Evidence for Coexistence of Superconductivity and Magnetism in Single Crystals of Co-doped SrFe 2 As 2
Jun Sung Kim, Seunghyun Khim, Liqin Yan, N. Manivannan, Yong Liu, Ingyu Kim, G. R. Stewart, Kee Hoon Kim
aa r X i v : . [ c ond - m a t . s up r- c on ] D ec Evidence for Coexistence of Superconductivity andMagnetism in Single Crystals of Co-doped SrFe As Jun Sung Kim, Seunghyun Khim, Liqin Yan, N. Manivannan,Yong Liu, Ingyu Kim, G. R. Stewart † , and Kee Hoon Kim ∗ FPRD, Department of Physics and Astronomy, Seoul National University, Seoul151-747, Republic of KoreaE-mail: [email protected]
Abstract.
In order to investigate whether magnetism and superconductivity coexistin Co-doped SrFe As , we have prepared single crystals of SrFe − x Co x As , x = 0and 0.4, and characterized them via X-ray diffraction, electrical resistivity in zero andapplied field up to 9 T as well as at ambient and applied pressure up to 1.6 GPa,and magnetic susceptibility. At x = 0.4, there is both magnetic and resistive evidencefor a spin density wave transition at 120 K, while T c = 19.5 K - indicating coexistentmagnetism and superconductivity. A discussion of how these results compare withreported results, both in SrFe − x Co x As and in other doped 122 compounds, is given.PACS numbers: 74.70.Dd, 74.25.Ha, 74.25.Fy, 74.62.Fj
1. Introduction
The recent discoveries of ever-mounting transition temperatures in the superconductingiron arsenside 2-dimensional layered compounds, coupled with the goal of understandingthe pairing mechanism(s) of this newly discovered class of superconducting compounds,has lead to a surge of activity in materials-based condensed matter physics. ¿Froma superconducting transition temperature T c = 26 K in LaFeAs(O − x F x )[1] the valueis now up to T c = 55 K in SmFeAs(O − x F x )[2]. Of particular help in the quest forunderstanding this new physics has been the widening range of compounds in whichthe ”iron arsenide (FeAs)” based superconductivity has been found, moving from therather difficult materials synthesis of the original 1111 compounds with F-doping tothe more-easily-prepared 122 compounds (non-superconducting prototype BaFe As )discovered by Rotter et al. [3] These latter compounds, as was pointed out by Ni et al. [4] can be rather easily grown from a Sn flux as well as from an FeAs ’selfflux’[5]. Thus, much of the recent effort for elucidating the physics has focused onthese 122 compounds, with both polycrystalline and single crystal work. Single crystalsof course allow greater homogeneity and the possibility of following the anisotropy of thefundamental properties - often important in distinguishing the underlying mechanismsof superconductivity[6].A central question[7] for deciding on the superconducting pairing mechanism inthese FeAs superconductors has been the interplay/relationship between the ubiquitousmagnetic behavior in the undoped, non-superconducting starting compounds (either the1111 family or the A Fe2As2, where A = Ca, Sr, Ba, and Eu) which is then suppressedby the doping. In the 122 family, K/Na/Cs, or hole doping, on the A -site or Co/Ni -electron doping[5] - on the Fe site) induces superconductivity. Whether the magnetic(spin density wave, ’SDW’) behavior is coupled to the occurrence of superconductivityin SrFe As doped with Co is a main subject of the present work, using single crystalsprepared in Sn flux.The question ”does the SDW coexist with superconductivity in FeAs superconduc-tors?” might seem straightforward to answer. However, even in just the 122 compounds,there exist at present four starting compounds A Fe As ( A = Ca, Sr, Ba, and Eu) withboth hole (including work on Na, K and Cs) and electron (Co and Ni) doping, and aswell the very important materials aspects of both single- and poly-crystalline samples.Even a cursory review of the current status of this 4 (Ca, Sr, Ba, Eu) × × T SDW and induces superconductivity varies widely between the var-ious A atoms and either hole- or electron-dopants, which is a sign of the richness ofthis new class of materials. However, there are also conflicts in some results on thesame A atom and