T-XAFS

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CONDENSED-MATTER

PHYSICS

SUPERCONDUCTINGPROPERTIES

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THEORYANDCOMPUTATION

QUANTUMMECHANICSIronIsotopeEffectandLocalLatticeDynamicsinthe(Ba,K)Fe2As2SuperconductorStudiedbyTemperature-DependentEXAFSWangshengChu1,2,JieCheng1,2,ShengqiChu2,TiandouHu2,AugustoMarcelli4,1,XianhuiChen3&ZiyuWu1,2

1

2Received9October2012

Accepted

16April2013

Published

29April2013NationalSynchrotronRadiationLaboratory,UniversityofScienceandTechnologyofChina,Hefei,Anhui230029,P.R.China,BeijingSynchrotronRadiationFacility,InstituteofHighEnergyPhysics,ChineseAcademyofSciences,Beijing100049,P.R.China,3HefeiNationalLaboratoryforPhysicalScienceatMicroscaleandDepartmentofPhysics,UniversityofScienceandTechnologyofChina,Hefei230026,China,4IstitutoNazionalediFisicaNucleare,LaboratoriNazionalidiFrascati,00044Frascati,Italy.

Correspondenceand

requestsformaterials

shouldbeaddressedto

Z.Y.W.(wuzy@http://wendang.chazidian.com)CurrentlyavailableinvestigationsoftheisotopeeffectinFe-basedsuperconductorsareallbasedonmacroscopicresistivityandsusceptibilitymeasurementsandresultsarehighlycontroversial.HerewepresentaninvestigationoftheFeisotopeeffectinthe(Ba,K)Fe2As2superconductorbytemperature-dependentEXAFSintheframeworkofEinsteinmodel.Thismethodseparatesthestaticstructuralisotopecontributionandthelocallatticeisotopecontribution.Resultspointouttheisotopesubstitutionhasanegligibleeffectoninter-atomicdistances.Onthecontrary,boththelocalvibrationsoftheFe-AsandtheFe-Febondsshowstrongironisotopedependence,whichisamarkerofan‘‘intrinsic’’isotopeeffect.Thisbulkironisotopecontributiontothelocallatticedynamicssuggeststhelocalphononsplayanimportantroleinthe(Ba,K)Fe2As2superconductorandathree-dimensionalelectron-phononinteractionshouldbeconsideredtounderstandthesuperconductingmechanismofiron-basedsuperconductors.

Sincethediscoveryofcupratesuperconductorsin1986,oneofthestillopenandchallengingproblemsin

condensedmatterphysicsistheoriginofthesuperconductivityinhigh-temperaturesuperconductors(HTSCs)1.Twodecadesofeffortsbasedonthousandsofexperimentalandtheoreticalresearchespointedoutthatelectron-electroninteraction(EEI)ratherthantheelectron-phononinteraction(EPI)playsthekeyroletotriggerthesuperconductivityinHTSCsmaterials2,3.However,iron-basedsuperconductorsthenewfamilyofHTSCsdiscoveredin2008doesnotshowanagreementwiththisassumption4–6.Indeed,aclearironisotopeeffectonTcwasexperimentallyobserved7–10.Chenetal.7firstlyreportedalargeandpositiveironisotopeeffect(Fe-IE)ontheTcofbothLnFeAs(O,F)and(Ba,K)Fe2As2superconductors.Khasanovetal.10foundalargerFe-IEexponent,wellbeyondtheBCSvalue.FinallyShirageetal.8,9observedaninverseironisotopeeffectinthe(Ba,K)Fe2As2superconductor.RecentlyYanagisawaetal.11presentedamodelexplainingthesigninversionoftheIEexponent.Inhismodel,the(Ba,K)Fe2As2systemischaracterizedbyamultibandsuperconductivityoriginatingbytwodifferentparingchannels:aphononiccontributionandanantiferromagnetic(AF)fluctuationmech-anism.However,hisapproachhasbeenquestionedbyBussmann-HolderandKeller12thatclaimedthataninversionoftheFe-IEexponentcouldnotoccur.Morerecently,Khasanovetal.13triedsolvingthediscrepancyintheFe-IEmeasurementsbydecomposingtheexponentintwoterms:oneassociatedtostructuralchangesandoneintrinsic.TheyalsopointedoutthattheFeisotopesubstitutionmayinducesmallstructuralchangeswhich,inturn,mayaffectTc.Actually,thecontroversystillonpointsoutthat:

1)

2)latticevibrations(i.e.,phonons)playanimportantroleinthesuperconductivemechanism(s)ofiron-basedsuperconductormaterials;theunderstandingoftheIEintheseiron-basedmaterialsremainsachallengingissuetoclarify.

