Table Of ContentNodal superconductivity in isovalently substituted SrFe (As P ) pnictide
2 1−x x 2
superconductor at the optimal doping, x =0.35
J. Murphy,1 C. P. Strehlow,1 K. Cho,1 M. A. Tanatar,1 N. Salovich,2
R. W. Giannetta,2 T. Kobayashi,3 S. Miyasaka,3,4 S. Tajima,3,4 and R. Prozorov1,∗
1The Ames Laboratory and Department of Physics and Astronomy, Iowa State University, Ames, IA 50011, USA
2Loomis Laboratory of Physics, University of Illinois at Urbana-Champaign, 1110 West Green St., Urbana, IL 61801
3Department of Physics, Graduate School of Science, Osaka University, Osaka 560-0043, Japan
4JST, Transformative Research-Project on Iron Pnictides (TRIP), Tokyo 102-0075, Japan
3
(Dated: January 24, 2013)
1
0 Temperature-dependent London penetration depth, λ(T), was measured in optimally - doped,
2 x =0.35, as-grown (T ≈25 K, RRR=ρ(300K)/ρ(T )=4.5) and annealed (T ≈35 K, RRR=6.4)
c c c
single crystals of SrFe (As P ) iron - based superconductor. Annealing increases the RRR and
n 2 1−x x 2
a decreasestheabsolutevalueoftheLondonpenetrationdepthfromλ(0)=300±10nminas-grown
J sample to λ(0) = 275±10 nm. At low temperatures, λ(T) ∼ T indicating superconducting gap
with line nodes. Analysis of the full - temperature range superfluid density is consistent with the
3
line nodes, but differs from the simple single - gap d−wave. The observed behavior is very similar
2
to that of BaFe2(As1−xPx)2, showing that isovalently substituted pnictides are inherently different
] from the charge - doped materials.
n
o PACSnumbers: 74.70.Xa,74.20.Rp,74.62.Dh
c
-
r
p Superconductivity in the AEFe As (AE = Ba, Sr gredients, such as the interplay of magnetism and super-
2 2
u or Ca) - based compounds can be induced by substitu- conductivity[24–26]andtheinfluenceofdisorder[27–30].
s
tion of each constituent element in the formula [1–5]. In
. Still, it seems that BaP122 is unique. Why does the
at BaFe2As2 (Ba122),hole-(KsubstitutionofBa,BaK122) superconducting gap of this material remain nodal irre-
m andelectron-(e.g.,CosubstitutionofFe,BaCo122)dop-
spective of doping level? Is its uniqueness linked to the
ing produces superconductors with the maximum super-
- high purity as suggested by the observation of quantum
d conducting transition temperature, T of 38 K and 24
c oscillations [18, 31]? This raises the question of whether
n
K, respectively. The isovalent substitution of As with P
o other isovalently substituted 122 materials also have a
(BaP122)inducessuperconductivitywithmaximumT of
c c nodalgapatoptimaldopingandifso,howdotheirprop-
[ 31 K [6]. erties compare to BaP122?
1 Despite proximity to an identical ground state of the SuperconductivityinSrFe (As P ) withT =27K
2 1−x x 2 c
v parent compound and similar values of Tc , supercon- attheoptimaldopingofx=0.35wasdiscoveredin2010
2
ductors produced by isovalent doping have a gap struc- [32]. Soon thereafter, single crystals with T = 30 K
2 c
6 tureentirelydifferentfromthatproducedbyholeorelec- were obtained [33]. Currently, optimized annealing pro-
5 trondoping. Thesuperconductinggapincharge-doped tocol results in high quality crystals with T = 35 K
c
. BaK122 and BaCo122 is full and isotropic at the opti- [34]. ThefirststudiesofSrP122havealreadyshownitto
1
0 mal doping [7, 8]. It becomes anisotropic upon depar- be an unconventional superconductor. Specific heat and
3 ture from the optimal doping toward either end of the NMRstudiesareconsistentwiththenodalsmallgapand
1 “superconducting dome” and even develops nodes at the nodeless larger gaps [35]. Microwave measurements find
v: extreme doping levels [9–15]. In sharp contrast, the su- fractional power law behavior of the London penetration
i perconducting gap of isovalently substituted BaP122 re- depth. These studies, together with an analysis of flux
X
veals line nodes irrespective of the doping level [16–18]. flow resistivity, suggested a nodal gap in one band and
r
a To-date, the dominant theory of iron - based super- stronganisotropyintheother[36]andthisbehaviorwas
contrasted to that of BaP122.