the same dopant which involve disagreements in concentration de-pendence of, e. g. , T SDW , in whether the SDW transition is first or second order in, e.g. , SrFe As [8, 9, 10, 11], and even in the quite fundamental question of coexistence ofmagnetism and superconductivity itself (see Table 1). Our work on the electron dopedSrFe As is the first to be done on single crystals in this compound (with one report onpolycrystalline samples[12] and one on thin films[13]), bringing an initial data set forthe 4 × × As single crystals show a structural phasetransition from a high-temperature tetragonal phase to a low-temperature orthorhombicphase at the same temperature as the SDW, T o = 198 K [11] similar to the behaviorobserved in the BaFe As compound[4].As summarized in Table 1, the relation between magnetic behavior andsuperconductivity in the 122 FeAs superconductors has been addressed quite thoroughlyfor A = Ba, but somewhat less so for A = Ca, Sr, and Eu. There is also growing workon electron doping (primarily Co replacing Fe) for all the A species listed. As detailedin Table 1, at present the question of whether magnetism in the form of a SDW coexistswith superconductivity in doped A Fe As is still controversial.Some of the disagreement in resolving the issue of coexistence of magnetism andsuperconductivity in the doped 122 A Fe As materials made apparent by the summaryin Table 1 can be resolved as merely based on interpretation. For example, some authors( e. g. see Refs. [17, 18]) have stated that the SDW transition is suppressed based on thelack of sharp structure in ρ vs T data, although a shoulder that might be indicative of aweak transition exists in their data. However, some of the disagreements appear to befundamentally unresolvable at this time. One example of this involves contrasting T SDW vs x results even in high quality single crystals of BaFe − x Co x As by X. F. Wang et al. [21]and by J.-H. Chu et al. [22] Such disagreement is independent of any interpretation.Two important lessons to be drawn from the summary in Table 1 on single crystalSrFe − x Co x As are: (1) a fine gradation in composition in BaFe − x Co x As was shownto be necessary for determining whether T SDW has been suppressed to T = 0 whensuperconductivity first appears[21, 22, 23] (2) Some of the work on polycrystallinesamples has been found to disagree with single crystal work, partly at least for reasonsstill under discussion, thus obscuring any possible conclusions. In general, althoughsingle crystals grown in Sn-flux can have small inclusions of Sn[4], single crystals shouldbe more homogeneous than sintered polycrystalline material. In order to addresspoint (1), we are working on single crystals of SrFe − x Co x As , x = 0.1, 0.2, 0.3,and 0.5 in addition to the work on x = 0 and 0.4 reported here. However, as willbe discussed below, T SDW is suppressed much less rapidly with Co in SrFe − x Co x As than in BaFe − x Co x As , and our present work on x = 0 and 0.4 is sufficient to showthe coexistence of magnetism and superconductivity in SrFe − x Co x As - contradictingconclusions based on polycrystalline SrFe − x Co x As [12](see Table 1).
2. Experimental
Single crystals of Co-doped SrFe As were grown using high temperature solution growthtechniques with a Sn flux[4]. Stoichiometric amounts of the elemental Sr, Fe, Co andAs were added to Sn with the ratio of [SrFe − x Co x As ] : Sn = 1 : 20 and placed in an Table 1.
Survey of previous doping results in 122 FeAs superconductors. Units oftemperature are Kelvin; results are for either single- or poly-crystalline samples. Itis worth noting that some authors, well-focused on the difficulty of answering thecoexistence question precisely, have used more precise determination of T SDW ( e. g. ,Wang et al. [21] used specific heat; Zhang et al. [16] used band splitting measured byphotoemission). A − x A ′ x Fe − y Co y As Dopant x,y T SDW T c Coexistence single [Ref.]( A ′ = K, Na) (yes/no) /poly A = Ca Co . none 17 no single [14]Na . none 20 no poly [15] A = Sr K .
135 25 yes single [16]K . none 38 no single [17]K . none 20 ∗ no poly [18]Na .
