Observation of Multi-Gap Superconductivity in GdO(F)FeAs by Andreev Spectroscopy

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by  T. E. Shanygina

Observation of Multi-Gap Superconductivity in GdO(F)FeAs by Andreev
Spectroscopy
T. E. Shanygina▽⋄, Ya. G. Ponomarev⋄, S. A. Kuzmichev⋄, M. G. Mikheev⋄,
S. N. Tchesnokov⋄, O. E. Omel’yanovskii▽†, A. V. Sadakov▽†, Yu. F. Eltsev▽,
A. S. Dormidontov▽, V. M. Pudalov▽△, A. S. Usol’tsev▽△, E. P. Khlybov(cid:3)†
▽P. N. Lebedev Physical Institute RAS, 119991 Moscow, Russia
⋄Department of Low Temperature Physics and Superconductivity,
1 Moscow State University, 119991 Moscow, Russia
1 †International Laboratory of High Magnetic Fields and Low Temperatures, Wroclaw, 53-421, Poland
0 △Moscow Institute of Physics and Technology, 141700 Moscow, Russia
2 (cid:3)Institute for High Pressure Physics RAS, Troitsk, Moscow district, 142190, Russia
(Dated: January 13, 2011)
n
a We have studied current-voltage characteristics of Andreev contacts in polycrystalline
J GdO0.88F0.12FeAs samples with bulk critical temperature Tc = (52.5 ± 1)K using break-junctions
2 technique. Thedataobtainedcannot bedescribed withinthesingle-gap approachandsuggests the
1 existenceofamulti-gapsuperconductivityinthiscompound. Thelargeandsmall superconducting
gap valuesestimated at T =4.2K are ∆L = 10.5±2 meV and ∆S = 2.3±0.4meV,respectively.
]
n
PACSnumbers: 74.70.Xa,74.20.Fg,74.25.-q,74.45.+c,74.62.Dh
o
c
-
r Novel superconducting compounds of 1111 family Correspondingly, many of the available theoretical and
p
based on rare-earth oxypnictides REOFeAs (RE = La, experimental data indicate that iron-based layered ma-
u
s Sm, Gd etc.) [1, 2] are currently in the focus of research terials are multiband superconductors with s-type sym-
. interest. Some oftheir featuressuch aslayeredstructure metry of the order parameter [3]. Knight shift measure-
t
a
and spatial separation of the carrier reservoir layers and ments in 1111-class compounds [16] have proven unam-
m
thesuperconductingpairinglayersaresimilartothoseof biguouslythespin-singlettypepairinginthesematerials.
- cuprates. However,manyotherpropertiesdiffersubstan- Severaldata were reportedin favorof s± [17, 18] or s++
d
n tiallyandpromisenewinterestingphysics[3]. Atpresent, orderparametersymmetry[19],makingtheexperimental
o the key issues under investigations are the effect of vari- situation regarding 1111-compounds uncertain.
c oustypesofdoping,pairingmechanism,symmetryofthe
The magnitude and structure of the superconducting
[
orderparameter,quasiparticle energyspectrum, and the gap ∆ is intimately related to the pairing mechanism.

2 superconducting energy gap(s).
ARPESmeasurementsarenotsensitiveenoughtoresolve
v
The stoichiometric compounds of the 1111-family are unambiguously such fine details as ∆, on the scale of a
0
4 antiferromagnetic metals with spin density wave ground few meV, that makes this parameter accessible nearly
4 state [4]. Partial deficiency of oxygen or fluorine substi- exclusively from point contact spectroscopy, such as
4 tutionforoxygeninducessuperconductivityintheFeAs- scanning tunneling spectroscopy (STS), tunneling- and
2. layers. Replacement of rare-earth elements also affects point-contact Andreev reflection (PCAR) spectroscopy
1 the superconducting critical temperature, Tc. In par- (the latter in the regime of SN-, or symmetrical SNS-
0 ticular, Tc of Gd-based oxypnictide may be lowered by junctions). The available experimental reports are how-
1
partial replacement of Y for Gd [5] or gained up by ever rather inconsistent for 1111 class compounds [20–
:
v introducing Th instead of Gd [6]. Tc=56 K found in 23], even for the most intensively studied SmO(F)FeAs.