conductorsisbasedontheextendeds model[19],which
±
originallypredictedequal fullisotropicgaps. Therewere In this Letter we present high - resolution measure-
many refinements of this idea to describe the numerous ments of London penetration depth in as-grown and an-
experimentsthatshowedanisotropicandevennodalgaps nealed single crystals of SrFe (As P ) superconduc-
2 1−x x 2
[14,20–23]. Itwasfoundthatnodalandnodelessground tor. The low temperature behavior suggests a super-
states are energetically very close to each other and the conducting gap with vertical line nodes, similar to the
fine tuning of the inter- and intra- band interactions and cuprates. However, the full temperature dependence of
band-structureparametersmayshiftthebalancetowards the superfluid density shows that the gap structure is
one of them. In addition, there are other important in- morecomplicatedthanasimplesinglebandd−waveand
2
2 5 S rF e (A s P ) 3 0
2 0.65 0.35 2
K)1.0 SrP122 BaRsaR PgR1r o2=w2 4n.8 u n a n n e a le d ua nn an ne na ele ad le, td*, =t* 0=.1 00.2 8
2 0 (3000.5 aRsR gRr o=w 4n.5 aa nn nn ee aa llee dd a n n e a le d , t* = 0 .0 7
)/R SrP122 annealed
(T BaP122 annealed 2 0
1 5 R0.0 R R R = 5.2
0 100 200 300 400 )
mm) 0.4 BaP122 as groTw n(,K R)R R = 4.8 (nm shifted up by 12 nm
(
Dl 1 50 R(T)/R(300 K)0000....01230SRrRPR1 2=22 04 a.5s gro4w0SRnrRBRPRa1RP 2R=621 06 2=a.2 4n5 na.2enan8lee0adled Dl 1 0 shifted up by 6 nm D (cid:1) = A t * t +2 t
T (K ) 0
0
0 1 0 2 0 3 0
0 .0 0 .1 0 .2
T ( K )
T / T
c
FIG. 1. Main pane: In-plane London penetration depth in
FIG. 2. Low - temperature part of ∆λ(T) for the three sam-
singlecrystalsofSrFe (As P ) , x=0.35, inthefulltem-
2 1−x x 2
ples of SrFe (As P ) , x =0.35. Solid lines are the best
peraturerangeshowingoneas-grownandtwoannealedsam- 2 1−x x 2
fits to the Hirschfeld - Goldenfeld model [41], see text for
ples. Top inset shows normalized resistivity, R(T)/R(300 K)
discussion.
for as-grown and annealed SrP122 (this work) and BaP122
(Ref.34). Lowerinsetshowsthesamedatazoomedoninthe
vicinity of T .
c
and references therein). A tunnel diode supports self
- oscillations in a high stability LC tank circuit at
√
maybemoreconsistentwithagapwithaccidentalnodes the resonant frequency f0 = 1/2π LC ≈ 14 MHz in
(see, e. g., [20]). The absolute value of the London pen- our setup. The sample is mounted on a movable sap-
etration depth at the optimal doping decreases upon an- phire rod whose temperature can be controlled indepen-
nealing from λ(0) = 300±10 nm in as-grown samples dently of the coil and oscillator components. The exci-
to λ(0) = 275±10 nm. Importantly, - the overall nodal tation magnetic field of the coil is ≈ 20 mOe, well be-
behavior of SrFe (As P ) was found to follow closely low first critical field. Insertion of the sample causes a
2 1−x x 2
that of BaP122. frequency shift by an amount ∆f(T) = −G4πχ(T) =
G[1−(λ(T)/R)tanh(R/λ(T))], which allows us to mea-
Single crystals of SrFe (As P ) were grown us-
2 1−x x 2
sure the change in London penetration depth, ∆λ(T) =
ing the self-flux method [33]. Samples were charac-
λ(T)−λ(0)witharesolutionofnearly1˚A.Thegeomet-
terized by x-ray, magnetization and transport measure-
ric constant G depends on the coil and sample volumes,
ments and the composition was determined using EDX
demagnetizationandemptycoilresonancefrequencyand
analysis, which yielded x =0.35. For London penetra-
is measured directly by extracting the sample from the
tiondepthmeasurementssampleswereselectedfromdif-
inductor coil while at base temperature [38].