160 35 yes single [19]Co . none 19.2 no poly [12] A = Ba K .
70 37 yes single [19]K . , . .
75 9 yes single [21]Co . . none 22 no single [5,23] A = Eu K . none 32 no poly [24]Pressure 115 30 yes single [25] ∗ Annealed polycrystalline Sr . K . Fe As changes T c from 38 to 20 K. In theunannealed state, there is an anomaly in ρ at 200 K indicative of SDW and T c =38 K [18]. alumina crucible, which was sealed in a silica ampoule in vacuum. All the handling ofthe elements was performed in a glove box with an Ar atmosphere (oxygen < O < o C (duration of 4 hours), thento 1100 o C (duration of 4 hours). After this, the sample was slowly cooled down to 500 o C at the rate of 4 o C /hour and then the plate-like single crystals of typical dimensions10 × × were removed from the Sn flux by centrifuging[4].Resistivity measurements were made by a standard 4-wire ac method, using aQuantum Design PPMS TM system in fields up to 9 T. Due to the large flat faces ofthe crystals, where the c-axis is perpendicular to the face, alignment of the field eitherparallel to the c-axis or in the ab-plane was straightforward. Magnetic susceptibilitymeasurements were performed in the same Quantum Design PPMS TM system.
3. Results and discussion
X-ray diffraction measurements were carried out on a single crystal from both of thecompositions x = 0 and 0.4. As shown in Fig. 1, only (00 l ) reflections with even l appear, indicating that the c -axis is perpendicular to the crystal plate. The addition ofCo decreased the c -axis lattice parameter in these single crystals. However, for reasons
20 30 40 50 60 70 80
20 40 60 80 100 ( )( )( )( ) x = 0 x = 0.4 I n t en s i t y ( a r b . un i t s ) (a) ( ) D c ( A ) x (b) * ** o q (deg.) o * o : Sn* : Fe Figure 1. (Color online) X-ray diffraction patterns for (a) single crystal and (b)crushed powder of SrFe − x Co x As with x = 0 and 0.4. The upper left inset shows thedecrease of the c -axis lattice constant (∆ c ) in the single crystals of the present work dueto Co doping (solid circle). For comparison, we also plot the ∆ c vs x for polycrystallineSrFe − x Co x As ( × symbols)[12]. The upper middle inset shows a photo of a x = 0.4single crystal. Note the marked second phase lines in the powder pattern (b), wherein addition to a slight amount of Sn inclusion from the flux, some excess Fe is seen. that will become clear below in the discussion of the resistivity data for x = 0.4, in orderto investigate possible crystal inhomogeneity and impurities below the rather shallowpenetration of the X-ray beam ( ∼ a few µ m) in the single crystals, we undertook X-raydiffraction of powders made of individual single crystals. These data, shown also in Fig.1, provide a measurement of both the a - and c -axis lattice parameters and are morecharacteristic of the bulk of the crystal. These powder diffraction lattice parameterresults agree with the single crystal results. The results for x = 0 were a = b = 3.928(3)˚A, c = 12.392(9)˚A, while a = b = 3.925(3) ˚A, c = 12.33(1) ˚A for x = 0.4. For x = 0, the c -axis lattice parameter is consistent with some previous reports on poly-[8]and single-crystal materials[17], but is slightly larger than the polycrystalline resultsof Leithe-Jasper et al. [12] With Co doping, it has been shown that the c-axis lattice x = 0 x = 0.4, S1 x = 0.4, S2 / K T (K) (b) x = 0 x = 0.4 M / H ( - e m u / g ) Figure 2. (Color online) Resistivity vs temperature (a) for single crystalSrFe − x Co x As , x = 0 and 0.4 (two samples from the same growth batch, labeledS1 and S2), showing the anomalies at T SDW (202 K for x = 0 and ≈
120 K for x =0.4). On an expanded plot, not shown, extrapolations of the higher temperature andseparately the lower temperature resistivity data from the data for S1 intersect at thetemperature marked by the red arrow. The plot in (b) shows the M/H (measured in7 T), where the SDW anomalies for both x = 0 and 0.4 are clear. parameter decreases linearly with Co concentration[12, 21, 22].Considering the inconsistency in the the absolute value of the c -axis latticeparameters in the literature, we focused here on a comparison of the change (contraction)of the c -axis in our Co-doped crystals with that found in polycrystalline works[12] (seeFig. 