We studied the temperature-pressure phase diagram of EuFe2As2 by measurements
of the electrical resistivity. The antiferromagnetic spin-density-wave
transition at T_0 associated with the FeAs-layers is continuously suppressed
with increasing pressure, while the antiferromagnetic ordering temperature of
the Eu 2+ moments seems to be nearly pressure independent up to 2.6 GPa. Above
2 GPa a sharp drop of the resistivity, \rho(T), indicates the onset of
superconductivity at T_c \approx 29.5 K. Surprisingly, on further reducing the
temperature \rho(T) is increasing again and exhibiting a maximum caused by the
ordering of the Eu 2+ moments, a behavior which is reminiscent of re-entrant
superconductivity as it is observed in the ternary Chevrel phases or in the
rare-earth nickel borocarbides

Gegenwart, P.

Geibel, C.

Hossain, Z.

Jeevan, H. S.

Kasinathan, D.

Miclea, C. F.

Nicklas, M.

Rosner, H.

Steglich, F.

English

arXiv

C. F. Miclea

M. Nicklas

H. S. Jeevan

D. Kasinathan

Z. Hossain

H. Rosner

P. Gegenwart

C. Geibel

F. Steglich

Crossref

Evidence for a reentrant superconducting state inEuFe2As2under pressure

We studied the temperature-pressure phase diagram of EuFe2As2 by electrical resistivity measurements. The spin-density-wave transition at T-0 associated with the FeAs-layers is continuously suppressed with increasing pressure, while the antiferromagnetic ordering temperature of the Eu2+ moments seems to be nearly pressure independent up to 2.6 GPa. Above 2 GPa a sharp drop of the resistivity, rho(T), indicates the onset of superconductivity at T-c approximate to 29.5 K. Surprisingly, on further reducing the temperature, rho(T) is increasing again and exhibiting a maximum caused by the ordering of the Eu2+ moments, a behavior which is reminiscent of reentrant superconductivity as it is observed in the ternary Chevrel phases or in the rare-earth nickel borocarbides

Jeevan, Hirale S.

Kasinathan, Deepa

Rosner, Helge

Gegenwart, Philipp

Geibel, Christoph

Steglich, Frank

GRO.publications (Univ. Göttingen)

Evidence for a reentrant superconducting state in EuFe2As2 under pressure

MPG.PuRe

Max Planck Institute for Chemical Physics of Solids, Max Planck Society

Evidence for a reentrant superconducting state in
                    
                      
                        
