Commensurate Spin Dynamics in the Superconducting State of a(2)

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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