1). According to Leithe-Jasper et al. a c -axis contraction of -0.07(1)˚A is expectedfor x = 0.4, which is comparable with our result of -0.06(2) ˚A for our Co-doped singlecrystal. This provides a bulk proof for the presence of approximately x = 0.4 Co in ourCo-doped single crystals.Resistivity and susceptibility data for x = 0 and 0.4 are shown in Figure 2.Discussing the normal state properties first, as shown clearly in Fig. 2, our singlecrystals for x = 0.4 show differing resistivity behaviors below T SDW : one crystal (S2)shows evidence for strong scattering below T SDW while another crystal (S1) shows onlya slight change in slope (marked by the red arrow). [We have measured a total of6 different single crystals out of the same growth batch for x = 0.4, and the strongincrease in ρ below T SDW is found in two samples. We are continuing to investigate this.]This sample dependence is of course reminiscent of early sample dependence problemsin ρ in YBa Cu O − δ . However, in both crystals (as well in all the other crystalsmeasured from this x = 0.4 batch), the superconducting transition is consistently at T c = 19.5 K. Clearly, the magnetic anomaly for SrFe . Co . As is a clearer evidencefor a SDW transition at 120 K than the slight change of slope/broad hump in theresistivity data that is characteristic of most of our samples. In the polycrystalline workon SrFe − x Co x As [12], the resistivity curve for a nonsuperconducting sample of x = 0.1increases below T ∼
130 K similar to the S2 data for the x = 0.4 single crystal in Fig.2. The polycrystalline resistivity data[12] for x ≥ T c = 19.2 K for x = 0.2) showpositive curvature vs temperature between T c and 300 K, i.e. unlike both the S2 andS1 resistivity curves for our single crystal SrFe − x Co x As shown in Fig. 2. Thus, if itwere not for the good agreement in the lattice contraction for the same compositions( x = 0 and 0.4) in the present single crystal work compared to the polycrystallinework[12], both the difference in the behavior of T SDW and T c would have called thecomparability of the Co-compositions into question. As it is, it would be useful formagnetic susceptibility data to higher temperatures than 25 K (the upper limit in Ref.12) to be measured on the polycrystalline samples. At this time it is not clear why thereis disagreement between T SDW ( x ) results determined by resistivity data on poly-[12] andsingle-crystalline (present work) samples of SrFe − x Co x As .As stated in the Introduction, the field of FeAs superconductivity is in a state offlux at present. The variation of the resistivity seen in our single crystals for x = 0.4,and the disagreement between our single crystal compositional dependence of T SDW and T c compared to polycrystalline[12] results is perhaps one reason why some of these openquestions must remain open until better understanding of sample quality is achieved.The superconducting transition temperature for x = 0.4 SrFe − x Co x As singlecrystals is 19.5 K, which is comparable with the maximum T c achieved by Co-doping inpolycrystalline SrFe As [12] and coexists with the magnetic transition at T SDW ≈ As , the magneticphase is more robust in Co-doped SrFe As . For BaFe As , the T SDW is decreasedrapidly with a relatively small amount of Co substitution, i. e. , x = 0.12, which issufficient to fully suppress the SDW transition and induce the maximum T c ≈
24 K. Incontrast, we still observe the clear magnetic transition at T SDW ≈
120 K with x = 0.4in SrFe − x Co x As with T c ≈
20 K. This result may be related to the higher T SDW ≈
202 K in SrFe As than that of BaFe As ( T SDW ≈
140 K).The temperature-dependence of H c ( T ), defined by 90% of the resistive transition
16 18 200246810 ab ( m c m ) H//ab(a)
H//c
T (K)(b) H // ab H c ( T ) T (K) H // c Figure 3. (Color online) Temperature dependence of the ab -plane resistivity of singlecrystal SrFe . Co . As with different magnetic fields along the ab -plane (a) and the c -axis (b). The inset shows the H c ( T ) curves near T c for the two field directions, H k c and H k ab . is shown in the inset of Fig. 3. Both H abc and H cc showed almost linear temperaturedependence with slopes of dH abc /dT = -3.9 T/K and dH cc /dT = -2.2 T/K. Thezero temperature upper critical fields can be estimated using the Werthamer-Helfand-Hohenberg formula, H c (0) = -0.69 T c ( dH c /dT ) | T c , yielding H cc (0) = 30 T and H abc (0)= 53 T. The corresponding coherence lengths are 33 ˚A and 19 ˚A along the ab -plane andthe c -axis, respectively. The c -axis coherence length is comparable with the distancebetween two adjacent FeAs layers, d ∼ H abc /H cc derived from the data inFig. 