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

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