Physica C 480 (2012) 71–74

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Physica C

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Enhancement in superconducting transition temperature (Tc) and upper criticalfield (Hc2) in new Yb-doped Ce1�xYbxO0.9F0.1FeAs superconductors

Gohil S. Thakur a, Jai Prakash a, M. Kanagaraj b, S. Arumugam b, Ashok K. Ganguli a,⇑a Department of Chemistry, Indian Institute of Technology, New Delhi 110 016, Indiab Centre for High Pressure Research, School of Physics, Bharathidasan University, Tiruchirapalli 620 024, India

a r t i c l e i n f o a b s t r a c t

Article history:Received 16 February 2012Received in revised form 14 March 2012Accepted 20 April 2012Available online 3 May 2012

Keywords:OxypnictidesUpper critical fieldCoherence length

0921-4534/$ - see front matter � 2012 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.physc.2012.04.031

⇑ Corresponding author. Tel.: +91 11 26591511; faxE-mail address: ashok@chemistry.iitd.ernet.in (A.K

We report the synthesis and characterization of new oxypnictides of the type Ce1�xYbxO0.9F0.1FeAs(‘x’ = 0.3, 0.4 and 0.5) for the first time. All these compounds crystallize in the tetragonal ZrCuSiAs typestructure (space group: P4/nmm). Reduction in both the lattice parameters (a and c) was observed onsubstitution of smaller Yb ions at Ce sites in CeO0.9F0.1FeAs. All these compounds were found to be super-conducting with maximum Tc of 48.7 K for ‘x’ = 0.5 composition which is highest among Ce(O/F)FeAsbased superconductors. Temperature dependence of resistivity under magnetic field has been studiedto evaluate the upper critical field (Hc2(0)) of these superconductors. Enhancement in upper critical field(Hc2) was observed for these Yb doped Ce1�xYbxO0.9F0.1FeAs superconductors (max Hc2(0) = �142 T for‘x’ = 0.3). The average coherence length of all Yb doped samples was estimated to be �23 Å.

� 2012 Elsevier B.V. All rights reserved.

1. Introduction

The recent discovery of superconductivity at 26 K in an oxyp-nictide La(O/F)FeAs [1] received enormous attention among solidstate chemists and condensed matter physicist. Following this re-port, many new pnictogen and chalcogen based superconductingfamilies were discovered with the general formula of AFFeAs,AFe2As2, AFeAs, A4M2M0

2Pn2O6, A3M2M02Pn2O5 and FeSe/Te. Com-

pounds with the general formula LnOFeAs (1111 system) adoptthe tetragonal ZrCuSiAs type structure (space group: P4/nmm) [2]which is a filled variant of well known PbFCl structure (spacegroup: P4/nmm). Its structure consists alternate layers of namely[La–O] (charge reservoir) and [Fe–As] (conducting layer) [3]. Un-doped CeOFeAs is an antiferromagnetic semimetal and exhibitsstructural transition from tetragonal (space group: P4/nmm) toorthorhombic structure (space group: Cmma) [4]. Resistivity andmagnetic studies of LnOFeAs shows an anomaly at 150–160 Kdue to structural transition followed by a spin density wave dueto antiferromagnetic ordering of iron spins. Superconductivity inthese pnictides can be induced by suppressing structural transitionand antiferromagnetism through electron or hole doping [1,5–8].Electrons in LnOFeAs can be doped through substitution of fluorideions at oxygen site [1,5] while holes are doped through substitu-tion of strontium ions at rare-earth site [7,8]. Enhancement in

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: +91 11 26854715.. Ganguli).

superconducting transition temperature of La(O/F)FeAs wasachieved by replacing La by other smaller rare-earth metals likeCe [9], Pr [10], Nd [11], Sm [12] etc. with maximum Tc of 55 K inSm(O/F)FeAs [13]. The hole doped Ln1�xSrxOFeAs superconductorsshow maximum Tc of 25 K in La0.8Sr0.2OFeAs [7]. The substitutionof Co at Fe sites in LnOFeAs also dopes electrons and induces super-conductivity [14–16] with maximum Tc of 15 K in SmOFe0.9Co0.1As[15].

In this report we have investigated the effect of substitution ofsmaller ytterbium ions at Ce site in CeO0.9F0.1FeAs superconductoron its structural and superconducting properties. We have alsoestimated upper critical field of these novel superconductors.