Xi Gd0.8Th0.2OFeAscompoundistodaythehighestonefor Various types of conclusions have been reported includ-
iron-based superconductors. ing d-wave like, single gap-like, and multi-gap behavior.
r
a According to band structure calculations, the total The ambiguity of the experimental information is
densityofstatesattheFermilevelN(0)isformedmainly partly due to an inevitable inhomogeneity of the 1111-
byFe3d-states[7–9]. AsshowninRef.[10],theTc values type polycrystalline samples and lack of large size 1111-
for different iron-based superconductors correlate with typesingle-crystalssuitableforthesemeasurements. An-
N(0), thus giving support to the BCS-like coupling in other cause for the divergency of the point contactspec-
these compounds. troscopy data is inherent in those experimental tech-
The theoretically calculated Fermi surface for 1111- niques, where the sample surface is not cleaved in high
system [11–13] consists of quasi-two-dimensional (2D) vacuum or cryogenic environment. In order to resolve
hole sheets centered at the Γ point and two electron theexperimentalambiguity,evidently,novelsetsofcom-
sheetsattheM pointsofthefirstBrillouinzone. Within prehensive experimental data are needed, which would
the so called minimal two-band model, these four bands compriseself-consistencycheck,substantialstatisticsand
may be considered as two effective 2D bands [14, 15]. provide local probing at various points of the in-situ
2
cleaved surfaces.
Herewereportthesuperconductinggapmeasurements
in nearly optimally doped GdO0.88F0.12FeAs samples by
SNSAndreevspectroscopyusingthebreak-junctiontech- 20 )
s
nique [24]. Until now these measurements havenot been t
ni
doneforGd-1111,ananaloguetoSm-1111withapproxi- u
nmiqatueelyotpheenssaamenicTec o∼ppo5r3tKu.niTtyhetobrpearekpajurencitnionhetleiuchm- nits)15 10 arb.
afotrmmoisnpghAerned,raetevliqcuonidta4cHt.eAtnemotpheerraatudrveasn,taclgeeanissauprfoascseis- arb.u10 /dT (
( R
bility of fine mechanical readjusting the contact during R 5 d
experiment, that enables to collect multiple data from 5
different local areas of the same sample. Using this
method we have unambiguously detected the presence
of two superconducting gaps, whose best fit values aver- 0
aged over about 30 spectra are ∆ = 10.5±2meV and 0
L
∆S =2.3±0.4meV at T =4.2K. 44 48 52 56
PolycrystallinesamplesGdO(F)FeAswerepreparedby
T (K)
high pressure synthesis [25]. The chips of high purity
Fe, and powders of single-phase FeF3, Fe2O3, and GdAs
FIG.1: Fig.1. Superconductingtransitionforpolycrystalline
were mixed together in the nominal ratio and pressed
GdO0.88F0.12FeAs sample measured prior a microcrack for-
into pellets of 3mm diameter and 3mm height. The pel- mation(dots). ThebulkTc =(52.5±1)Kwasdeterminedat
lets were placed in boron nitride crucible and synthe- a maximum in dR(T)/dT-curve(solid line).
sizedatpressureof50kbandtemperature1350◦Cduring
60min. The X-ray diffraction pattern averaged over the
sample area showed a polycrystalline compound with a
4.2K using a micrometric screw.
dominating desired 1111-phase (with lattice parameters
Current-voltage dependence, I(V), and its derivative,
a = 3.902(2)˚A, c = 8.414(5)˚A ) and an admixture of in-
dI(V)/dV,weremeasuredautomaticallyusingthe16-bit
cidental FeAs and Gd2O3 phases. The subsequent local
AT-MIO-16X(NationalInstruments) digitalboard. The
EDS analysis (JSM-7001FA) has revealed that the inci-
amplitude of a low-level 820 Hz modulation voltage at
dental phases are concentrated in grains of about 1mkm
potential leads of a sample was maintained stable using
sizewhicharescatteredinthebulkmajorityphase. This
a lock-in nanovoltmeter (operated as null- detector) and
fact opens a possibility to probe properties of the true
acomputercontrolleddigitalbridgewithaproportional-
majority phase using local techniques, such as PCAR.