Therefore,additionalandmoreaccurateexperimentalmethodstomeasuretheIE,probingnotonlymac-roscopicaveragedpropertiessuchasthosebasedonTcdeterminations,butalsomicroscopicinformationofthesystemarerequired.

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HerewepresentaninvestigationoftheFe-IEinthe(Ba,K)Fe2As2superconductorsystembytemperature-dependentextendedx-rayabsorptionspectroscopy(T-EXAFS).Duetothecorrelationbetweentheabsorberandthebackscatterthroughtheemittedphotoelectronsexcitedbyx-rays,EXAFSprobesthelocalstructurearoundtheselectedphotoabsorber,suchasthenumberofneighboringatomsN,theaveragebondlengthbetweenabsorbingatomsandneighbor-ingatoms,r.andthevariances214,15.TheDebye-Wallerfactors2correspondstotheparallelmeansquarerelativedisplacement(MSRD)amongtwoatomsanditisthesumoftwocontributions:

2

s2~s2statzsvib:

ð1Þ

Thefirsttermistemperatureindependentandassociatedtothestatic

disorder,e.g.,tothedispersionofbondlengthsneglectingthevibra-tionaldynamics.Thesecond(dynamical)contributionisduetothermalvibrationsandcanbecorrelatedtothevibrationalspectrumofthesystem.ThelatterisusuallywelldescribedbytheEinsteinmodel16

?? TEh21??2

svib~coth:ð2Þ

2kmTE2Twheremisthereducedmassoftheatompair,Tisthetemperature,k

isBoltzmann’sconstantand??histhePlank’sconstant.IntheEq.2,

h=kÞvEistheEinsteintemperatureandvEisthecorrespond-TE~ð??

ingEinsteinfrequency.Thesvib2contributionisdifferentfromzeroalsoat0K.

MeasurementsofT-EXAFSmayallow,viatheanalysisoftheDebye-Wallerfactor,toseparatethestaticandthethermaldisordercontributionspresentinasample.Thefirsttermisrelativetothelocalstructureandaffectstheelectronicandthemagneticbehaviorofthesystem,whiletheseconddependsonlybythelocalvibrationbehavior(i.e.,phonons).ByusingtheT-EXAFStechnique,wemayevaluateatthemicroscopicscalethecontributionoftheFe-IEonboththestaticstructureandthelatticedynamicsiniron-basedSCs.BecauseEXAFSisanelement-specifictechnique,inthesesystemswemayindependentlyachieveinformationoftheindividualatomicpairssuchastheFe-AsandtheFe-Fepairs.PowdersamplescanbealsousedinanEXAFSexperimentandsinglecrystalsarenotnecessarytoaccuratelydeterminethemean-squarerelativedisplace-ment(MSRD).

BothFeandAsK-edgeextendedx-rayabsorptionfinestructurespectraoftheBa0.6K0.4Fe2As2systembyreplacing56Fe(56Fe-sample)withtheisotope54Fe(54Fe-sample)weremeasuredvs.temperaturedownto10K.AfteranaccurateanalysisandcomparisonofEXAFSspectraofthetwoisotopicsubstitutionswepointoutthatitdoesnotinducedetectablechangeoftheaveragedistancesofbothFe-AsandFe-FeatomicpairswhileslightlymodulatestheFe-AsandFe-Fepairdistributions.Moreover,itaffectsthelocalvibrationofbothFe-AsandFe-Febonds.