2. Experimental

Polycrystalline samples with nominal compositions ofCe1�xYbxO0.9F0.1FeAs (‘x’ = 0.3, 0.4 and 0.5) were synthesized by atwo step solid state method using high purity Ce, CeO2, Yb2O3,CeF3, and FeAs as starting materials. FeAs was obtained by reactingFe chips and As powder at 800 �C for 24 h. The raw materials weretaken according to stoichiometric ratio and then sealed in evacu-ated silica ampoules (10�4 torr) and heated at 900 �C for 30 h.The powder was then compacted (5 tons) and the disks werewrapped in Ta foil, sealed in evacuated silica ampoules and heatedat 1150 �C for 30 h. All the chemical manipulations were per-formed in an Argon-filled glove box. The samples were character-ized by powder X-ray diffraction using Cu Ka radiation. Refinedlattice parameters were calculated by least square fit method.

0.3 0.4 0.5

3.965

3.970

3.975

3.980

3.985

3.990

3.995

Yb concentration (x)

Ce1-xYbxO0.9F0.1FeAs

8.57

8.58

8.59

8.60

8.61

8.62

8.63

a la

ttice

par

amet

er (Å

) c lattice parameter (Å

)

Fig. 2. Plot of variation of lattice parameters (a and c) with Ytterbium content (x) inCe1�xYbxO0.9F0.1FeAs.

0 40 80 120 160 200 240 2800.00

0.01

0.02

0.03

0.04

0.05

x = 0.5

Ce1-xYbxO0.9F0.1FeAs

ρ (m

Ω−c

m)

ρ (m

Ω−c

m)

Temperature (K)

x = 0.3 x = 0.4 x = 0.5

x = 0.3

x = 0.4

(a)

0.0025

0.0050

0.0075Ce0.5Yb0.5O0.9F0.1FeAs

Tc=48.7 K

(b)

72 G.S. Thakur et al. / Physica C 480 (2012) 71–74

The temperature dependence of normal state resistivity (q) ofYb-doped Ce1�xYbxO0.9F0.1FeAs (‘x’ = 0.3, 0.4 and 0.5) polycrystal-line samples were measured by conventional four probe techniqueusing PPMS (Quantum Design, USA) with an excitation current of1 mA. For resistivity measurements under applied magnetic field(0–4 T) data were recorded during warming runs at a rate of1 K/min. The dc magnetic susceptibility as a function of tempera-ture was also performed by PPMS–VSM module under zero fieldcooling with applied field of 5 Oe at temperature between 2 and300 K.

3. Results and discussion

Powder X-ray diffraction patterns for Ce1�xYbxO0.9F0.1FeAs(‘x’ = 0.3, 0.4 and 0.5) are shown in Fig. 1. Majority of the observedreflections could be indexed on the basis of tetragonal CeOFeAsstructure (space group: P4/nmm). Along with major tetragonalCeOFeAs phase, some amount of Yb2O3, Fe2As and CeAs were alsoobserved as secondary phases. Both the lattice parameters (a andc) were found to be decreasing with increase in Yb substitution(Fig. 2) which is expected since the ionic size of Yb3+(0.985 Å) issmaller as compared to Ce3+ (1.143 Å) in eightfold coordination.Hence the substitution of Yb-ions in CeO0.9F0.1FeAs increases thechemical pressure. The lattice parameters for all the Yb-dopedcompositions were compared and found to be smaller than thatof the parent CeO0.9F0.1FeAs (a = 3.991(1) Å and c = 8.613(3) Å)reported earlier [17]. This indicates successful substitution of Ybat the rare earth sites. ‘x’ = 0.3 composition (a = 3.991 Å andc = 8.625 Å) however does not follow the trend, indicating a devia-tion of stoichiometry from the loaded composition. The variation ofresistivity as a function of temperature for Ce1�xYbxO0.9F0.1FeAs(‘x’ = 0.3, 0.4 and 0.5) is shown in Fig. 3a. For ‘x’ = 0.3 compositiontemperature dependence of resistivity show metallic behavior be-fore a superconducting transition at 42.7 K which is higher thanthe ytterbium free CeO0.9F0.1FeAs (Tc = 38 K [17]). The supercon-ducting transition temperature (Tc) was determined by the inter-section of two extrapolated lines from 90% and 10% of theresistivity curve (schematically shown in Fig. 3b). Further increasein Yb substitution (x) in Ce1�xYbxO0.9F0.1FeAs leads to gradualenhancement of Tc to 47.1 K and 48.7 K for ‘x’ = 0.4 and ‘x’ = 0.5respectively. The maximum Tc was observed for ‘x’ = 0.5 (48.7 K)composition which is highest among the Ce(O/F)FeAs supercon-ductors [9,17]. The calculated residual resistivity values(RRR = R300K/R50K) were found to be 3.6, 4.7 and 3.5 for ‘x’ = 0.3,0.4 and 0.5 compositions, respectively. The small value of RRR indi-

20 30 40 50 60

##

Ce1-xYbxO0.9F0.1FeAs

(003

)

(101

)

(102

)

x= 0.3

x = 0.4

x = 0.5

Cou

nts

2θ (Degrees)

(110

)(1

11) (112

)

(004

) (200

)(1

04) (114

)

(212

)

##

*#

*

#

#

o

Fig. 1. Powder X-ray diffraction patterns of Ce1�xYbxO0.9F0.1FeAs. The impurityphases are Fe2As (�), Yb2O3 (#) and CeAs (0).

