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

FIG.1:(coloronline)Observed(crosses)andcalcu-lated(solidline)neutronpowderdi ractionintensitiesforthesuperconductorLaO0.87F0.13FeAsat1.6KusingspacegroupP4/nmm.VerticallinesareBraggpeakpositionsforLaO0.87F0.13FeAs(lower)andAlsampleholder(upper).ThedatawerecollectedonDCSwithanincidentbeamwave-length

λ=1.8 A.Thestructurewasre nedusingtheGSASprogram[33].Inset:TheresistivityofLaO0.87F0.13FeAsshow-ingthesuperconductingtransitionatTC=26K.

neticmomentM=0.36(5)µBperFeat8K[13]canbeexplainedbyanassociatedstructuretransition[34].ThestrongestmagneticBraggpeak(1/2,1/2,3/2)isonly1.1%oftheintensityofthestructural(002)peak[13].ForsuperconductingsamplesLaO0.92F0.08FeAsandLaO0.89F0.11FeAs,magneticpeaksoftheSDWorderarenotobservedabovemeasurementstatisticslevelofabout0.5%ofthe(002)peakat8K[13]and70K[14],respectively.NeitherdoesoursuperconductingsampleLaO0.87F0.13FeAsshowanydetectableSDWorderdownto1.6Kinthesuperconductingstate(Fig.1),norat30Kinthenormalstate(Fig.2).Applyingamagnetic eldof9Talsodoesnotinduceanymagneticpeakstrongerthan0.5%ofthe(002)Braggpeak.Theseresultssup-porttheproposalthattheSDWandsuperconductingorderparametersarecompetingforitinerantelectronsandholesontheFermisurface[12,19],anddonotfa-vorthetheoryofcoexistenceofantiferromagnetismwithsuperconductivityinLa(O,F)FeAs.

Conventionalsuperconductivityismediatedbyphonons,andthephononspectrumhasbeencalcu-

TABLEI:Re nedstructureparametersofLaO0.87F0.13FeAsat1.6K.Spacegroup:P4/nmm(No.129).a4.0245(3) A,c=8.713(1) A,V=141.126(9) A3=

.Rp=11.72%,

wRp=15.53%.Atomsitexyz

B( A2)La2c1/41/40.1442(8)1.0(2)Fe2b3/41/41/2

0.3(1)As2c1/41/40.6541(8)0.02(2)O/F

2a

3/4

1/4

0.9(2)

2

FIG.2:(coloronline)Neutronpowderdi ractionintensitiesofLaO0.87F0.13FeAsat30Kandzero eldinthenormalstate(openred)andat1.6Kand9Tmagnetic eldinthesuperconductingstate(solidblue).Thedataarecollectedwithaneutronwavelengthλ=4.8 AtofocusonthesmallQrangeformagneticsignals.Theredsymbolshavebeenshiftedupforclarity.ThearrowsindicatemagneticBraggpeakpositionsoftheSDWorder.Nomagneticpeakstrongerthan0.5%ofthe(002)existsinthespectra.

latedforLa(O,F)FeAs[15,21].Ithasbeenusedtocalculatetheelectron-phononcoupling,andtheTCfromthismechanismismuchlowerthantheobservedvalue[19,21].Tovalidatethetheoreticalcalculations,wehavemeasuredinelasticneutronscatteringfromphononsinLaO0.87F0.13FeAs.Forpolycrystallinesamples,theintensityisgivenby

I(Q,ω)=

σi Q2

exp( 2WDi)

i(ω)

[n(ω,T)+i

2m1],

iω(1)

whereσiandmiaretheneutronscatteringcrosssec-tionandatomicmassoftheithatom(La,O/F,Fe,As),n(ω,T)istheBosefactor,WitheDebye-Wallerfactor[35]. Theweightedphonondensityofstates(PDOS)D(ω)=iDi(ω)inEq.(1)di ersfromthebarePDOScalculatedin[15,21]byafactorofthesquaredmodulusofphononeigenvectors.ThemeasuredneutronscatteringintensityI(Q,ω)isfurtherweightedbyσi/miofdi erentatoms.Butthepeakpositionsinthemea-suredandbarePDOSusuallyremainthesame[35].

Inthetopframeof gure3,themeasuredI(ω)=dQI(Q,ω)at1.6and110Kusingneutronsofwave-length1.8 A,integratedfromQ=2.5to7 A 1,isshown.InthemiddleframeisshownthetheoreticalintensitycalculatedfromthebarePDOSofSinghandDu[15],convolutedwithinstrumentresolution.At1.6K,theBosefactorleadstozerointensityfornegativeen-ergytransfer,andmeasurementsthereservetodeter-mine

thebackground.TheintegratedintensityI(Q)=dωI(Q,ω)fromthenegativeenergyside,showninthebottomframe,demonstratestheexpectedbehaviorforphononscattering,whichisapproximatelyproportional