ferent batches by measuring the transition curves and
finding the sharpest transition. The best samples were Figure 1 shows the full temperature range varia-
cut to a typical sample size of 0.5 × 0.5 × (0.02-0.1) tion of the in-plane London penetration depth, ∆λ(T),
mm3. Annealing was shown to improve T from 31 K measured in an as - grown (T = 27 K) and
c c
to 34.8 K and to increase the residual resistivity ratio, two annealed (T = 34.8 K) single crystals of
c
RRR=R(300K)/R(T )from4.5to6.4. Alesserincrease SrFe (As P ) , x =0.35. The insets show normal-
c 2 1−x x 2
of T from 30 K to 31 K and of RRR from 4.8 to 5.2 izedresistivity,R(T)/R(300K)foras-grownandannealed
c
after the annealing was reported for close to the opti- SrP122(thiswork)andBaP122(Ref.34)samples. Lower
mal doping (x=0.32) BaP122 samples [34], see insets in inset shows the data zoomed in the vicinity of T . Over-
c
Fig. 1. Furthermore, if we extrapolate linearly the resis- all, theresistivitycurvesforSrP122andBaP122arevir-
tivity curves to T = 0, we obtain RRR(0) = 10.2 and tually the same showing clear deviation from the Fermi
15.1 for as-grown and annealed SrP122, and RRR(0) = liquid T2 dependence at all temperatures, indicating
7.1 and 8.1 for as-grown and annealed BaP122, respec- proximity to the quantum critical point at the optimal
tively. By these measures, SrP122 appears to be cleaner doping [18, 31, 33, 39, 40].
than BaP122. Figure 2 shows the low temperature behavior of the
London penetration depth was measured by using a penetrationdepthforthreesamplesofSrFe (As P ) .
2 1−x x 2
tunnel diode resonator (TDR) technique (see Ref. [37] Two of the curves are shifted vertically by 0.06 and 0.12
3
nmtoavoidoverlap. Thelineartemperaturedependence
300
is evident. Some rounding off at the low temperatures 271
is due to impurity scattering as was shown for nodal 250 a n n e a le d s a m p le
cuprate superconductors by Hirschfeld and Goldenfeld 1 5 200 (cid:1) (0 )=2 7 5 – 1 0 n m T c(A l) = 1.28 K c o a te d
[41]. Within their model the behavior at low temper-
150
atures can be approximated by ∆λ(T) = At2/(t∗ + t) )
where t∗ is a crossover temperature scale determined by (nm100 D L = 293 nm
)1 0 l
unitary limit impurity scattering. Solid red curves in m D 50
Fig.2showbestfitstothedataresultinginthecrossover m( s-w ave B C S
temperatures t∗ =0.068, 0.101 and 0.285 for the three Dl 0 lAl(0) - d = 21.3 nm
curves from bottom up. The amplitude A also increases 5
0.0 0.5 1.0 1.5 2.0
fromthebottomtothetopcurve,A=88,97and130nm, T (K )
respectively. A straightforward interpretation is that we
aredealingwithsampleswithdifferentdegreesofscatter- A l c o a tin g tra n s itio n
0
ing from the cleanest (lowest curve) to the dirtiest (top
curve) and such assignment is in line with the effect of 0 1 0 2 0 3 0
annealing on resistivity and T . Good quality fits to the T ( K )
c
Hirschfeld-Goldenfeldformula,asshowninFig.2,would
appear to indicate the presence of line nodes. However, FIG. 3. Main panel: full - temperature ∆λ(T) of the same
this is not sufficient for the determination of the topol- sample before and after aluminum coating showing that the
curves are indistinguishable for T > TAl. Inset shows the
ogy of the nodal lines on the multi-band warped Fermi c
region of the Al transition. The curves are offset vertically
surface.
by λ (T) = d−λ(TAl) providing a rough visual estimate
eff c
For a full analysis we must determine the superfluid
ofλ(0)≈271nmuponextrapolationoftheuncoatedsample
density over the entire temperature range. Knowing the curve to T = 0. Numerical solution of Eq. 1 gives λ(0) ≈
variation of ∆λ(T), the superfluid density is given by 275±10 nm.