20 25 30 35 40 45 50 550.0000

Temperature (K)

Fig. 3. Temperature dependence of resistivity (q) for (a) Ce1�xYbxO0.9F0.1FeAs(‘x’ = 0.3, 0.4 and 0.5) and (b) q vs T plot for Ce0.5Yb0.5O0.9F0.1FeAs showing thecriterion for evaluating Tc.

cates inhomogeneity in samples. Decrease in lattice parameters(chemical pressure) through substitution at Ln sites leading to in-crease in Tc as has been reported earlier [18,19]. On comparisonwith other superconducting Ln-1111 phases [18,20] it is clear thatthe oxygen-deficient and yttrium substituted Ln-1111 phasesshow a gradual increase in Tc with decrease in ‘a’- lattice parameterup to a certain level of doping. Reduction in the lattice parameterson substitution of smaller Ln ions lead to shorter Fe–As bondlengths which increases the Fe–As hybridization and changes thebandwidth leading to the enhancement in Tc. The temperature

0 50 100 150 200 250 300-0.30

-0.25

-0.20

-0.15

-0.10

-0.05

0.00

0.05

χ dc x

10-2

(em

u/g*

Oe)

Temperature (K)

x = 0.5

(c)

Tc = 44 K

0.30 0.35 0.40 0.45 0.5038

40

42

44

46

48

50

T c(K

)

Yb content (x)

TC

TC

Ce1-xYbxO0.9F0.1FeAs(d)

0 50 100 150 200 250 300-0.15

-0.12

-0.09

-0.06

-0.03

0.00

Temperature (K)

χ dcx 1

0-2

(em

u/g*

Oe)

Tc = 39 K

(a)

x= 0.3

0 50 100 150 200 250 300

-0.20

-0.15

-0.10

-0.05

0.00

0.05

Temperature (K)

Yb = 0.4

Tc = 42 K

χdc

x 10

-2(e

mu/

g*O

e)

(b)

Fig. 4. The variation of dc magnetic susceptibility with temperature for (a) Ce0.7Yb0.3O0.9F0.1FeAs, (b) Ce0.6Yb0.4O0.9F0.1FeAs, (c) Ce0.5Yb0.5O0.9F0.1FeAs and (d) variation ofmagnetic and resistive Tc with Ytterbium content (x).

G.S. Thakur et al. / Physica C 480 (2012) 71–74 73

dependence of mass magnetic susceptibility (emu/gOe) with ap-plied field of 5 Oe for Ce1�xYbxO0.9F0.1FeAs (‘x’ = 0.3, 0.4 and 0.5)are shown in Fig. 4a–c respectively. The magnetic susceptibilitygradually decreases with reducing temperature and sharp diamag-netic transition corresponding to onset of superconductivity wasobserved at Tc 39.8 K (‘x’ = 0.3), 42 K (‘x’ = 0.4) and 44 K (‘x’ = 0.5).The lower value of Tc is observed in magnetization measurementsas compared to Tc estimated by resistivity measurements for all theYb doped superconductors (0.3 6 x P 0.5) (Fig. 4d) indicate smallinhomogenities in the samples.

We have also investigated the temperature dependence of theresistivity under different magnetic fields in order to study theupper critical field (Hc2) and flux pinning properties ofCe1�xYbxO0.9F0.1FeAs (‘x’ = 0.3, 0.4 and 0.5) superconductors. Asthe magnetic field is increased the onset of superconducting tran-sition temperature weakly shifts towards lower temperature, butzero transition temperature (irreversibility field (H�)) decreasessharply with significant broadening of superconducting transition(Fig. 5). The 90% and 10% value of normal state resistivity (qn) cri-teria was used to estimate the value of upper critical field (Hc2) andirreversibility field (H�). The zero temperature upper critical field(Hc2(0)) was calculated using Werthamer–Helfand–Hohenberg(WHH) formula [21]:

Hc2ð0Þ ¼ �0:693TcðdHc2=dTÞT¼Tc:

