Neutron scattering study of the oxypnictide superconductor La(O,F)FeAs

The newly discovered superconductor La(O,F)FeAs (Tc = 26 K) was investigated using the neutron scattering technique. No spin-density-wave (SDW) order was observed in the normal state nor in the superconducting state, both with and without an applied magnet

NeutronscatteringstudyoftheoxypnictidesuperconductorLaO0.87F0.13FeAs

Y.Qiu,1,2M.Kofu,3WeiBao,4, S.-H.Lee,3Q.Huang,1T.Yildirim,1J.R.D.Copley,1J.W.Lynn,1T.Wu,5G.Wu,5andX.H.Chen5

1

arXiv:0805.1062v1 [cond-mat.supr-con] 7 May 2008

NISTCenterforNeutronResearch,NationalInstituteofStandardsandTechnology,Gaithersburg,MD20899

2

Dept.ofMaterialsScienceandEngineering,UniversityofMaryland,CollegePark,MD20742

3

DepartmentofPhysics,UniversityofVirginia,Charlottesville,VA22904

4

LosAlamosNationalLaboratory,LosAlamos,NM87545

5

HefeiNationalLaboratoryforPhysicalScienceatMicroscaleandDepartmentofPhysics,

UniversityofScienceandTechnologyofChina,Hefei,Anhui230026,China

(Dated:May7,2008)

ThenewlydiscoveredsuperconductorLaO0.87F0.13FeAs(TC≈26K)wasinvestigatedusingtheneutronscatteringtechnique.Nospin-density-wave(SDW)orderwasobservedinthenormalstatenorinthesuperconductingstate,bothwithandwithoutanappliedmagnetic eldof9T,consistentwiththeproposalthatSDWandsuperconductivityarecompetinginthelaminarmaterials.Whileourinelasticmeasurementso ernoconstraintsonthespindynamicresponsefromd-wavepairing,anupperlimitforthemagneticresonancepeakpredictedfromanextendeds-wavepairingmechanismisprovided.Ourmeasurementsalsosupporttheenergyscaleofthecalculatedphononspectrumwhichisusedinelectron-phononcouplingtheory,andfailstoproducethehighobservedTC.

Anewfamilyofsuperconductorshasbeendiscov-eredinlaminaroxypnictideLa(O,F)FeP(TC≈4K)[1],LaONiP(TC≈3K)[2],andLa(O,F)FeAs(TC≈26K)[3].EnormousexcitementhasbeengeneratedsinceTCwasraisedabove40KwhenLawasreplacedbySm(TC≈43K)[4],Ce(TC≈41K)[5],Nd(TC≈52K)[6],orPr(TC≈52K)[7]inLn(O,F)FeAs.Gd(O,F)FeAsisalsoasuperconductor[8]anditsTchasbeenraisedto36K[9].Sofar,Sm(O,F)FeAshasthehighestTC～55Kinthenewfamilyofsuperconductors[10,11],andthetransitiontemperatureisthehighestamongallsuper-conductorsexceptinsomecuprates.

TheparentcompoundsLnOFeAs(Ln=La[3],Sm[11],Ce[5],Nd,Gd[8])arenotsuperconductors.Instead,aspin-density-wave(SDW)duetoFermisurfacenest-ingdevelopsbelow～150K[12,13,14].InadditiontoaddingelectronsinLn(O,F)FeAs,removingelectronsin(La,Sr)OFeAsalsoshiftstheFermisurfaceoutofthenestingcondition[12,15]andleadstosuperconduc-torsofsimilarlyhighTC[16].Pressurealsostronglyaf-fectsTC[17,18],buttheoreticalcalculationsdonotfa-vorthephononmechanism[19,20,21].Varioustheo-reticalpossibilitiesinvolvingmagnetic uctuationshavebeenproposed[19,22,23,24,25,26,27].Inpartic-ular,apronouncedresonancepeakinthespinexcita-tionsispredictedforanextendeds-wavesuperconduct-ingorderparameter,whiled-waveorderparameteronlymodestlyenhancesspinexcitationsoverthenormalstateresponse[27].