Results

Fig.1(a)comparesAsK-edgeEXAFSdataat10Kandatroomtemperature(RT)showingacleardampingoftheEXAFSoscillationsvs.temperature.ThisbehaviorofthelocalstructureisalsoclearlookingatthecomparisonoftheFouriertransforms(FTs)oftheEXAFSspectrainFig.2(b).Inthisiron-basedsuperconductorwiththe122-structurenoanomaliesaredetectedatlowtemperatures,especiallyaroundTc.Inadditiontoourinvestigation,toourknow-ledgeothertwomanuscriptspublishedtemperature-dependentEXAFSmeasurementsoniron-basedsuperconductors.TheyarefromtheOyanagiandTysonteams.Bothinvestigatedthe(1111)iron-basedsuperconductorsLaFeAsO12xFxobtainingdifferentresults.Oyanagietal.foundananomalousbehavioracrossTSDWandTc17.NosuchfinebehaviorhasbeenobservedinthedataofTyson18.Weareconvincedthatinadditiontoalwayspossiblesystematicerrorsorlimitedtemperaturepoints,thedifferences

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observedcanbeassociatedtothedifferentFconcentration:x50.07andx50.11.Actually,thetemperature-dependenceofFe-AsandFe-FeMSRDofoursystemissimilartothatobservedbyTysonetal.,butourworkisinvestigatingadifferenttypeofiron-basedsuper-conductors:the(Ba,K)Fe2As2systembelongingtothe(122)familywhosestructureiswelldifferentfrompnictidesofthe(1111)family.Onthecontrary,aswewilldiscussinthenext,theironisotopeeffectcanbeclearlyseenlookingatthemagnitudeoftheFTs,whichiscorrelatedwiththes2term,i.e.,theEXAFSDebye-Waller(DW)factor.

InFig.1(c)and(d)wecomparethefirst-shellpeak(As-Fepair)oftheFTsofthe54Fe-and56Fe-samplesat10KandatRT,respectively.Thepositionofthefirst-shellpeak,i.e.,theAs-FepairintheFTs,isconstantinthetwosamplesandthesameoccursfortheFe-Fepair(paneld).DatapointoutthattheFe-IEdoesnotaffecttheaveragedistancesofbothFe-AsandFe-Featomicpairsinthetime-domainoftheEXAFSexperiments.AdifferenceintheFTsassociatedtotheisotopicsubstitutionappearsonlylookingatthemagnitudeofthefirst-shellpeak.Asitiswellknow,foragivencrystalstructurethemagnitudeoftheFTpeakdependsbythes2term.AsshowninFig.1(c)adifferenceintheFTmagnitudefortheAs-Fepairisevidentatthelowesttemperature(10K-panelc).ItgraduallyreduceswiththetemperatureandatRTthetwocurvesalmostoverlap(paneld).Toobtainaquantitativedependenceofs2vs.T,weextractedtheDWofbothAs-FeandFe-FepairsbyfittingEXAFSsignalsvs.Tfrom10KtoRTatbothAsandFeK-edges,respectively.FortheAsK-edge,weconsideredtheEXAFSsignalsofthenearestAs-Febond.FortheFeK-edge,bothnearestneighboring(Fe-As)andnext-nearestneigh-boring(Fe-Fe)werefilteredout.WehavetounderlineherethattheAs-FecorrelationisidenticaltotheFe-AscorrelationinanEXAFSprocess.Therefore,inordertoobtainexactinformationontheFe-FebondwefixthedistanceandtheDWfactoroftheFe-AsbondusingparametersobtainedbytheAsK-edgeEXAFSfitduringtheanalysisattheFeK-edgeEXAFS.Fig.2ashowsthequalityoftheEXAFSfitoftheAsK-edgeEXAFSoscillationsatRT.InFig.2bwecomparetheaveragedistancesofbothFe-AsandFe-Febondsof54Fe-and56Fe-samples,respectively,inthewholetemperaturerange(10,300K).DataareconsistentwithwhatshowedinFig1cand1d.WemayemphasizethattheresultisdifferentfromthereportofKhasanovetal.10ontheFeSe12xsystemthatinvestigatedthetemperaturedependenceofthelatticestructurebyNeutronPowderDiffraction(NPD).ThisworkalsoshowsalargeFe-IE(aFe50.81)bymeansofSQUIDmeasurements.AsclaimedbyKhasanovetal.,itisthetinymodulationofthestructuralparameterscausedbytheisotopesub-stitutionthatinducesthelargeFe-IE,beyondtheframeworkoftheBCStheory.However,thesefindingsindicatethataFeisotopeeffectonTcisreallypresentandrelatedwiththeatomicorlatticestructureintheFe-basedsuperconductorsofthe(11)family.Inthesuper-conductor(Ba,K)Fe2As2[(122)family]wedetectedanFe-IEonlyontheatomicdistributionsandnotonbonddistances.Moreover,datapointsoutthattheisotopesubstitutionhasnoeffectonlatticedis-tancesinthesesamplesofthe(122)family.Asaconsequence,itrepresentsanidealmodeltocharacterizetheisotopeeffectonthevibrationalcontributions,afundamentalissuetounderstandtheelectron-phononinteractioninasuperconductor.