Using Hc2 vs T plot, upper critical field with respect to temper-ature was estimated by slope (dHc2/dT). The estimated value ofslopes (dHc2/dT) were found to be �4.81, �2.11 and �1.97 T/K for‘x’ = 0.3, 0.4 and 0.5 compositions, respectively. Using these valuesof Tc = 42.6 K, 47.1 K and 48.7 K, we found Hc2(0) = 142 T, 68 T and

66 T for ‘x’ = 0.3, 0.4, 0.5 compositions, respectively. Upper criticalfield of these superconductors decreases with increasing the ytter-bium substitution. The upper critical field for ‘x’ = 0.3 composition(�142 T) is higher than ytterbium free CeO0.9F0.1FeAs supercon-ductor (�94 T) [17]. Decrease in Hc2 with an increase in Tc was ob-tained due to the multiband nature of the superconductingsamples, which is common for most iron pnictide superconductors[17]. A similar dependence of Hc2 on increased doping levels is ob-served in F-doped La-1111 system, (Hc2 = 73 T for LaO0.95F0.05FeAs)where Hc2 decreases systematically with an increase in the dopantconcentration (x) in the underdoped region [22]. In our investiga-tion the maximum Hc2 of 142 T was obtained fromCe1�xYbxO0.9F0.1FeAs (‘x’ = 0.3) under doped sample which has thelowest Tc among the three compositions (‘x’ = 0.3, 0.4 and 0.5). Alsoa higher Hc2 corresponds to a stronger flux pinning potential whichindicates the stabilization of flux lines under a certain low dopinglevel (Yb = 0.3) and it might be unstable at over-doped region. Var-iation in Hc2 due to dopant concentration may also be understoodin terms of an increase in density of states due to lattice contrac-tion (Yb-doping) resulting in an enhancement in Tc and decreasein Hc2. Very recently Zhang et al. [23] reported that the multi-bandsuperconductivity in Fe-superconductors might complicate thebehavior of upper critical field, attributed to the disorder by chem-ical doping and anisotropy.

By using the values of upper critical field, the mean-fieldGinzberg–Landau coherence lengths (nGL = (u0/2pHc2)1/2) forCe1�xYbxO0.9F0.1FeAs (‘x’ = 0.3, 0.4, 0.5) were calculated to be�23.18 Å, �22.14 Å and �22.45 Å for ‘x’ = 0.3, 0.4, 0.5 composi-tions, respectively. The averaged coherence length of �23 Å, wasfound to be lower than La(O/F)FeAs (33 Å) [24].

27 30 33 36 39 42 450.00

0.03

0.06

0.09

0.12

0.15(m

−cm

)

T (K)

0T 1T 2T 3T 4T

28 30 32 34 36 38 40 42 44

0

1

2

3

4 dHc2 / dTT=Tc= - 4.81 T/K

H c2, H

*

Tc (K)

(a)

27 30 33 36 39 42 45 480.000

0.025

0.050

0.075

0.100

T (K)

0T 1T 2T 3T 4T

30 33 36 39 42 45 48 51

0

1

2

3

4

H c2, H

*

Tc (K)

dHc2 / dT T=Tc= - 2.11 T/K

(m−c

m)

(b)

27 30 33 36 39 42 45 48 510.00

0.02

0.04

0.06

30 33 36 39 42 45 48 51

0

1

2

3

4

Tc (K)

H c2, H

*

dHc2 / dT T=Tc= - 1.97 T/K

(m−c

m)

T (K)

0T 1T 2T 3T 4T

(c)

Fig. 5. The magnetic field dependence of resistivity as a function of temperature for(a) Ce0.7Yb0.3O0.9F0.1FeAs, (b) Ce0.6Yb0.4O0.9F0.1FeAs and (c) Ce0.5Yb0.5O0.9F0.1FeAs.Inset shows temperature dependence of upper critical field (j) and irreversibilityfield (d).

74 G.S. Thakur et al. / Physica C 480 (2012) 71–74

4. Conclusions

We have successfully synthesized ytterbium-dopedCe1�xYbxO0.9F0.1FeAs (‘x’ = 0.3, 0.4, 0.5) novel superconductors by

sealed tube method. Enhancement in superconducting transitiontemperature was observed with increase in Yb substitution inCeO0.9F0.1FeAs with maximum Tc of �49 K in ‘x’ = 0.5 compositionwhich is highest among known Ce(O/F)FeAs superconductors. Avery high value of upper critical field of �142 T was estimatedfor ‘x’ = 0.3 composition by WHH formula which is significantlyhigher than Yb free CeO0.9F0.1FeAs (�94 T).

Acknowledgements

A.K.G and S.A. thank DST and UGC Govt. of India financial sup-port. G.S. Thakur and J.P. thank DST and CSIR, Govt. of India, for fel-lowships. Authors thank DST for providing the national SQUIDfacility at IIT Delhi.

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