ρ (T)=λ2(0)/λ2(T)=(1+∆λ(T)/λ(0))−2, so we need
s
to know the absolute value of zero - temperature pene-
tration depth, λ(0). To obtain this value we used TDR
reproduce each other perfectly for T > TAl indicating
measurements of Al coated samples [42]. After initial c
a good repeatability and stability of our measurements.
measurement of ∆λ(T) each sample is uniformly coated
The low - temperature part in the vicinity of the su-
withAlusingmagnetronsputteringandthenremeasured
perconducting transition of the aluminum layer is shown
[42, 43]. To ensure a uniform Al film thickness the sam-
in the inset in Fig. 3. The curves are offset vertically,
ple is suspended by a fine wire from a rotating stage
so that BCS extrapolation (shown my the solid line) to
inside the sputter deposition chamber. The thickness of
T = 0 gives effective penetration depth of λ (T) =
the Al layer, d, was measured using focused - ion beam eff
d−λ(TAl) = 21.3 nm. The difference between the un-
cross-sectioning and imaging in SEM [43]. In our case c
coated sample and the coated sample at T = 0 gives a
d =73 nm is greater than the Al London penetration
depth, λAl(0) = 52 nm. At T < TAl, the effective pene- rough visual estimate of λ(0) = 271 nm and the numer-
c
ical solution of Eq. 1 (with the discussed above uncer-
tration depth is given by:
taintyof10nm)finallygivesλ(0)≈275±10nm. Apply-
ingthesameprocedure,weobtainedλ(0)=300±10nm
λ(T)+λAl(T)tanh d for the as-grown sample, consistent with the assumption
λeff(T)=λAl(T)λAl(T)+λ(T)tanhλAld(T) (1) ofanenhancedpair-breakingcomparedtotheannealed
λAl(T) samples. In BaP122 at the optimal doping, x =0.30, we
obtainedacomparablemagnitudeofλ(0)≈330nm,but
where λ(T) is the London penetration depth of the ma-
the situation is complicated by the strong doping depen-
terial of interest. When Al becomes normal at TAl ≈
c dence of λ(0) due to the quantum critical point hidden
1.28 K, λ (T) = d−λ(TAl). Extrapolation of ∆λ(T)
eff c beneath the dome [18]. Whether the same features exist
to T =0 shows that λ(TAl)≈λ(0)+0.7 nm and by us-
c in SrP122 requires a systematic doping study.
ingtheBCSs-waveformof λ(T)forAl, wecanestimate
the difference,L = λ(0)−λ (0). Solving numerically Combining the results presented in Fig. 2 and Fig. 3
eff
Eq. 1 we obtain λ(T). Considering all the uncertainties, we can compare the rate of change of the penetration
we estimate the accuracy as ±10 nm. depth with temperature observed in other clean nodal
Figure 3 illustrates the procedure to estimate the ab- superconductorswiththecurrentwork. Inad−wavesu-
solutevalueofλ(0). Mainpanelshowsfulltemperature- perconductor with vertical line nodes, the amplitude of
range∆λ(T)forthesameannealedsamplemeasuredbe- the (linear) low - temperature variation of the penetra-
fore and after aluminum coating. Evidently, the curves tion depth is given by [44]:
4
at higher temperatures. This difference must be due to
d irty d - w a v e
s - w a v e
1 .0 the difference in the gap magnitudes and anisotropies in
these multi-gap systems, but the low - temperature be-
c le a n d - w a v e
havior is determined by the nodal quasiparticles and the
0 .8
similarity of the data implies that the nodal structure
tw o a n n e a le d a s -g ro w n s a m p le
s a m p le s l (0 ) = 3 0 0 n m of SrP122 and BaP122 is similar. The deviation from
0 .6 l (0 ) = 2 7 5 n m the 2D d−wave could be due to geometry of the nodal
lines, - perhaps forming the loops in the electron bands
s dirty d - wave s - wave
r 1.0 [17, 20, 31, 47–49].
0 .4
s In conclusion, measurements of the London penetra-
r as-grown
clean d - wave l(0) = 300 nm tion depth, λ(T), in optimally - doped as - grown and
0.8
0 .2 annealed annealed single crystals of SrFe (As P ) iron - based
l(0) = 275 nm B a P 1 2 2 2 1−x x 2
x = 0 .3 0 superconductor provide clear evidence for line nodes.
