Commensurate Spin Dynamics in the Superconducting State of an Electron-Doped Cuprate Superconductor

NCCO superconductor HTS 高温超导

VOLUME90,NUMBER13

PHYSICALREVIEWLETTERS

weekending4APRIL2003

CommensurateSpinDynamicsintheSuperconductingState

ofanElectron-DopedCuprateSuperconductor

K.Yamada,1,*K.Kurahashi,2, T.Uefuji,1M.Fujita,1S.Park,3,4S.-H.Lee,3andY.Endoh5

InstituteforChemicalResearch,KyotoUniversity,Uji611-0011,Japan

DepartmentofPhysics,TohokuUniversity,AramakiAoba,Sendai980-8577,Japan

3

NISTCenterforNeutronResearch,NationalInstituteofStandardsandTechnology,Gaithersburg,Maryland20899-8652

4

DepartmentofMaterialsandNuclearEngineering,UniversityofMaryland,CollegePark,Maryland20742

5

InstituteofMaterialsResearch,TohokuUniversity,Sendai980-8577,Japan

(Received29October2002;published4April2003)

2

1

Wereportneutronscatteringstudiesontwosinglecrystalsamplesoftheelectron-doped(n-type)superconducting(SC)cuprateNd2ÿxCexCuO4(x 0:15)withTc 18and25K.Unlikethehole-doped(p-type)SCcuprates,whereincommensuratemagnetic uctuationscommonlyexist,then-typecuprateshowscommensuratemagnetic uctuationsatthetetragonal(1=21=20)reciprocalpointsbothintheSCandinthenormalstate.Aspingapopensupwhenthen-typecupratebecomesSC,asintheoptimallydopedp-typeLa2ÿxSrxCuO4.Thegapenergy,however,increasesgraduallyuptoabout4meVasTdecreasesfromTcto2K,whichcontrastswiththespinpseudogapbehaviorwithaT-independentgapenergyintheSCstateofp-typecuprates.

DOI:10.1103/PhysRevLett.90.137004

PACSnumbers:74.25.Ha,74.72.Jt

High-Tcsuperconductivityemergeswhenchargecarriers,holes,orelectronsaredopedintoanantiferro-magnetic(AF)Mottinsulator[1–3].Themechanismofthesuperconductivityliesontheircommontwo-dimensionalCuO2planesintowhichthechargecarriersgo.Oneoftheissuesinunderstandingthemechanismisthequestionoftheelectron-holesymmetry.Theelec-tronicstructureoftheoptimallydopedcupratesshowsevidencefortheelectron-holesymmetry[4,5].Theirphasediagramsasafunctionofdoping,however,areasymmetric[6].Forholedoping,antiferromagnetismrapidlyweakensandisreplacedbyaspin-glass-likephasewithcharacteristicsofincommensuratespincorre-lationsandpseudogapintransportmeasurements.Thepseudogaptemperature,T ,iswellde nedintheunder-dopedregionanddecreaseswithdoping.Thesystembecomessuperconducting(SC)overawiderangeoftheholeconcentration,x,aroundtheoptimalx 0:15.TheSCstatehasincommensuratespincorrelationswithaT-independentspingap.ThenormalstateoftheunderdopedandoptimallydopedSCregionalsoshowsunusualnon-Fermi-liquid(FL)behaviors.Thereisincreasingevidenceforaquantumcriticalpoint(QCP)aroundtheoptimaldopingwhichisresponsiblefortheunusualpropertiesoftheSCandthenormalphase[7–10].Fortheelectron-doped(n-type)cuprates,ontheotherhand,antiferromagnetismsurvivesuntilthesuper-conductivityappearsoveranarrowrangeofxaroundtheoptimalx 0:15.Thenormalstateofthen-typecupratesshowsFermi-liquidT2behaviorinresistivityratherthanthelinearbehaviorofthehole-doped(p-type)cuprates.Therefore,investigatingsimilaritiesanddifferencesofthen-typeandp-typecuprateswouldbecrucialtounder-standingphysicsofthehigh-Tcsuperconductivity.Inthispaper,wereportneutronscatteringmeasure-mentsonsinglecrystalsofthen-typeSCcuprateNd2ÿxCexCuO4(NCCO)(x 0:15)withTc 18and25K.Wehavefoundcommensuratemagnetic uctuationsatthetetragonal 1=21=20 reciprocalpointsintheSCandnormalstates.AspingapopensupwhenthesystembecomesSC,asintheoptimallydopedp-typeLa2ÿxSrxCuO4[11–13].Thegapenergy,however,in-creasesgraduallyuptoabout4meVasTdecreasesfromTcto2K,whichcontrastswiththeT-independentspinpseudogapenergyfortheSCstateofthep-typecupratenearTc.Ourresultsindicatethatdopedelectronsself-organizeinadifferentwaythanholesthatformstripessuchasin La;Nd 2ÿxSrxCuO4[14,15],La2ÿx Sr;Ba xCuO4[16],andtheisostructuralinsulatorLa2ÿxSrxNiO4[17,18].