Inthisneutronscatteringstudy,wechosesupercon-ductingLa(O,F)FeAsasoursubject.AlthoughotherLn(O,F)FeAshavehigherTC,theLacompoundisthemostthoroughlystudiedanditalsoavoidscomplex-itycausedbymagneticrare-earthelements.Addition-ally,thesuperconductinggapinLa(O,F)FeAshasbeenestimatedat 0≈3.7(8)meVininfrared[28],spe-

ci cheat[29],andpoint-contacttunneling[30]experi-ments.Theexperimentalvaluesof 0setthetheoret-icalresonanceenergyoftheextendeds-wavesupercon-ductivityatthein-planeantiferromagneticwavevectorQ=(1/2,1/2,0)andat5.6±1.3meV[27],withinthecon-venientrangeofneutronscatteringspectroscopy.Atthismoment,onlypolycrystallinesamplesofLn(O,F)FeAsareavailable,andanidealinstrumentforinvestigatingsuchsamplesisthetime-of- ightdiskchopperspectrom-eter(DCS)attheNISTCenterforNeutronResearch.Acryomagnetcontrolledthesampletemperatureandmag-netic eld.Theintensityofmagneticinelasticneutronscatteringwasnormalizedtoabsoluteunitswithincoher-entnuclearscattering.Ourmeasurementsprovidevalu-ableinsightsintothenewsuperconductorsandimposeexperimentalconstraintsfortheoreticalwork.

ApolycrystallinesampleofLaO0.87F0.13FeAs,mass2g,wassynthesizedbythesolidstatereaction[3].Theobservedpowderdi ractionspectrumat1.6Kisshownin gure1.ItiswellaccountedforbythetetragonalZr-CuSiAsstructureasindicatedbythere nedpro lealsoshowninthe gure.Thesampleisofhighpurity.Onlyminuteimpuritypeaksarediscernibleinthedi ractionspectrum.Re paringtoLaO0.92F0.08FeAsatasimilartemperature[13],bothlatticeparametersaandcofLaO0.87F0.13FeAsincreaseby0.11%.OnlytheAspositionhasasmalldetectabledi erence.Theresistiv-ityρshownintheinsettoFigure1wasmeasuredusingthestandardfour-probemethod.ThesuperconductingtransitionstartsatTC=26K,dρ/dTpeaksat24Kandρreacheszeroat22K.Thetransitionisamongthenar-rowestforLa(O,F)FeAsmaterials[3,28,29,30,31,32].TheSDWofLaOFeAsischaracterizedbythewavevec-tor(1/2,1/2,1/2)[13,14].Theweakstaggeredmag-

The newly discovered superconductor La(O,F)FeAs (Tc = 26 K) was investigated using the neutron scattering technique. No spin-density-wave (SDW) order was observed in the normal state nor in the superconducting state, both with and without an applied magnet

FIG.1:(coloronline)Observed(crosses)andcalcu-lated(solidline)neutronpowderdi ractionintensitiesforthesuperconductorLaO0.87F0.13FeAsat1.6KusingspacegroupP4/nmm.VerticallinesareBraggpeakpositionsforLaO0.87F0.13FeAs(lower)andAlsampleholder(upper).ThedatawerecollectedonDCSwithanincidentbeamwave-length

λ=1.8 A.Thestructurewasre nedusingtheGSASprogram[33].Inset:TheresistivityofLaO0.87F0.13FeAsshow-ingthesuperconductingtransitionatTC=26K.

neticmomentM=0.36(5)µBperFeat8K[13]canbeexplainedbyanassociatedstructuretransition[34].ThestrongestmagneticBraggpeak(1/2,1/2,3/2)isonly1.1%oftheintensityofthestructural(002)peak[13].ForsuperconductingsamplesLaO0.92F0.08FeAsandLaO0.89F0.11FeAs,magneticpeaksoftheSDWorderarenotobservedabovemeasurementstatisticslevelofabout0.5%ofthe(002)peakat8K[13]and70K[14],respectively.NeitherdoesoursuperconductingsampleLaO0.87F0.13FeAsshowanydetectableSDWorderdownto1.6Kinthesuperconductingstate(Fig.1),norat30Kinthenormalstate(Fig.2).Applyingamagnetic eldof9Talsodoesnotinduceanymagneticpeakstrongerthan0.5%ofthe(002)Braggpeak.Theseresultssup-porttheproposalthattheSDWandsuperconductingorderparametersarecompetingforitinerantelectronsandholesontheFermisurface[12,19],anddonotfa-vorthetheoryofcoexistenceofantiferromagnetismwithsuperconductivityinLa(O,F)FeAs.