0.0 0.1 T0/.2T 0.3 0.4 l (0 ) = 3 0 0 n m The absolute value of London penetration depth de-
0 .0 c
0 .0 0 .2 0 .4 0 .6 0 .8 1 .0 creases with annealing from λ(0) = 300 ± 10 nm to
λ(0) = 275±10 nm. The slope dλ/dt = 0.28 is consis-
T /T
c tent with the expectations for the superconducting gap
with line nodes, dλ/dt = ln2/2.14 = 0.32 and compara-
FIG.4. Comparisonofthesuperfluiddensity,ρ(T),forthree
samples of SrFe (As P ) with the prediction of a two - ble to the measured values in YBCO and BSCCO2212.
2 1−x x 2
dimensional d−wave pairing (short-dashed line - clean and The superfluid density ρ(T) differs from the prediction
dashed line - dirty limits) and isotropic s−wave (dot-dashed for the vertical infinite line nodes (as in a simple single
line). We also show ρ(T) for BaP122 (gray line, x =0.30, - band d−wave) and requires an analysis withing a full
λ(0) =330 nm). (Inset) Expanded view of low temperature
three - dimensional band-structure. Overall, our results
region.
indicate that SrFe (As P ) behaves very similarly to
2 1−x x 2
BaFe (As P ) bothfromtransportandsuperfluidre-
2 1−x x 2
sponse points of view and it seems that isovalently sub-
stitutedpnictidesareinherentlydifferentfromthecharge
d(λ/λ(0)) dλ 2ln2 - doped materials.
≡ = (2)
d(T/T ) dt (d∆/dϕ) The work at Ames was supported by the U.S. Depart-
c ϕ→node
mentofEnergy,OfficeofBasicEnergySciences,Division
where (d∆/dϕ) is the slope of the angle - depen- ofMaterialsSciencesandEngineeringundercontractNo.
ϕ→node
dent superconducting gap approaching the node posi- DE-AC02-07CH11358. Work at UIUC was supported by
tion on the Fermi surface. In the case of d−wave pair- the Center for Emergent Superconductivity, an Energy
ing, ∆(ϕ) = ∆(0)cos(2ϕ) and dλ/dt = Tcln2/∆(0) = Frontier Research Center funded by the US Department
ln2/2.14 = 0.32. For YBCO, the measured dλ/dt = of Energy, Office of Science, Office of Basic Energy Sci-
0.33 [42, 45] and for BSCCO2212 the observed value ences under Award No. DE-AC0298CH1088. Work at
is dλ/dt = 0.39 [42, 46], - both are quite close to Osaka was partly supported by a Grant-in-Aid IRON-
the theoretical prediction. In the present case of SEA from the Japan Science and Technology Agency
SrFe2(As1−xPx)2 , we obtained dλ/dt = 0.28. For com- (JST).
parison, in BaP122, dλ/dt =0.42 and 0.38 for x = 0.30
(λ(0) =330 nm) and x = 0.33 (λ(0) =215 nm), respec-
tively [18]. These values are in a reasonable agreement
with the theoretical value of 0.32 showing that the node
topology is not much different from a standard d−wave ∗ Corresponding author: [email protected]
[1] D. C. Johnston, Adv. Phys. 59, 803 (2010).
symmetry.
[2] P. C. Canfield and S. L. Bud’ko, Annu. Rev. Condens.
Figure 4 shows experimental superfluid densities con-
Matter Phys. 1, 27 (2010).
structed with the estimated values of λ(0). The data
[3] J. Paglione and R. Greene, Nat. Phys. 6, 645 (2010).
are compared with the expectations for d−wave pairing [4] G. R. Stewart, Rev. Mod. Phys. 83, 1589 (2011).
(short-dashed line - clean and dashed line - dirty limits) [5] H.-H.WenandS.Li,Annu.Rev.Condens.MatterPhys.
and isotropic s−wave (dot-dashed line). The data are 2, 121 (2011).
in a complete disagreement with the exponentially sat- [6] S. Kasahara, K. Hashimoto, R. Okazaki, H. Shishido,
M. Yamashita, K. Ikada, S. Tonegawa, N. Nakata,
urating s−wave curve. Instead, the data show a clear
Y. Sensyu, H. Takeya, K. Hirata, T. Shibauchi,
T−linear variation at low temperatures. For compari-
T. Terashima, and Y. Matsuda, Physica C 470, S462
son, the data for BaP122 are also shown by the gray
(2010).
line. The curves for BaP122 and SrP122 overlap at the [7] R. Prozorov and V. G. Kogan, Rep. Prog. Phys. 74,
low temperatures (below 0.2 T , see inset), but deviate 124505 (2011).
c
5
[8] J.-P. Reid, A. Juneau-Fecteau, R. T. Gordon, S. R. [28] D. V. Efremov, M. M. Korshunov, O. V. Dolgov, A. A.
de Cotret, N. Doiron-Leyraud, X. G. Luo, H. Shak- Golubov, andP.J.Hirschfeld, Phys.Rev.B84,180512
eripour, J.Chang, M.A.Tanatar,H.Kim,R.Prozorov, (2011).