SizablesinglecrystalsofNCCOwithx 0:15weregrownbyatraveling-solvent- oating-zone(TSFZ)method.Theas-growncrystalisanAFinsulatorwith

´eltemperatureTNofaround125to160Kdepend-theNe

ingontheexcessoxygenconcentrationinthecrystal.Bulksuperconductivityappearsonlywithproperheattreatmentsontheseas-growncrystals.Thedetailedpro-cedureofthecrystalgrowthandtheheattreatmentisdescribedinaseparatepaper[19].FortheSCsample,Tcisdeterminedfromzero- eld-cooleddiamagneticsus-ceptibilitiesmeasuredbyaSQUIDmagnetometer.Forthepresentstudy,weusedtwoSCsampleswithTc 18and25K.SCtransitiontemperaturewidthis 3Kand 5KforsampleswithTc 18and25K,respectively.AnalmostfullMeissnerfractionwasobtainedinbothsamples[19].

Neutronscatteringexperimentswereperformedonthethermalneutrontriple-axisspectrometerofTohoku

NCCO superconductor HTS 高温超导

University,TOPAN,andthecoldneutrontriple-axisspec-trometeroftheUniversityofTokyo,HER,installedatJRR-3MinJAERI,TokaiEstablishment.Weperformedtheexperimentwithlowenergyexcitationsbelow1meVonthecoldneutrontriple-axisspectrometer,SPINS,attheNationalInstituteofStandardsandTechnology(NIST)CenterforNeutronResearch.Incidentneutronenergiesof13.7meVforTOPANand5meVforHERandSPINSwereselectedusingthe 002 re ectionofpyrolyticgraphitemonochromators.Additionally,inor-dertoeliminatethehigher-orderre ectedbeams,apy-rolyticgraphite lterforthermalneutronsandaBe lterforcoldneutronswereplacedintheupordownstreamofthesampleposition.Previously,afewattemptshavebeenmadetostudyspindynamicsinNCCOusinganeutronscatteringtechnique[20].However,nowell-de nedmag-neticsignalhasbeenfoundintheSCphase.Inthepreviousmeasurements,thetotalvolumeofthesamplewas0:5cm3andthe hhl scatteringplanewasexam-ined.Forourstudy,wehaveinvestigatedmostlythe hk0 scatteringplanetoincreasethesignalfromtwo-dimensionalmagneticrodalongthecaxisbyusingthelargerverticalangulardivergenceofthebeam.Furthermore,wecutalongsinglecrystallinerodwithatotalvolumeof2cm3intothreepiecesandstackedthoseverticallyusinganaluminumsampleholder.Theholderwasmountedinanaluminumcanattachedeithertothecoldplateofa4He-closedcyclerefrigerator,ortoatop-loadingliquid-Hecryostat,whichcoolsdownto1.5K.Figure1(a)showstemperaturedependenceofelasticneutronscatteringintensityat 3=21=20 re ection,ob-tainedfromthreesamples:theas-growninsulatingsam-pleandthetwoheat-treatedSCsampleswithTtheas-growninsulatingsample,uponc 18and25K.ForcoolingtheintensitystartsgraduallyincreasingbelowT140K,signalingtheAFlongrangeorderofCu2 N mo-ments.ItfurtherincreasesbelowT3 moments.WhenNd 20KduetotheparticipationofNdthesamplegetstheproperheattreatmentandbecomesSC,thestaticmag-neticorderisdrasticallysuppressed.EventhoughthemagneticelasticpeaksremainevenintheSCphase,theSCsampleshavelowerTtheinsulatingN 60to80KandTsample.Thesuppres-Nd,andweakerintensitythansionismoreseverefortheSCsamplewiththehigherTc 25KthanfortheonewithTstaticpeakwasresolutionlimitedc 18K.ThewidthofthefortheinsulatingsampleandwasbroadenedfortheSCsamples.By ttingthestaticpeakwithaLorentzianconvolutedwiththeinstrumentalresolution,weobtainedthein-planeandout-of-plane the