3 is Γ ≈ ≈ As andK-doped SrFe As [17] but significantly lower than Γ ≈ . Co . As , we present the pressure dependence of T c in Fig. 4. Gooch et al. [26]reported on T c ( P ) in polycrystalline Sr . K . Fe As , T onsetc = 37 K, and find an increasein T onsetc at 0.9 GPa of about 1.2 K, or about 3 %, compared to our result for electron-doped SrFe As where T c increases by about 1.8 K, or about 9 % with 0.9 GPa. Gooch et al. also see some saturation in the rise of T c with pressure in their 1.7 GPa datacomparable to what we observe (see inset to Fig. 4). From previous pressure work on
15 20 25 30 350.00.20.40.60.81.0 / K T (K) T c ( K ) P (GPa)
Figure 4. (Color online) Superconducting transition temperatures for single crystalSrFe . Co . As determined by the resistivity as a function of hydrostatic pressure the K-doped A Fe As ( A = Ba, Sr) compounds, it has been found that the pressuredependence of T c reflects the ”dome” shape of the doping dependence of T c [26]. Theunderdoped and overdoped samples show positive and negative pressure dependence,respectively, while almost no pressure dependence of T c is observed in the optimallydoped sample. As mentioned already, our x = 0.4 crystal shows T c = 19.5 K, close tothe maximum T c of Co-doped polycrystalline SrFe As [12], thus in the optimal dopingregime. The sizable pressure dependence of T c in our crystal, therefore, suggests thatthere is still room for improving the superconducting transition temperature by furthertuning, e. g. , using external pressure. Similar behavior has been also observed inoptimally Co-doped BaFe As [27]. This different behavior between K-doped and Co-doped 122 compounds indicates that the pressure dependence of T c is determined notjust by the doping level of the FeAs layer but also reflects more complex interplay withother parameters such as the degree of hybridization between the Fe and As states thatcan be tuned by, e. g. , a bonding angle of the Fe-As-Fe network[28].
4. Summary and Conclusion
Our present work on single crystal SrFe − x Co x As ( x = 0 and 0.4) shows clear signaturesin both electrical resistivity and magnetization curves for the presence of a spin densitywave at 202 and 120 K, respectively. The x = 0.4 sample shows superconductivity at19.5 K, which - in the spirit of the work on the FeAs superconductors to date (seeTable 1) - allows the conclusion that superconductivity is coexistent with magnetism0(SDW) in single crystal SrFe . Co . As . Of course, a microscopic determination ofthe coexistence below T c is further required. Both the single crystal and powder X-ray diffraction characterization of our samples show internal consistency as well asagreement of the lattice contraction with Co doping, compared to the polycrystallinework on SrFe − x Co x As [12]. In contrast, our compositional dependence of both T SDW and T c disagree with the polycrystalline data in Ref. 12 which does not report magneticsusceptibility. The anisotropy of the upper critical field H c in our single crystals ofSrFe . Co . As is consistent with K- or Co-doped BaFe As and K-doped SrFe As [17].The pressure dependence of T c of our single crystalline SrFe . Co . As is, when expressedas a percentage of T c ( P = 0), much larger than that observed[26] in K-doped SrFe As .An important conclusion that can be drawn from our present work is that evenin single crystals there appears to be significant sample dependence at least in theresistivity below T SDW , while T SDW and T c themselves did not show any sampledependence. Our results clearly show sample dependence in the resistivity, as well as anunexplained difference between our single crystal and Ref. 12’s polycrystalline values of T SDW and T c as a function of Co-concentration. This may be a useful cautionary noteabout the rush to make firm conclusions in the early stages of the fascinating study ofsuperconductivity in the 122 FeAs compounds. Acknowledgments
This work was supported by the National Research Lab program (M10600000238)and KICOS through a grant provided by the MEST (K20702020014-07E0200-01410).Work at Florida supported by the US Department of Energy, contract no. DE-FG02-86ER45268. JSK is supported by BK21 Frontier Physics Research Division.