T. Saito, H. Fukazawa, Y. Kohori, K. Kihou, C. H. Lee, [29] M. G. Vavilov and A. V. Chubukov, Phys. Rev. B 84,
A. Iyo, H. Eisaki, B. Shen, H.-H. Wen, and L. Taillefer, 214521 (2011).
Superc. Sci. Techn. 25, 084013 (2012). [30] Y. Bang, Supercond. Sci. Technol. 25, 084002 (2012).
[9] K.Cho,M.A.Tanatar,H.Kim,W.E.Straszheim,N.Ni, [31] A. Carrington, Rep. Prog. Phys. 74, 124507 (2011).
R. J. Cava, and R. Prozorov, Phys. Rev. B 85, 020504 [32] H.L.Shi,H.X.Yang,H.F.Tian,J.B.Lu,Z.W.Wang,
(2012). Y. B. Qin, Y. J. Song, and J. Q. Li, J. Phys.: Condens.
[10] K. Cho, M. A. Tanatar, N. Spyrison, H. Kim, Y. Song, Matter 22, 125702 (2010).
P.Dai,C.L.Zhang, andR.Prozorov,Phys.Rev.B86, [33] T.Kobayashi,S.Miyasaka, andS.Tajima,J.Phys.Soc.
020508 (2012). Jpn. 81SB, SB045 (2012).
[11] C. Martin, H. Kim, R. T. Gordon, N. Ni, V. G. Kogan, [34] M. Nakajima, S.-I. Uchida, K. Kihou, C.-H. Lee, A. Iyo,
S.L.Bud’ko,P.C.Canfield,M.A.Tanatar, andR.Pro- and H. Eisaki, J. Phys. Soc. Jpn. 81, 104710 (2012).
zorov, Phys. Rev. B 81, 060505 (2010). [35] T. Dulguun, H. Mukuda, T. Kobayashi, F. Engetsu,
[12] J. P. Reid, M. A. Tanatar, X. G. Luo, H. Shakeripour, H.Kinouchi,M.Yashima,Y.Kitaoka,S.Miyasaka, and
N. Doiron-Leyraud, N. Ni, S. L. Bud’ko, P. C. Canfield, S. Tajima, Phys. Rev. B 85, 144515 (2012).
R. Prozorov, and L. Taillefer, Phys. Rev. B 82, 064501 [36] H.Takahashi,T.Okada,Y.Imai,K.Kitagawa,K.Mat-
(2010). subayashi, Y. Uwatoko, and A. Maeda, Phys. Rev. B
[13] M. A. Tanatar, J. P. Reid, H. Shakeripour, X. G. Luo, 86, 144525 (2012).
N. Doiron-Leyraud, N. Ni, S. L. Bud’ko, P. C. Canfield, [37] R. Prozorovand R. W. Giannetta,Superc. Sci. Technol.
R. Prozorov, and L. Taillefer, Phys. Rev. Lett. 104, 19, R41 (2006).
067002 (2010). [38] R.Prozorov,R.W.Giannetta,A.Carrington, andF.M.
[14] S.Maiti,M.M.Korshunov,T.A.Maier,P.J.Hirschfeld, Araujo-Moreira, Phys. Rev. B 62, 115 (2000).
and A. V. Chubukov, Phys. Rev. Lett. 107, 147002 [39] Y. Nakai, T. Iye, S. Kitagawa, K. Ishida, H. Ikeda,
(2011). S.Kasahara,H.Shishido,T.Shibauchi,Y.Matsuda, and
[15] J.S.Kim,B.D.Faeth,Y.Wang,P.J.Hirschfeld,G.R. T. Terashima, Phys. Rev. Lett. 105, 107003 (2010).
