Neutron scattering study of the oxypnictide superconductor L

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-