c 80 30 A

correlationlength, ab 150 60 A

and,respectively,atT 8KfortheT25Ksample.Thisindicatesthatsuperconductivitycom-c peteswiththemagneticorderinthen-typecuprateasinp-typecuprates.

Toinvestigatetherelationshipbetweenthesupercon-ductivityandthedynamicspincorrelations,wehave

performedconstantenergyscansaroundtheAF 1=21=20 point.Figure2(a)showstheresultsforthenormalstateobtainedwith! 2meV,withscansalongthe 100 and 110 directions,whichareparallelanddiagonaltoCu-ObondsintheCuOInbothdirections,acommensurate2plane,respectively.peakappears.Theseresultssharplycontrastwiththeincommensuratepeaksfoundinthep-typeSCcuprates[14].Solidlinesarethebest tstoaGaussianconvolutedwiththeinstrumentalresolutionfunctionyieldingtheintrinsichalfwidthathalfmaximum(HWHM)of0:025 3 a .Indeed,thedatashowninFig.2(a)arethe rstdirectexperimentalevi-denceforthecommensuratespin uctuationsinthen-typecuprate.ThepeakwidthsoftheSCsamplesaresubstantiallybroaderthanthoseoftheAFphase,whiletheq-integratedpeakintensitiesarecomparablebetweentheAFandSCsamplesexceptatlowtemperaturesbelowT25c.Furthermore,thepeakwidthisbroaderfortheTKsamplethantheTc thatfor! 0:8c 18Ksample.Figure1(b)showsmeVthecommensuratespin uctuationsdiminishasTdecreasesbelowTaspingapintheSC

c,whichindicatesopeningofstate.

NCCO superconductor HTS 高温超导

Figure2(b)clearlyshowsthedepletionofthespectralweightfor! 3meV.Tostudyenergydependenceofthedynamicspin uctuationsindetail,wehaveper-formedconstant!scansshowninFig.2withvariousenergytransfers.Thedatawere ttedtoaGaussiancon-volutedwiththeinstrumentalresolutionfunction.TheintegratedintensityoftheGaussianwasconvertedtotheimaginarypartofdynamicsusceptibilityviathe uctua-tiondissipationtheorem,I ! / 00 ! 1 n ! = ,wheren ! istheBosethermalpopulationfactor.Figure3showstheresulting 00 ! asafunctionoftheenergytransfer,!.ForbothSCsamples,atT 2K, 00 ! hasagapof2 3meVand4meVfortheT18and25Ksamples,respectively.Thefactthatc thesamplewiththehigherTchasthelarger2 ,whereasithastheweakerNdorderingandthelowerTthegappedcommensuratespin uctuationNd,tellsusthatisanintrinsicpropertyofthespindynamicsintheoptimallydopedn-typesuperconductor.Suchaspingaphasalsobeenreportedinthep-typeLaspingapof2 62ÿ xSr7xCuOmeV4nearoptimaldop-ing.AwasobservedintheoptimallydopedLaofFig.4,their2ÿmaximumxSrxCuO4[12,13].Asshownintheinsetgaps2 withtheSCtemperaturescaleCkmaxbehaveline-arly2:0 2 ,irrespectiveofcarriertype.ThereBTis,cwithC however,aqualitativedifferenceofthespindynamicsbetweenthen-typeandthep-typecuprates:Forthen-typeNd2ÿxCexCuO4(x 0:15),asshowninFig.3,uponwarming 00 Q;! shiftstowardlowerenergiesandeventually llsupthegapinthenormalphase.Theshiftofthespectralweightintheenergyspectrumissummar-izedinFig.4:2 decreaseswithincreasingTandeven-tuallydisappearswhenthesystementersthenormalphase.ThissuggeststheabsenceordegradationofaspinpseudogapstateandisinsharpcontrastwiththoseobservedintheLaQ;! diminishes2ÿxSrxCuO4(x 0:15)inwhich,forT Tc 00 forall!<2 ,andasTapproachesTc, 00 Q;! smearsintolower!withoutremarkablechangein2 [11,12].RecentNMRmeasure-mentsonsinglecrystallinen-typesuperconductorPr1ÿxLaCexCuO4alsosuggestedtheabsenceofspinpseudogap[21].