Stewart,K.Gofryk,F.Ronning,A.S.Sefat,K.Y.Choi, [40] T. Iye, Y. Nakai, S. Kitagawa, K. Ishida, S. Kasahara,
and K. H. Kim, Phys. Rev. B 86, 014513 (2012). T.Shibauchi,Y.Matsuda, andT.Terashima,Phys.Rev.
[16] K. Hashimoto, M. Yamashita, S. Kasahara, Y. Senshu, B 85, 184505 (2012).
N. Nakata, S. Tonegawa, K. Ikada, A. Serafin, A. Car- [41] P.J.HirschfeldandN.Goldenfeld,Phys.Rev.B48,4219
rington, T. Terashima, H. Ikeda, T. Shibauchi, and (1993).
Y. Matsuda, Phys. Rev. B 81, 220501 (2010). [42] R. Prozorov, R. W. Giannetta, A. Carrington,
[17] K. Suzuki, H. Usui, and K. Kuroki, J. Phys. Soc. Jpn. P. Fournier, R. L. Greene, P. Guptasarma, D. G. Hinks,
80, 013710 (2011). and A. R. Banks, Appl. Phys. Lett. 77, 4202 (2000).
[18] K. Hashimoto, K. Cho, T. Shibauchi, S. Kasa- [43] R. T. Gordon, H. Kim, N. Salovich, R. W. Giannetta,
hara, Y. Mizukami, R. Katsumata, Y. Tsuruhara, R. M. Fernandes, V. G. Kogan, T. Prozorov, S. L.
T.Terashima,H.Ikeda,M.A.Tanatar,H.Kitano,N.Sa- Bud’ko,P.C.Canfield,M.A.Tanatar, andR.Prozorov,
lovich, R. W. Giannetta, P. Walmsley, A. Carrington, Phys. Rev. B 82, 054507 (2010).
R.Prozorov, andY.Matsuda,Science336,1554(2012). [44] D. Xu, S. K. Yip, and J. A. Sauls,NoStop
[19] I.I.Mazin,D.J.Singh,M.D.Johannes, andM.H.Du, [45] K. Zhang, D. A. Bonn, S. Kamal, R. Liang, D. J. Baar,
Phys. Rev. Lett. 101, 057003 (2008). W. N. Hardy, D. Basov, and T. Timusk,NoStop
[20] P.J.Hirschfeld,M.M.Korshunov, andI.I.Mazin,Rep. [46] T. Jacobs, S. Sridhar, Q. Li, G. D. Gu, and
Prog. Phys. 74, 124508 (2011). N. Koshizuka,NoStop
[21] S.Maiti,M.M.Korshunov,T.A.Maier,P.J.Hirschfeld, [47] S.Graser,A.F.Kemper,T.A.Maier,H.-P.Cheng,P.J.
and A. V. Chubukov, Phys. Rev. B 84, 224505 (2011). Hirschfeld, andD.J.Scalapino, Phys.Rev.B81,214503
[22] A. Chubukov, Annu. Rev. Condens. Matter Phys. 3, 57 (2010).
(2012). [48] S. Maiti and A. V. Chubukov, Phys. Rev. B 82, 214515
[23] M. Khodas and A. V. Chubukov, Phys. Rev. B 86, (2010).
144519 (2012). [49] T. Shimojima, F. Sakaguchi, K. Ishizaka, Y. Ishida,
[24] D. Parker, M. G. Vavilov, A. V. Chubukov, and W. Malaeb, T. Yoshida, S. Ideta, A. Fujimori, T. Kiss,
I. I. Mazin, Phys. Rev. B 80 (2009), 10.1103/Phys- M. Okawa, T. Togashi, C.-T. Chen, S. Watanabe,
RevB.80.100508. Y. Nakashima, A. Ino, H. Anzai, M. Arita, K. Shimada,
[25] R. M. Fernandes and J. Schmalian, Phys. Rev. B 82 H.Namatame,M.Taniguchi,S.Kasahara,T.Terashima,
(2010), 10.1103/PhysRevB.82.014521. T. Shibauchi, Y. Matsuda, M. Nakajima, S. Uchida,
[26] S. Maiti, R. M. Fernandes, and A. V. Chubukov, Phys. K. Kihou, C. H. Lee, A. Iyo, H. Eisaki, A. Chainani,
Rev. B 85, 144527 (2012). and S. Shin, Solid State Commun. 152, 695 (2012).
[27] A. F. Kemper, C. Cao, P. J. Hirschfeld, and H.-P.
Cheng,NoStop