                          
                            EuFe

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OPUS Augsburg

Evidence for a reentrant superconducting state in EuFe2As2 under pressureC. F. Miclea,1,* M. Nicklas,1,† H. S. Jeevan,2 D. Kasinathan,1 Z. Hossain,3 H. Rosner,1 P. Gegenwart,2 C. Geibel,1 andF. Steglich11Max Planck Institute for Chemical Physics of Solids, Nöthnitzer Str. 40, 01187 Dresden, Germany2I. Physik. Institut, Georg-August-Universität Göttingen, 37077 Göttingen, Germany3Department of Physics, Indian Institute of Technology, Kanpur 208016, IndiaReceived 6 March 2009; revised manuscript received 26 May 2009; published 15 June 2009We studied the temperature-pressure phase diagram of EuFe2As2 by electrical resistivity measurements. Thespin-density-wave transition at T0 associated with the FeAs-layers is continuously suppressed with increasingpressure, while the antiferromagnetic ordering temperature of the Eu2+ moments seems to be nearly pressureindependent up to 2.6 GPa. Above 2 GPa a sharp drop of the resistivity, T, indicates the onset of supercon-ductivity at Tc29.5 K. Surprisingly, on further reducing the temperature, T is increasing again andexhibiting a maximum caused by the ordering of the Eu2+ moments, a behavior which is reminiscent ofreentrant superconductivity as it is observed in the ternary Chevrel phases or in the rare-earth nickelborocarbides.DOI: 10.1103/PhysRevB.79.212509 PACS numbers: 74.62.Fj, 74.10.v, 74.25.Dw, 74.70.DdThe recent discovery of high-temperature superconductiv-ity SC in the RFeAsO compounds R=La,Ce,Pr,Nd,Sm,Gd1–6 with superconducting transition temperature val-ues above to 50 K Refs. 3 and 4 has attracted strong inter-est in the scientific community. The RFeAsO compoundsforming in the ZrCuSiAs-type tetragonal structure areclosely related to AFe2As2 A=Ca,Sr,Ba7–9 forming in theThCr2Si2-type tetragonal structure. Both share a similar ar-rangement of Fe2As2 layers assumed to be the key for the SCoccurrence in this class of compounds. Materials from bothfamilies show at T0150–210 K a structural transitionfrom a tetragonal to an orthorhombic phase which is closelyrelated to the formation of a spin-density-wave SDW typemagnetic instability.7,8,10–12In contrast to the AFe2As2 A=Ca,Sr,Ba compoundswhere only the iron possesses a magnetic moment, inEuFe2As2 an additional large, local magnetic moment of7.5B is carried by Eu2+.13 Like the A=Ca,Sr,Ba membersof the AFe2As2 family, EuFe2As2 exhibits a SDW transitionaround T0=190 K related to the Fe2As2 layers but in addi-tion, the magnetic moments of the localized Eu2+ order atTN=20 K10,13,14 in a so-called A-type antiferromagneticAF structure.15 SrFe2As2 has similar structural properties toEuFe2As2, its unit-cell volume being only 3% larger, and acomparable value of T0=210 K.8,16 Furthermore, aside fromthe Eu-4f part, the electronic density of states DOS ofEuFe2As2 is almost identical to that of SrFe2As2.10 There-fore, SrFe2As2 can be considered a nonmagnetic homolog ofEuFe2As2. Provided that the two different kinds of magneticordering phenomena are reasonably well decoupled, the re-sults of previous doping and pressure studies on SrFe2As2Refs. 17–19 would suggest the appearance of a supercon-ducting phase on doping and/or at sufficiently high pressurein EuFe2As2, too. In fact, a recent investigation20 confirmedthe first prediction: K0.5Eu0.5Fe2As2 is superconducting be-low Tc=30 K. Further on, as a result of the replacement ofhalf of the Eu by K, no clear signature of the ordering of theEu2+ is present anymore.Our electrical resistivity measurements under hydrostaticpressure on single crystalline EuFe2As2 indicate that theSDW transition is continuously suppressed upon applyingpressure, while the magnetic ordering of the Eu2+ momentsseems to be very robust against pressure. Above 2 GPa, asharp drop of the resistivity indicates the emergence of asuperconducting phase below Tc29.5 K this value beingclose to the one reported for K0.5Eu0.5Fe2As2.20 Remarkably,we found an indication of reentrant SC as observed for ex-ample in the ternary Chevrel phases, e.g., HoMo6S8 Ref. 21or in the rare-earth nickel borocarbides, e.g., HoNi2B2C.22Single crystals of EuFe2As2 were synthesized using theBridgman method.10 Powder x-ray diffraction confirmed theproper ThCr2Si2-type tetragonal structure and the singlephase nature of the sample. Measurements of the electricalresistance were carried out using a standard four-probe tech-nique with current flowing in the a ,b-plane and magneticfield applied parallel to the current. The investigations weredone from room temperature down to 1.8 K and in magneticfields up to 7 T using a physical property measurement sys-tem PPMS, Quantum Design. Pressures up to 2.6 GPa havebeen generated using a double-layer piston-cylinder-typepressure cell with an inner cylinder made from MP35N. Sili-cone oil was used as pressure transmitting medium. The tem-perature dependence of the superconducting transition tem-perature of a Pb sample mounted along the whole samplespace, served as a pressure gauge. The narrow and pressureindependent SC transition width of Pb confirmed the quasi-hydrostatic pressure conditions inside the pressure cell. Den-sity functional band-structure calculations within the localspin