InFig.3wealsoshowthedependenceoftheDWfactorsoftheAs-FeandtheFe-Febondsvs.temperatureforboththe54Fe-and56Fe-samples.Aclear‘‘isotopic’’contributionappearsatlowtemperaturewhileathightemperature,thedependenceoftheDWfactorapproachestheclassicallinearbehavior.

Asoutlinedabove,thes2termcontainstwocontributions:thestaticdisorderandthethermaldisorder.InordertoevaluatethecontributionofthestaticdisorderandofthevibrationalfrequencyoftheAs-FeandFe-Febondsofisotopicsubstitutedsamples,weusedthecorrelationEinsteinmodeltofitthes2functionvs.

temperature:

2

Figure1|Temperature-dependentEXAFSdatafortheFeisotopesubstitutedsamples.(a)RepresentativeexperimentalEXAFSoscillationsattheAs(upperpanel)andFe(lowerpanel)K-edgesofthe54Fe-sample.(b)FTsoftheAsK-edgeEXAFSdatameasuredcoolingthesampledownto10K.(c)Comparisonbetweenthefirst-shellpeakoftheFTsofthe54Fe-and56Fe-samplesat10K.(d)Thesameof(c)atRT.

?? 2

1TEh??

:s2(T)~s2coth0z2kmTE2T

ð3Þ

InEq.3s20isthestaticcontributiontothebondandthequalityofour

simulationsisshownbysolidlinesinthesamefigure.Thecorres-pondingvaluesofthestaticcontributionandofthevibrationalfre-quencyofthetwosamplesaresummarizedintable1forbothAs-FeandFe-Febonds.DatashowthattheisotopicsubstitutiongivesrisetoaweakmodulationofthedistributionoftheinteratomicdistancesofbothFe-AsandFe-Fepairs.Moreover,theFe-IEaffectsslightlymorethestaticstructuraldisorderoftheFe-Fepair(it

?2)thanthatoftheAs-Fe(orFe-changesfrom0.0056to0.0053A

?2.Inaddition,theAs)pairthatdecreasesfrom0.0025to0.0023A

staticdisorderoftheFe-FepairismuchlargerthanthatoftheFe-Aspair.Bondswithalargers20value(i.e.,theFe-Fepair)andmodu-lationduetotheisotopicsubstitution,pointoutthatthesebondsaremoreflexible.Thisresultisinagreementwithotherexperimentaldata19andbecausedecreasingtemperaturedowntothestructuraltransitiontemperature(Ts)theFe-Febondwillundergotoasplit-ting20,dataarealsoconsistentwiththeoccurrenceofastructuralphasetransitionintheironlayeroftheBaFe2As2system.AlsobyusingT-EXAFStechnique,intheK0.8Fe1.6Se2[122-chalcogenide]asimilarlatticepropertyhasbeendetected.AlsothisworkpointsoutthattheFe-FebondismoreflexiblethantheFe-Asbond21.Theiron

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isotopeeffectweobserveons20isduetothestaticstructuralmodu-lationandcorrespondstoelectronicandmagneticpropertiesoftheiron-basedSCs.