integral-derivative feedback signal. As a result, the dif-
Superconductingpropertiesofthesamplesweretestedby
ferential conductance of a contact was proportional to
measurementsoftemperaturedependenceofac-magnetic
the amplitude of the ac feedback current through the
susceptibility andresistivityR(T). Bothshoweda sharp
contact.
superconductingtransitioninourpolycrystallinesamples
Figure 2 represents I(V), dI(V)/dV and d2I(V)/dV2
with T ≈52.5K (the latter value was defined at a max-
c
characteristics for individual Andreev (SNS) break-
imum of dR(T)/dT-curve). Figure 1 shows typical tem-
perature dependence of resistance and its derivative. junction in polycrystalline GdO0.88F0.12FeAs sample
measured at T = 4.2K. The observed experimental IV-
For point-contact spectroscopy we used two meth-
curves are typical for the clean classical SNS-contacts
ods: (i) multiple Andreev reflections spectroscopy of
with excess-current characteristics [27, 29], therefore,
individual superconductor-constriction-superconductor
the theoretical model of Ku¨mmel et al. [27] is sup-
Sharvin-type contacts [26–28] and (ii) intrinsic Andreev
posed to be applicable to our break-junctions. Accord-
specrtoscopy of stack contacts that usually exist due to
ing to the Ku¨mmel model, the IV-characteristics at low
the presence of steps and terraces on clean cryogenic
bias voltages should show a subharmonic gap structure
cleaves in layered crystals.
(SGS) with a series of dips in the dynamic conductance
Thin plates of about 2 ×1× 0.12 mm3 in size were
dI(V)/dV at bias voltages
cut from the synthesized pellets. At room temperature,
theplate-likesamplewasmountedontoanelasticbronze V =2∆/en, (1)
n
holder and the two current and two potential leads were
attachedtothe sample byliquidIn-Gaalloy. The holder with an integer n = 1,2..., due to multiple AR effect.
with the sample was placed in the measuring cell and For a two-gap superconductor, two independent SGSs
cooled down to 4.2K. A microcrack in the sample was corresponding to the large ∆ and small ∆ gaps are
L S
generated by precise bending the sample holder at T = anticipated.
3
They may be caused by excitation of collective modes
and require additional studies.
0,08
L = 11 meV EL1D06
0,06 S = 2.5 meV
1,5
TC = 53 K
) EL3D01
b. un. 0,04 dI/dV S = 2.15 meV
ar nS=1 1,0
(
2 0,02
V
d
2 dI/ n.)
V; 0,00 b.u0,5
dI/d d2I/dV2 (ar
A); -0,02 V
m
I (-0,04 I(V) dI/d0,0
nL=1
2 L/kTC = 4.81 nS=1
-0,06 2 S/kTC = 1.1 nL=2 -0,5 nS=2
-30 -20 -10 0 10 20 30
V (mV)
-1,0
-8 -6 -4 -2 0 2 4 6 8
V (mV)
FIG. 2: Fig. 2. I(V), dI(V)/dV and d2I(V)/dV2-curves for
asingleSNS-contact1D06atT =4.2K.Background(apoly-
nomial function) is subtracted. The set of dips in the dif-
ferential conductance at bias voltages VnL = 2∆L/en (verti- FIG. 3: Fig. 3. Differential conductance of the SNS contact
cal solid lines) determines the energy of the large supercon- 3D01 in GdO0.88F0.12FeAs sample at T =4.2K. Background
ducting gap, ∆L ≈ 11meV. Peculiarities on the dI(V)/dV is subtracted. Two differential conductance dips define the
andd2I(V)/dV2-curves,markedbydashedlines,indicatethe energy of the small superconducting gap ∆S ≈ 2.15meV.
presence of small superconducting gap, ∆S ≈2.5 meV. Theanticipated biasvoltages VnS =2∆S/enaredepictedby
vertical dashed lines.