CommensurateshortrangespincorrelationsintheSCphaseofthen-typecupratesuggestthatthedopedelectronsmaybeinhomogeneouslydistributedorformdroplets/bubblesintheCuO2planes,ratherthanorganiz-ingintoone-dimensionalstripesasthedopedholesseemtodointhep-typecuprates.Theirdifferentorbitalchar-actersmightberesponsibleforthedifferentbehaviors.Dopedholesaredominantlyintroducedintothe2porbitalofoxygenionsinCuOxy2planesandinducethefrustratedmagneticinteractionsbetweentheneighboringCuspinswhichstabilizetheformationofstripes.Theelasticincommensuratemagneticsignalincreaseswhenanexternalmagnetic eldisappliedperpendiculartotheCuO2planes[22–24].Ontheotherhand,dopedelectronspredominantlyenterintothe3dx2ÿy2orbitalsofCuions

to

NCCO superconductor HTS 高温超导

maketheCusitenonmagnetic,whichreducesthesizeoftheAFdomainswithoutintroducingfrustrationorchang-ingthecommensurability.Forthep-typeSCcuprates,thequasi-two-dimensionalstripephase(orthepseudogapphase)competeswiththeSCphase,whereasforthen-typeSCcupratesitisthethree-dimensionalAFstatethatcompeteswiththesuperconductivity.Ourresultsshowthatforn-typecupratestheantiferromagnetismcoexistswithsuperconductivityattheoptimalregion.Thismayindicatethatthetransitionfromtheantiferro-magnetismtosuperconductivityupondopingisa rstorderinnatureandthereforeitlacksaQCP.Thisdiffer-encemayberesponsiblefortheirdifferentpropertiesinSCandnormalstates[25–27].

Insummary,werevealedbyaneutronscatteringstudyonthesinglecrystalsofNdofgappedcommensuratespin1:85Ce uctuations0:15CuO4thecoexistencewiththeelec-tron-dopedsuperconductivity.Theenergygapofaround4meVclosesatornearTc,whichsuggeststheabsenceordegradationofthespinpseudogapstateintheelectron-dopedsuperconductor.Amorecomprehensivestudyontheelectron-dopedsuperconductorfreefromtheeffectofrare-earthmagneticmomentsishighlyrequiredforthefullunderstandingoftheuniversalpropertiesofspin uctuationsirrespectiveoftypesofcarrier.

WethankY.Kojima,I.Tanaka,S.Hosoya,K.Hirota,H.Y.Kee,J.M.Tranquada,P.A.Lee,G.Shirane,andR.J.Birgeneaufortheirvaluablediscussions.ThisworkwassupportedbytheJapaneseMinistryofEducation,Culture,Sports,ScienceandTechnology,Grant-in-AidforScienti cResearchonPriorityAreasContract,Scienti cResearch(A)and(B),EncouragementofYoungScientists,CreativeScienti cResearch,theJapanScienceandTechnologyCorporation,theCoreResearchforEvolutionalScienceandTechnologyProject(CREST),andtheNationalScienceFoundationunderAgreementNo.DMR-9986442.

*Present

address:yamada@scl.kyoto-u.ac.jpPresentaddress:KohzuPrecisionCo.Ltd.,Setagaya,Tokyo154-0005,Japan.

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