density approximation LSDA have been performedusing a full potential code FPLO.24 The strong Coulomb re-pulsion in the Eu 4f orbitals have been included in a mean-field level using the atomic limit, double-counting scheme25in the LSDA+U approximation. A value of U=8 eV wasused for the Eu 4f orbitals, but the resultant conclusions didnot change within a range from 6 to 10 eV. The total energieswere calculated on a dense mesh of k points using thePerdew-Wang26 exchange-correlation potential.Figure 1 shows the electrical resistivity, T, ofEuFe2As2 for different applied pressures. The absolute valueof the room-temperature resistivity at ambient pressure,PHYSICAL REVIEW B 79, 212509 20091098-0121/2009/7921/2125094 ©2009 The American Physical Society212509-1300 K1.6 m cm, is typical for the AFe2As2 materials. Inthe whole investigated pressure range the resistivity de-creases continuously on decreasing temperature, with the ex-ception of a clear anomaly indicating a SDW type of mag-netic transition at T0 at low pressures. With increasingpressure this anomaly broadens. At p=2.3 GPa only achange in slope in T is remaining; however, T0 can be stilldetermined from the local minimum in the second derivativeof the resistivity, 2 /T2. At higher pressures this anomalycannot be any longer detected unambiguously. At low tem-perature a second anomaly appears around TN20 K, indi-cating the magnetic ordering of the Eu2+ moments. TNpseems to be nearly pressure independent. At p=2.03 GPa forthe first time a sharp drop of the resistivity appears aroundTc=29.5 K. With increasing pressure this drop becomeseven sharper and more pronounced, while its position staysalmost constant. We ascribe this feature to the onset of SC.The complete formation of the superconducting state is in-terrupted by the ordering of the Eu2+ sublattice at TNTc.The ordering of the Eu2+ ions causes an initial increase inT followed by a maximum on lowering the temperature.Our finding is reminiscent of reentrant SC in magnetic su-perconductors as has been previously observed for examplein HoMo6S8 Ref. 21 and in HoNi2B2C.22 In the pressurerange p2.03 GPa we take TN as the temperature where theresistivity reaches its maximum below Tc. In the rare-earthnickel borocarbides this has been shown to be the properprocedure to determine TN from resistivity, in this region ofthe phase diagram. To exemplify, in the inset of Fig. 1 thepositions of Tc and TN for two different pressures, 2.03 and2.3 GPa, are marked by arrows. The resulting pressure-temperature phase diagram is presented in Fig. 2.To get further insight in the relation of magnetic orderingof the Eu2+ moments and SC in EuFe2As2 under pressure wemeasured T at p=2.16 GPa in different external magneticfields cf. Fig. 3. As discussed before, the zero-field resis-tivity data show the typical behavior found in reentrant su-perconductors: after a drop at Tc, T is increasing again,showing a maximum at TN, before further decreasing uponcooling. On increasing the magnetic field, the drop in resis-tivity shifts to lower temperatures. Already at a field of B=0.5 T, no maximum in T is visible anymore. Although adrop in resistivity is present in the whole investigated fieldrange up to B=7 T, there is a qualitative difference betweenmagnetic fields B3 T and B3 T. In the former caseT is decreasing much stronger compared with the latter,dividing the data sets in two distinct groups. The small re-duction in the resistivity in large magnetic fields is mostlikely still due to superconductivity since the AF phaseseems to be suppressed at lower fields, B2 T.15,23 To con-struct a T-B phase diagram we define Tx as the local mini-mum of the second derivative of T, 2 /T2, minimumwhich corresponds to the kink in T. The phase diagram isdepicted in the inset of Fig. 3. Upon reducing T, at lowfields, the upper critical field Bc2 is increasing linearly. Be-tween 3 and 7 T, BxT shows a strong upturn. The resultingT-B phase diagram shows a clear enhancement of Bc2 forfields above B=3 T and temperatures below T20 K. Wefind an initial slope of Bc2 /TTc =0.368 T /K much smallerthan the value found for SrFe2As2Bc2 /TTc =2.05 T /K.17This would yield, for the dirty limit, an orbital critical field0 100 200 30000.20.40.60.81.01.21.41.61.80 10 20 30 4000.20.4p (GPa)0.211.302.032.162.30(mcm)T (K)EuFe2As2pTN TcTN(mcm)T (K)TcFIG. 1. Color online Electrical resistivity vs temperature fordifferent applied pressures for single crystalline EuFe2As2. Inset:T at low temperature for p=2.03 GPa and p=2.3 GPa. The su-perconducting and the antiferromagnetic are marked by arrows.0 0.5 1.0 1.5 2.0 2.5050100150200EuFe2As2T(K)p (GPa)T0TcTNFIG. 