References ∗ Corresponding author. † On sabbatical from Department of Physics, University of Florida. [1] Kamihara Y, Watanabe T, Hirano M, and Hosono H 2008
J. Am. Chem. Soc.
Chin. Phys. Lett. Phys. Rev. Lett.
Phys. Rev. B Phys. Rev. Lett.
Rev. Mod. Phys. et al. Priprint arXiv0811.0034 who state ”Our result strongly suggeststhe magnetic fluctuation as the pairing mechanism for the superconducting ground state” inthese Fe-based superconductors.[8] Krellner C, Caroca-Canales N, Jesche A, Rosner H, Ormeci A, and Geibel C 2008 Phys. Rev. B [9] Chen G F, Li Z, Li G, Hu W Z, Dong J, Zhang X D, Zheng P, Wang N L, and Luo J L 2008 Chin.Phys. Lett. Priprint arXiv0806.4782.[11] Yan J -Q, Kreyssig A, Nandi S, Ni N, Bud’ko S L, Kracher A, McQueeney R J, McCallum R W,Lograsso T A, Goldman A I, Canfield P C 2008
Phys. Rev. B. Priprint arXiv0807.2223.[13] Hiramatsu H, Katase T, Kamiya T, Hirano M, and Hosono H 2008
Appl. Phys. Express Priprint arXiv0810.0848.[15] Wu G, Chen H, Wu T, Xie Y L, Yan Y J, Liu R H, Wang X F, Ying J J, and Chen X H
Priprint arXiv0806.4279.[16] Zhang Y, Wei J, Ou H W, Zhao J F, Zhou B, Chen F, Xu M, He C, Wu G, Chen H, Arita M,Shimada K, Namatame H, Taniguchi M, Chen X H, Feng D L
Priprint arXiv0808.2738.[17] Chen G F, Li Z, Dong J, Li G, Hu W Z, Zhang X D, Song X H, Zheng P, Wang N L, and Luo JL 2008
Phys. Rev. B Chin.Phys. Lett. Priprint arXiv0808.1425.[20] Chen H, Ren Y, Qiu Y, Bao W, Liu R H, Wu G, Wu T, Xie Y L, Wang X F, Huang Q, and ChenX H
Priprint arXiv0807.3950.[21] Wang X F, Wu T, Wu G, Liu R H, Chen H, Xie Y L and Chen X H
Priprint arXiv0811.2920.[22] Chu J-H, Analytis J G, Kucharczyk C, Fisher I R
Priprint arXiv0811.2463.[23] Ning F L, Ahilan K, Imai T, Sefat A S, Jin R, McGuire M A, Sales B C, Mandrus D
Priprint arXiv0811.1617.[24] Jeevan H S, Hossain Z, Kasinathan D, Rosner H, Geibel C, and Gegenwart P 2008
Phys. Rev. B , 092406.[25] Miclea C F, Nicklas M, Jeevan H S, Kasinathan D, Hossain Z, Rosner H, Gegenwart P, Geibel C,and Steglich F Priprint arXiv0808.2026.[26] Gooch M, Lv B, Lorenz B, Guloy A M, and Chu C-W 2008