I. Lee, Phys. 41 W. K. Kwok, S. Fleshler, U. Welp, V. M. Vinokur,
Rev.Lett. 92, 057001 (2004). J. Downey, and G. W. Crabtree, and M. M. Miller, Phys.
19 J.D.Fletcher,A.Carrington,O.J.Taylor,S.M.Kazakov, Rev.Lett. 69, 3370 (1992).
and J. Karpinski, Phys.Rev. Lett. 95, 097005 (2005). 42 Y. Jia, P. Cheng, L. Fang, H. Yang, C. Ren, L. Shan,
20 V. Cvetkovic and Z. Tesanovic, arXiv:0804.4678 (unpub- C.-Z. Gu, and H.-H. Wen, Supercond. Sci. Technol. 21,
lished). 105018 (2008).
21 K. Seo, B. A. Bernevig, and J. Hu, Phys. Rev. Lett. 101, 43 V. G. Kogan, Phys. Rev.B 66, 020509(R) (2002).
206404 (2008). 44 P.Miranovic,K.Machida,andV.G.Kogan,J.Phys.Soc.
22 H. Ikeda,J. Phys. Soc. Jpn. 77, 123707 (2008). Jpn. 72, 221 (2003).
23 T. Nomura, arXiv:0811.2462 (unpublished). 45 H. Ding, P. Richard, K. Nakayama, T. Sugawara,
24 S. Weyeneth, U. Mosele, N. D. Zhigadlo, S. Katrych, T. Arakane, Y. Sekiba, A. Takayama, S. Souma, T. Sato,
Z. Bukowski, J. Karpinski, S. Kohout, J. Roos, and T. Takahashi, Z. Wang, X. Dai1, Z. Fang, G. F. Chen,
H. Keller, arXiv:0806.1024 (unpublished). J. L. Luo, and N. L. Wang. Europhys. Lett. 83, 47001
25 C.Martin,R.T.Gordon,M.A.Tanatar,M.D.Vannette, (2008).
M. E. Tillman, E. D. Mun, P. C. Canfield, V. G. Ko- 46 P. Szabo, Z. Pribulova, G. Pristas, S. L. Bud’ko, P. C.
gan, G. D. Samolyuk, J. Schmalian, and R. Prozorov, Canfield, and P.Samuely, arXiv:0809.1566 (unpublished).
arXiv:0807.0876 (unpublished). 47 K. Hashimoto, T. Shibauchi, S. Kasahara, K. Ikada, T.
26 L.Balicas,A.Gurevich,Y.J.Jo,J.Jaroszynski,D.C.Lar- Kato, R. Okazaki, C. J. van der Beek, M. Konczykowski,
balestier, R. H. Liu, H. Chen, X. H. Chen, N. D. Zhi- H. Takeya, K. Hirata, T. Terashima, and Y. Matsuda,
gadlo, S. Katrych, Z. Bukowski, and J. Karpinski, arXiv:0810.3506 (unpublished).
7
48 K.Matano, Z.A.Ren,X.L.Dong,L.L.Sun,Z.X.Zhao, Georges, Phys.Rev.B 78, 064518 (2008).
and G.-Q.Zheng, Europhys.Lett. 83, 57001 (2008). 51 Z. Pribulova, T. Klein, J. Kacmarcik, C. Marcenat, M.
49 L. Malone, J.D. Fletcher, A. Serafin, A. Carrington, N.D. Konczykowski, S. L. Budko, M. Tillman, and P. C. Can-
Zhigadlo, Z. Bukowski, S. Katrych, and J. Karpinski, field, arXiv:0812.3953 (unpublished).
arXiv:0806.3908 (unpublished).
50 V.Vildosola,L.Pourovskii,R.Arita,S.Biermann,andA.