2. Color online Pressure-temperature phase diagram ofEuFe2As2 obtained from T data. The lines are guides to the eyes.0 10 20 30 40 50 60 700.20.40246815 20 25 30B (T)2357B (T)00.51(mcm)T (K)EuFe2As2p=2.16 GPaTNT (K)B(T)FIG. 3. Color online Electrical resistivity at p=2.16 GPa indifferent magnetic fields. Inset: temperature-magnetic field phasediagram. See text for details.BRIEF REPORTS PHYSICAL REVIEW B 79, 212509 2009212509-2of only about Bc2orb0=0.69 TcBc2 /TTc 7.5 T. How-ever, for B3 T we find Bc2 /T=1.1 T /K.Recently it has been shown that there is a volume collapseunder pressure that precedes the onset of superconductivityin CaFe2As2.27,28 Unfortunately, structural or spectroscopicdata for EuFe2As2 under pressure are unavailable up to now.Thus the first problem we addressed with our band-structurecalculations as a function of pressure for EuFe2As2 was thepossibility of a similar volume collapse behavior and theramifications to the SDW of Fe suppressed or not and theEu valency possible valence transition from Eu2+ to Eu3+.During these calculations, the internal parameter for the As zposition was held fixed at the experimentally determined po-sition of the ambient pressure, while the c /a ratio was opti-mized at different reduced volumes. The calculations weredone with the Fe spin moments aligned in a columnar mag-netic structure pattern, as observed for the Sr homolog.16 Ourresults do not indicate any tendency to a sudden collapse ofthe c axis within the experimentally applied pressure range.The SDW is fully suppressed for pressures larger than 5 GPa,while the Eu is stable in the 2+ state up to pressures largerthan 10 GPa.Another interesting question to answer is the strength ofthe AF interaction between the Eu2+ planes as a function ofpressure. To evaluate this, we have calculated the energydifference between the ferromagnetic and antiferromagneticinterplanes arrangement of the Eu spins for various pres-sure values.29 First results from the LSDA+U calculationsshow that pressure stabilizes the AF ordering of the Eu2+planes along the c axis. The influence of spin-orbit couplingin this scenario is still under investigation. As a next step wehave compared the change in the magnitude of the Fe mo-ments as a function of pressure for both the Sr and Eu sys-tems. The magnitude of the Fe moments, ordered in the co-lumnar magnetic structure, decreases upon reducing thevolume and finally is suppressed in a similar way for bothsystems.Comparing the electronic structure under pressure ofEuFe2As2 to that of the Sr homolog, a very close similarityremains between the two systems. Tc for the 50% K substi-tuted EuFe2As2 is reduced by 6 K as compared to the 50% Ksubstituted SrFe2As2. Also, in the current pressure study ofthe pure Eu system, a similar reduction by 6 K of Tc isobserved compared to the Sr compound. But both the Sr andEu homologs behave very similar at high temperaturesabove the Eu magnetic ordering temperature. Therefore,one can conclude that the drop of the Tc together with thereduction of Bc2 /TTc are caused by the presence of para-magnetic Eu2+ ions at temperatures above the Néel orderingtemperature.To summarize, we have investigated the effect of hydro-static pressure on the peculiar properties of EuFe2As2. Thetransition at T0 corresponding to the lattice distortion and theformation of the SDW shifts with increasing pressure from180 K at p=0 GPa down to 90 K at p=2.3 GPa. The cor-responding anomaly in T has already become very weakat this pressure and cannot be observed any longer forp2.5 GPa, suggesting the critical point were this transitiongets completely suppressed to be very close to p=2.3 GPa.Above 2 GPa, a sharp drop appears in T at Tc=29.5 Kand becomes more pronounced upon further increasing pres-sure, indicating the onset of SC. The large and linear initialshift of Tc to lower temperatures with increasing field sup-ports the superconducting nature of this transition. Thus thetransition from the magnetic order of Fe moments to thesuperconducting state occurs in EuFe2As2 at a slightlysmaller pressure than in SrFe2As2, which correlates with aslightly smaller initial unit-cell volume. Further on, assumingthat the transition at T0 is of first order as suggested by mostcurrent investigations and applying the Clausius-Clapeyronrelation one finds a faster suppression of T0 with increasingp, in accordance with a larger volume change at T0 inEuFe2As2 	V−510−4 nm3 compared to SrFe2As2	V−310−4nm3.30 On the other hand, the latent heat atthe transition, 	H220 J /mol,10 is about the same as inSrFe2As2.8 In the superconducting state, we observed a mini-mum in T below Tc, followed by a clear increase leadingto a maximum at TN=20 K. This points to reentrant SC dueto the AF ordering of Eu. TN does not seem to change sig-nificantly with pressure, as typically observed in Eu systems.While this reentrant behavior is suppressed at low fieldsB0.5 T, between B=3 T and B=7 T we observed astrong reduction in the drop in T below the transition aswell as a strong decrease of the field dependence of the tran-sition temperature.Thus, our results suggest a very peculiar and interestinginteraction between the superconducting state and the mag-netism of the rare-earth ions in EuFe2As2 under pressure.Such reentrant SC has not been observed in the dopedRFeAsO compounds and this makes EuFe2As2 uniqueamong the layered FeAs systems. The occurrence of thesephenomena seems to be related to the fact that at B=0, TN isnot much smaller than Tc.We acknowledge the financial support of the DFG—MI1171/1-1, DFG—Research Unit 960, DFG—Sonderfor-schungsbereich 463, and BRNS Grant No. 2007/37/28.*miclea@cpfs.mpg.de†nicklas@cpfs.mpg.de1 Y. Kamihara, T. Watanabe, M. Hirano, and H. Hosono, J. Am.Chem. 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Miclea, C.

Jeevan, H.

Evidence for a reentrant superconducting state in EuFe<sub>2</sub>As<sub>2</sub> under pressure