The dips labeled on Figure 2 as n =1 and 2 on SGS The peculiarities in the dI(V)/dV and d2I(V)/dV2-
L
reflect the large gap; they are markedwith vertical solid characteristics shown in Figs. 2 and 3 clearly mani-
lines. The singularities shown by vertical dashed lines fest the existence of two gaps in our GdO(F)FeAs sam-
cannotbeattributedtothelargegapand,therefore,may ples. This behavior is similar to that observed earlier
reflecttheexistenceofasmallgap∆S ≈2.5 meV.Com- in the multi-band superconductor Mg1−xAlxB2 [30] and
paring the result for the large gap (Fig. 2) with Eq. (1) in LaO0.9F0.1FeAs [31] (an analog to our Gd-1111 sam-
one can easily obtain ∆ ≈11meV. ple with somewhat lower T ≈ 28K). The sharpest SGS
L c
Byreadjustingthecontact,wecouldobserveclearsets (like those in Figure 3) may be usually observedonly on
of dips on dI(V)/dV curve due to either large- or small- dI(V)/dV-characteristicsofAndreevcontactsofthehigh
gaps, or even to both (as shown in Fig. 2). Figure 3 quality and of small size, comparable to the quasiparti-
expandsthesmallbiasrange,wherethesubharmonicsof clesmeanfreepath(ballistic limit)[28]. Forsuchacase,
thesmallgapareclearlyseenindI/dV curvesforasingle a number of the observable gap peculiarities (up to 4 in
SNS-contact EL3D01. For clarity, the smoothly varying some samples) facilitates interpretation of the multigap
background is subtracted. Using expression (1), from subharmonic structure.
the setofdifferentialconductance dips onthe dI(V)/dV Our experimental data are summarized in Figure 4
curve we determined the energy of the small supercon- where the normalized to a single junction bias voltages
ducting gap ∆ = 2.15meV. In Fig. 3, one can also see V for four microcontacts are plotted versus 1/n .
S nL,S L,S
two extra features at≈3mV and 3.7mV, whichcannot According to expression (1), such dependences have to
be attributed to either large or small gap subharmonics. fall onto straight lines passing through zero. This is in-
4
2∆ /k T ≈1.1≪3.52suggeststhatthe“weak”super-
S B c
conductivity may be induced by interbandcoupling, due
to k-spaceinternalproximityeffectbetweentwo conden-
sates,wherethelargegapcondensateplaysthe“driving”
role. In particular, similar situation is believed to be re-
22 GdFeAsO1-xFx alized in MgB2 [30] and LaO0.9F0.1FeAs [31].
20 V
e
- EL3D01 m TABLE I: Table 1. Summary of the ∆ values (in meV)
18 - EL1D06 – 2 measured for 1111-family REO(F)FeAs compounds by point
- EL3D02 5 contact Andreev reflection (PCAR), break-junction Andreev
V) 16 - EL3D13 10. reflection (BJ), scanning tunneling spectroscopy technique
m 14 = (STS),and ARPES
( L

nS RE Tc(K) method 2∆L 2∆S ref.
V 12
Gd 53 BJ 10.5±2 2.3±0.4 thiswork
,
VnL 10 TC = 53 K Sm 53 ARPES 15±1.5 no [34]
Sm 52 PCAR 18±3 6.15±0.45 [35]
8
Sm 52 PCAR 19 5.7 [36]
6 Sm 52 STS 8-8.5 no [21]
4 = 2.3 – 0.4 meV SSmm 5511.5 PPCCAARR 2100 64.6 [[3262]]
S
2 Nd 51 PCAR 14±1 6 ±1 [37]
Nd 51 PCAR 12.5±0.5 6.3±0.3 [38]
0
0,0 0,2 0,4 0,6 0,8 1,0 Nd 48 BJ,STS 7-10 no [33]
1/n Tb 45 PCAR 8.8 5 [39]
Nd 45 PCAR 11±2 5 ±1 [40]
Sm 42 PCAR 15±1 4.9±0.5 [35]
Sm 42 PCAR 6.7±0.1 no [18]
FIG.4: Fig.4. NormalizedbiasvoltagesVn =2∆L,S/enver-
sus 1/nL,S for the studied SNS-arrays. The averaged values
of the superconducting gaps are ∆L = (10.5±2)meV and The presence of the large superconducting gap char-
∆S =(2.3±0.4)meV. Solid lines are guides to theeye. acterized by 2∆L/kBTc > 3.52 in the 1111-family com-
pounds REOFeAs (RE = La, Sm, Nd) was confirmed
by tunneling spectroscopy using break-junction tech-
nique[33],point-contactAndreevreflectionspectroscopy
deedfulfilledfor∆L inallsamples;for∆S,thishowever, [20, 22,35–40], scanning tunneling spectroscopy[21, 33],
could be verified only in sample EL3D01, because a rich andangle-resolvedphotoemissionspectroscopy(ARPES)
pictureofreproduciblefeaturesdetectedatlowbiasvolt- [34] (see Table 1). To the best of our knowledge, there
ages impeded the analysis for other samples. is no other available data for Gd-1111. Therefore, we
Based on the data obtained we conclude on the ex- compareinTable1ourdataforGd-1111withotherdata
istence of two distinct superconducting gaps with ener- availableforSm-,Nd-andTb-1111superconductorswith
gies ∆L = (10.5±2)meV and ∆S = (2.3±0.4)meV at similarTc. Weemphasizerathergoodagreementbetween
T = 4.2K in GdO1−xFxFeAs sample. The reproducibil- 2∆L/kBTc values determined from our study and those
ity of two SGSs detected at dI(V)/dV-characteristics of from STS [21], break-junction measurements [33], and
various Andreev arrays, formed by the break-junction some PCAR-measurements [22, 38–40].