Conventionalsuperconductivityismediatedbyphonons,andthephononspectrumhasbeencalcu-

TABLEI:Re nedstructureparametersofLaO0.87F0.13FeAsat1.6K.Spacegroup:P4/nmm(No.129).a4.0245(3) A,c=8.713(1) A,V=141.126(9) A3=

.Rp=11.72%,

wRp=15.53%.Atomsitexyz

B( A2)La2c1/41/40.1442(8)1.0(2)Fe2b3/41/41/2

0.3(1)As2c1/41/40.6541(8)0.02(2)O/F

2a

3/4

1/4

0.9(2)

2

FIG.2:(coloronline)Neutronpowderdi ractionintensitiesofLaO0.87F0.13FeAsat30Kandzero eldinthenormalstate(openred)andat1.6Kand9Tmagnetic eldinthesuperconductingstate(solidblue).Thedataarecollectedwithaneutronwavelengthλ=4.8 AtofocusonthesmallQrangeformagneticsignals.Theredsymbolshavebeenshiftedupforclarity.ThearrowsindicatemagneticBraggpeakpositionsoftheSDWorder.Nomagneticpeakstrongerthan0.5%ofthe(002)existsinthespectra.

latedforLa(O,F)FeAs[15,21].Ithasbeenusedtocalculatetheelectron-phononcoupling,andtheTCfromthismechanismismuchlowerthantheobservedvalue[19,21].Tovalidatethetheoreticalcalculations,wehavemeasuredinelasticneutronscatteringfromphononsinLaO0.87F0.13FeAs.Forpolycrystallinesamples,theintensityisgivenby

I(Q,ω)=

σi Q2

exp( 2WDi)

i(ω)

[n(ω,T)+i

2m1],

iω(1)

whereσiandmiaretheneutronscatteringcrosssec-tionandatomicmassoftheithatom(La,O/F,Fe,As),n(ω,T)istheBosefactor,WitheDebye-Wallerfactor[35]. Theweightedphonondensityofstates(PDOS)D(ω)=iDi(ω)inEq.(1)di ersfromthebarePDOScalculatedin[15,21]byafactorofthesquaredmodulusofphononeigenvectors.ThemeasuredneutronscatteringintensityI(Q,ω)isfurtherweightedbyσi/miofdi erentatoms.Butthepeakpositionsinthemea-suredandbarePDOSusuallyremainthesame[35].

Inthetopframeof gure3,themeasuredI(ω)=dQI(Q,ω)at1.6and110Kusingneutronsofwave-length1.8 A,integratedfromQ=2.5to7 A 1,isshown.InthemiddleframeisshownthetheoreticalintensitycalculatedfromthebarePDOSofSinghandDu[15],convolutedwithinstrumentresolution.At1.6K,theBosefactorleadstozerointensityfornegativeen-ergytransfer,andmeasurementsthereservetodeter-mine

thebackground.TheintegratedintensityI(Q)=dωI(Q,ω)fromthenegativeenergyside,showninthebottomframe,demonstratestheexpectedbehaviorforphononscattering,whichisapproximatelyproportional

The newly discovered superconductor La(O,F)FeAs (Tc = 26 K) was investigated using the neutron scattering technique. No spin-density-wave (SDW) order was observed in the normal state nor in the superconducting state, both with and without an applied magnet

FIG.3:(coloronline)Top:I(ω)=R

dQS(Q,ω)measuredat1.6and110K.Theintegrationrangeisfrom2.5to7 A 1.Middle:Calculatedbareintensitypro leat1.6and110K.Bottom:MeasuredI(Q)=R

dωI(Q,ω)at1.6and110K.Theintegrationrangeisfrom-15to-5meV.Theshadedpro leismeasuredS(Q,ω=0).

toQ2I(Q,ω=

0)[35].ThepeakpositionsofthebarePDOSofSinghandDuarewellreproducedinthemea-suredI(ω).ThecalculatedPDOSin[21]closelyresem-blesthatin[15].Thus,thephononspectrausedintheelectron-phononcouplingcalculationsin[19,21],whichdonotfavorthephononmechanismforsuperconductiv-ityinLa(O,F)FeAs,haveexperimentalsupportfromthiswork.

Unconventionalsuperconductivitymediatedbyvariousmagneticchannelshasbeeninvestigatedtheoretically[19,22,23,24,25,26,27].Bothd-waveandextendeds-wavehavebeenproposedforthesuperconductingorderpa-rameterofLn(O,F)FeAs.KorshunovandEreminhaveinvestigatedtheconsequencesofthesepairingsinspindynamics[27].Ford-wavepairing,thesuperconductingtransitiononlymodestlyredistributesthespinspectralweightbelow2 0,where 0isthesuperconductinggap