AsecondimportantresultweobtainedbytheanalysisoftheT-EXAFSexperimentsisthattheEinsteintemperature(TE)thatisaprobeofthevibrationalfrequencyofthesystem,exhibitsashiftforboththeAs-FeandFe-Febondsinducedbytheironisotopesubsti-tution.TheTEoftheAs-Febondchangesabout1.3%from331.5to327.1Kafterthesubstitutionwith54FeintheBa0.6K0.4Fe2As2.IntheFe-Febond,TEshifts,1.5%from169.0to166.5K.BothFe-FeandFe-Asvibrationshavethenasimilar,butnotnegligiblecontributiontothesuperconductivemechanismofiron-basedSCs.Althoughthedetectedchangesaresmall,similarchangesofTEoccurforbothFe-FeandFe-Asbonds.Actually,theEinsteintemperatureTEcanbeassociatedtoaneffectiveforceconstantk0normalizedtothereducedmassm:

p??????????TE~k0=m:ð4ÞActually,becausetheEinsteintemperatureislowerfortheFe-Fe

bond,thelatterissupposedtobesofterthantheFe-Asone.Thisexperimentalresultpointsoutagreaterflexibilityoftheironlayersinthe(Ba,K)Fe2As2system.ToevaluatethecontributionoftheisotopeeffectonthelocalvibrationoftheFe-FeandAs-Fecorrelations,weintroducedthecoefficient

a

3

Figure2|EXAFSfitsandresultofthebonddistancesvs.T.

(a)Experimental(blackcircles)andfit(redlines)EXAFSoscillationsofAs-Fe(uppercurve)andFe-Fe(lowerpanel)shells,respectively,forthe54Fe-sampleatRT.(b)TemperaturedependenceofFe-AsbonddistancesandFe-Febonddistancesforthe54Fe-andthe\\Fe-samples,

respectively.

Figure3|DWfactorss2forthebondofthe54Fe-andthe56Fe-samplesvs.T.(a)As-Febond;(b)Fe-Febond.SymbolsareexperimentaldataextractedfromtheEXAFSfitswhilesolidlinesaresimulationsbasedontheEinsteinmodel.Barsrefertostatisticalerrors.

a~{

MDTEE

ð5Þ

verysimilartothedefinitionoftheTcisotopeeffectcoefficient22.HereTEistheEinsteintemperatureobtainedbytheEXAFSexperi-ments,regardingbothFe-FeandFe-Asbonds(Table1)http://wendang.chazidian.comingvalueslistedintable1wegotvaluesof0.37and0.41fortheAs-FeandFe-Febonds,respectively,veryclosetotheTciron-isotope-effectcoefficientof0.37reportedbytheChen’sgroup7.Ouranalysissupportsthehypothesisthattheelectron-phononinteractionplaysaroleinthesupercon-ductivemechanismofiron-basedsuperconductorsandthatbothFe-FeandFe-Asvibrationscontributetothesuperconductingmechanism.

BecauseFe-Febondslieinthesuperconductinglayers(a-bplane)whileFe-Asbondsareorientedalongthecaxis,ourresultconfirmsintheiron-basedsuperconductorsthepresenceofaweakbulkironisotopeeffect.Inaddition,itpointsoutthepresenceiniron-basedsuperconductorsofanotnegligible3Delectron-phononinter-actionasshowninFig.4.Thisresultcanbealsousedtointerpretthenearlyisotropicsuperconductingpropertiesofiron-basedsuperconductors23,systemswelldifferentfromlayeredcupratesuperconductors.

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Discussion

AnaccurateEXAFSanalysisofthe(Ba,K)Fe2As2systemvs.temper-atureshowsthatanironisotopeeffectcorrelatedtothestaticstruc-turaldistributionandwithlocalvibrationscanbedetected.Changesofthestaticstructuraldistributionmayoccurviachangesoftheinter-atomicdistancesandoftheatomicdistribution(orofthedis-order).TheFe-IEdoesnotinduceappreciablechangesintheinter-atomicdistanceswhiletheatomicdistributionisslightlyaffected.Atpresent,itremainshoweverextremelydifficultidentifyaclearcor-relationamonglocaldisordercontributions(presentinapowdersample)andsuperconductingmechanisms.Inaddition,weclearlyshowedtheexistenceofaFe-IEcontributiontolocalvibrationaltermforbothFe-AsandFe-Febonds.Theirisotopicexponentsareinagreementwithpreviousexperiments7lookingattheiron

isotope

Table1|ValuesoflocalstaticdistortionsandEinsteintempera-turesassociatedtotheAs-FeandFe-Febondsinthe54Fe-and56