technique support this conclusion. In some cases we ob- Astothesmallgap,thereisevidentlyasizeablespread
served extra features in dI/dV-curves signalling the ex- initsvalue(2−5)meVobservedindifferentexperiments.
istence of the 3rd, smaller gap, ∆SS ∼1meV. We can not also exclude that the spread may be caused
Using the determined gap energies and bulk T = by the existence of three-gap superconductivity in 1111-
c
(52.5±1)K, one can estimate 2∆/k T ratio. For the systemwithmultiple-sheetFermisurface[13,41];forthe
B c
large gap, our experimental data lead to 2∆ /k T = smallest gap, we estimate 2∆ /k T ≈0.5.
L B c S B c
(4.8±1.0)thatexceedsthestandardBCSvalue,3.52,for In conclusion, we have studied the I(V)- and
single-gap superconductors in the weak coupling limit. dI(V)/dV-characteristics at T = 4.2K for vari-
This fact together with rather conventional exponent ous SNS Andreev break-junctions in polycrystalline
value for Fe isotope effect [32] resembles the BCS - GdO0.88F0.12FeAs samples with bulk critical tempera-
model behavior with strong electron-phonon coupling. turesT ≈52.5K.Theobtainedcharacteristicsdonotfol-
c
At the same time, the 2∆/k T ratio for the small gap lowthestandardsingle-gapmodelbehavior. Twoclearly
B c
5
observed independent subharmonic gap structures point [18] C.-T. Chen, C. C. Tsuei, M. B. Ketchen, Z.-A. Ren, Z.
at the existence of two distinct superconducting gaps, X. Zhao, Nature Physics 6, 260 (2010).
∆ = (10.5±2)meV and ∆ = (2.3±0.4)meV deter- [19] I. I. Mazin, J. Schmalian, arXiv: 0901.4790.
L S
[20] T.Y.Chen,Z.Tesanovic,R.H.Liu,X.H.Chen,andC.
mined at T = 4.2K. The estimated 2∆ /k T ratio ex-
L B c L. Chien, Nature453, 1224 (2008).
ceedsthestandardBCSvalue,3.52,forsingle-gapsuper-
[21] O. Millo, I. Asulin, O. Yuli, I. Felner, Z.-A. Ren, X.-Li
conductors and weak-coupling limit while for the small Shen, G.-C. Che, Z.-X. Zhao, Phys. Rev. B 78, 092505
gap the 2∆/Tc-ratio 2∆S/kBTc ≪3.52. (2008).
TheauthorsaregratefultoE.G.Maksimov,E.V.An- [22] Y.-L. Wang, L. Shan, L. Fang, P. Cheng, C. Ren, H.-H.
tipov, and S. M. Kazakov for valuable discussions and Wen, Supercond.Sci. Technol. 22, 015018 (2009).
S. M.Kazakovforhelp in X-raydiffractioncharacteriza- [23] M. H.Pan, X.B. He, G.R. Li, J. F. Wendelken,R.Jin,
A.S.Sefat,M.A.McGuire, B.C.Sales,D.Mandrus,E.
tion of the samples. The work was partially supported
W. Plummer, arXiv: 0808.0895.
bytheRussianFoundationforBasicResearchandbythe
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