3

FIG.4:(coloronline)Theoreticalχ (Q,ω)inthesuper-conductingstatewithanextendeds-waveorderparameter,from[27],isshownintheleftinsetinthebottomframe.Theχ (QAFM,ω)asafunctionofωinthenormalandsupercon-ductingstatewiththetwoorderparametersymmetries[27]areshowninthetop.MeasuredS(Q,ω)≡χ (Q,ω) n(T,ω)+1 /π≈χ (Q,ω)/πat1.6Kisshownintherightinsetinthebottomframe.Thecolorbarindicatesofµ2B/meVsrFe.TheI(ω)≈χ

intensityintheunits

(QAFM,ω)/πisshowninthebottomframe.

parameter.Forextendeds-wavepairing,astrongres-onancepeakwouldappearatthenestingwavevectorQAFM=(1/2,1/2,0)and ω～1.5 0.IntheleftinsetofthebottomframeandthetopframeofFig.4,thethe-oreticalimaginarydynamicspinsusceptibilityχ (Q,ω)from[27]isshownforvariouscases.Thevalueof 0ob-tainedfrominfraredmeasurementsisbetween3.1and3.7meV[28],inthespeci cheatstudy3.4(5)meV[29],andfromtunneling3.9(7)meV[30].Therefore,theresonancepeakatQAFMisbetween4.4and6.9meV.

Intherightinsettothebottomframeof gure4,mag-neticneutronscatteringintensityS(Q,ω)measuredinthesuperconductingstateat1.6Kisshowninthesame(ω,Q)rangeasintheleftinset.Below2.5meV,intensityisdominatedbyincoherentnuclearneutronscatteringandisnotshown.Theenergydependenceattheanti-ferromagneticpointisshowninthemainbottomframewiththesameenergyscaleasinthetopframe.Above2.5meV,theBosefactorn(ω,T)≈0at1.6K.Thus,

The newly discovered superconductor La(O,F)FeAs (Tc = 26 K) was investigated using the neutron scattering technique. No spin-density-wave (SDW) order was observed in the normal state nor in the superconducting state, both with and without an applied magnet

themeasuredS(Q,ω)≈χ (Q,ω)/πcanbecompareddirectlytothetheoreticalχ (Q,ω)inFig.4.Powderav-eragingwillenhancethemeasuredintensityatQlargerthan|QAFM|tosomeextent,however,thesharpreso-nancepeakwillbelittlea ected.

Wedidnotobservethestronglyenhancedsupercon-ductingresonancepeakinLaO0.87F0.13FeAsat1.6K.Theupperlimitforintensityofsucharesonancepeakis0.5(1)µ2theB/meVsrperFefromourdata(seethebluecurveinbottomframe).Ifthepredictedresonancepeakisasstrongasintheunconventionalsuperconduc-torCeCoIn5,～30µ2ofmagnitudestrongerB/meVsrperCo[36],beingtwoor-dersthanourmeasurementlimit,itwouldhavebeenobservedinourexperiments.Ontheotherhand,iftheintensityoftheresonancepeakinLa(O,F)FeAsissimilartothatinYBa2Cu3O6+x,～0.2µ2B/meVsrperCu[37],itwouldnotbeobservedinourmeasurements.Theoretically,thepeakintensityfortheresonanceinLa(O,F)FeAswiththeextendeds-wavepairingdependsonthechoiceofdampingfactorandcorrectionsbeyondtherandom-phase-approximation[27].Forthed-wavepairingalsodiscussedin[27],themod-estchangeinthespindynamicswouldbebeyondthesensitivityofthispolycrystallineexperiment.

Insummary,thespin-density-waveorderofLaOFeAsisdisplacedbysuperconductivityinLaO0.87F0.13FeAs.Thepeaksinthetheoreticalphonondensityofstatesat12and17meVareobservedinourphononmea-surements.Thetheoryofphononmediatedsupercon-ductivity,whichfailstoproducethehighTC≈26K,thusisbasedonreliablephononcalculation.Ourexperi-mentssetanupperlimitof0.5µ2B/meVsrperFefortheresonancepeakinthespinexcitationsattheantiferro-magneticwavevector.Unconventionalextendeds-wavesuperconductivitymediatedbyspin uctuationsiscon-strainedbythelimit.

WewouldliketothankD.J.SinghandM.-H.DuforprovidingtheoreticalPDOSdata.WorkatLANLissupportedbyU.S.DOE,atUSTCbytheNaturalScienceFoundationofChina,MinistryofScienceandTechnologyofChina(973ProjectNo:2006CB601001)andbyNationalBasicResearchProgramofChina(2006CB922005).TheDCSatNISTispartiallysup-portedbyNSFunderAgreementNo.DMR-0454672.

Electronicaddress:wbao@lanl.gov

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