Fe-samples

bondsAs-FeFe-Fe

Isotope

5456

?2)s02(A0.002560.0002

0.002460.00020.005660.00020.005360.0002

TE(K)331.562.0327.162.0169.061.0166.56

1.0

a0.370.41

FeFe54Fe56Fe

4

Figure4|LayoutofFe-FeandFe-Asvibrationsintheiron-basedsuperconductors.Fe-Febondsareinthesuperconductinglayersandtheirvibrationsattractelectronsinthecdirection,whileFe-Asbondsareorientedalongthecaxisandtheirvibrationsaffectelectronsinthea-blayers.

effectinthesesystems.ThisresultrepresentsthefirstexperimentallinkbetweentheFe-IEofTcexperimentsandthesuperconductingmechanismatamicroscopiclevel.

Methods

IsotopicallysubstitutedpolycrystallinesamplesweresynthesizedbythesolidstatereactionmethoddescribedindetailinRef.7.Previousinvestigationofthissuper-conductorshowedalargeironisotopeeffectontheTc7.AsandFeK-edgeabsorptionspectrawerecollectedusingaSi(111)doublecrystalmonochromatorattheXAFSstationofthe1W1BbeamlineoftheBeijingSynchrotronRadiationFacility(BSRF).Thestorageringwasworkingat2.5GeVwithanelectroncurrentdecreasingfrom240to160mAwithinabout8hours.Samplesweregroundintofinepowdersandbrushedontotapesthatwerestackedtogethertogiveapproximatelyonex-ray-absorptionlengthsattheircorrespondingmetaledges.Sampleswerecooledbyaclosed-cycleheliumrefrigeratorwithanerrorbaronthetemperaturedeterminationofabout0.1K.DatawereprocessedandanalyzedusingbothUWXAFS24andFEFF8.0

packages25,26togeneratescatteringamplitudesandphaseshiftsstartingfromavailableXRDrefinements(seesupplementarymaterialsofRef.7).

1.Bednorz,J.G.&Mu¨ller,K.A.PossiblehighTcsuperconductivityintheBa-La-Cu-Osystem.Z.Phys.B64,189–193(1986).

2.Anderson,P.W.TheoryofSuperconductivityintheHigh-TcCuprates.(PrincetonUniversityPress,Princeton,1997).

3.Kresin,V.,Morawitz,H.&Wolf,S.A.MechanismofConventionalandHighTcSuperconductivity.(OxfordPress,Oxford,1993).

4.Kamihara,Y.etal.Iron-BasedLayeredSuperconductorLa[O12xFx]FeAs(x50.05–0.12)withTc,26K.J.Am.Chem.Soc.130,3296–3297(2008).5.Chen,X.H.etal.Superconductivityat43KinSmFeAsO12xFx.Nature453,761–762(2008).

6.Takahashi,H.etal.Superconductivityat43Kinaniron-basedlayeredcompoundLaO12xFxFeAs.Nature453,376–378(2008).

7.Liu,R.H.etal.AlargeironisotopeeffectinSmFeAsO12xFxandBa12xKxFe2As2.Nature459,64–67(2009).

8.Shirage,P.M.etal.InverseIronIsotopeEffectontheTransitionTemperatureofthe(Ba,K)Fe2As2Superconductor.Phys.Rev.Lett.103,257003(2009).9.Shirage,P.M.etal.AbsenceofanAppreciableIronIsotopeEffectonthe

TransitionTemperatureoftheOptimallyDopedSmFeAsO12ySuperconductor.Phys.Rev.Lett.105,037004(2010).

10.Khasanov,R.etal.Ironisotopeeffectonthesuperconductingtransition

temperatureandthecrystalstructureofFeSe12x.NewJ.Phys.12,073024(2010).11.Yanagisawa,T.etal.IsotopeEffectinMulti-BandandMulti-ChannelAttractiveSystemsandInverseIsotopeEffectinIron-BasedSuperconductors.J.Phys.Soc.Jpn.78,094718(2009).

12.Bussmann-Holder,A.&Keller,H.Commenton‘‘IsotopeEffectinMulti-BandandMulti-ChannelAttractiveSystemsandInverseIsotopeEffectinIron-BasedSuperconductors’’J.Phys.Soc.Jpn.79,126001(2010).

13.Khasanov,R.,Bendele,M.,Bussmann-Holder,A.&Keller,H.Intrinsicandstructuralisotopeeffectsiniron-basedsuperconductors.Phys.Rev.B82,212505(2010).

14.Greegor,R.B.&Lytle,F.W.Extendedx-rayabsorptionfinestructure

determinationofthermaldisorderinCu:Comparisonoftheoryandexperiment.Phys.Rev.B20,4902–4907(1979).

15.Purans,J.etal.IsotopicEffectInExtendedX-Ray-AbsorptionFineStructureofGermanium.Phys.Rev.Lett.100,055901(2008).

16.Dalba,G.&Fornasini,P.EXAFSDebye-WallerFactorandThermalVibrationsofCrystals.J.SynchrotronRad.4,243–255(1997).

17.Zhang,C.J.etal.Low-temperaturelatticestructureanomalyinthe

LaFeAsO0.93F0.07superconductorbyx-rayabsorptionspectroscopy:Evidenceforastrongelectron-phononinteraction.Phys.Rev.B78,214513(2008).18.Tyson,T.A.etal.Temperature-dependentlocalstructureofLaFeAsO12xFx:Probingtheatomiccorrelations.J.Appl.Phys.108,123715(2010).

19.Cheng,J.etal.PotassiumdopingeffectonthelatticesofteningandelectronicstructureofBa12xKxFe2As2probedbyX-rayabsorptionspectroscopy.J.SynchrotronRad.17,730–736(2010).

20.Rotter,M.etal.SpinDensityWaveAnomalyat140KintheTernaryArsenideBaFe2As2.Phy.Rev.B78,020503(R)(2008).

http://wendang.chazidian.comdecola,http://wendang.chazidian.comrgelocaldisorderinsuperconductingK0.8Fe1.6Se2studiedbyextendedx-rayabsorptionfinestructure.J.Phys.:Condens.Matter24,115701(2012)

22.Bill,A.,Kresin,V.Z.&Wolf,S.A.TheIsotopeEffectinSuperconductors(Reviewarticle),inPairCorrelationsinMany-FermionSystems,,p.25.(PlenumPress,1998).

23.Yan,H.Q.etal.Nearlyisotropicsuperconductivityin(Ba,K)Fe2As2.Nature457,565–568(2009).

24.Stern,E.A.etal.TheUWXAFSAnalysisPackage:PhilosophyandDetails.PhysicaB208&209,117(1995).

25.Rehr,J.J.,MustredeLeon,J.,Zabinsky,S.I.&Albers,R.C.Theoreticalx-rayabsorptionfinestructurestandards.J.Am.Chem.Soc.113,5135–5140(1991).26.Zabinsky,S.I.,Rehr,J.J.,Ankudinov,A.,Albers,R.C.&Eller,M.J.MultipleScatteringCalculationsofX-rayAbsorptionSpectra.Phys.Rev.B52,2995–3009(1995).

Acknowledgements

WethankYiLuo,YaninXie,JinZhang,LirenZhengfortheirvaluablehelpanddiscussions.ThisworkwaspartlysupportedbytheNationalOutstandingYouthFund(ProjectNo.10125523toZ.Y.W.),theKnowledgeInnovationProgramoftheChineseAcademyofSciences(KJCX2-YW-N42)andtheNationalBasicResearchProgramofChina

(2012CB825800.Oneoftheauthors(W.S.C.)thankstheNaturalScienceFoundation(11179023;11275227)andtheFundamentalResearchFundsfortheCentralUniversities(WK2310000019).

Authorcontributions

Z.Y.W.designedtheresearch.W.S.C.performedtheexperimentandanalyzedthedata,withthehelpfromJ.C.andS.Q.C.T.D.H.providedthesupportforthedatacollection

and

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