JournalofthePhysicalSocietyofJapan DRAFT ObservationofPhonon-AssistedMagnonAbsorption inSpin-Orbit Coupling Induced MottInsulatorSr IrO 2 4 YasuyukiHirata,HiroyukiTajima,andKenyaOhgushi InstituteforSolidStatePhysics,UniversityofTokyo,Kashiwanoha5-1-5,Kashiwa,Chiba277-8581,Japan 3 KEYWORDS: Infraredspectroscopy,Two-dimensionalantiferromagnet,Mottinsulator,Iridiumoxides 1 0 2 300 K n 8000 260 K a Thestrongelectroncorrelationeffectintransitionmetalox- 240 K J idesisarichsourceofvariousintriguingphysicalproperties 220 K 180 K tr-el] 9 tsasihmuunepctpihefloaerscayrirosneonrgmmeddeuaaxcggpatnneimveerotitpitivsyclmseMkeimisatoenetctdrsogitnrpesususpcupewtlreuarhrtocieenxo,nginwddcsehautsaercrtrtsieeieuv.r1ctishthSyea.uarhcOesihgndLheeoax-p2tooeCeftmdiuctpOhipnee4rtrooamwpttuoeihtrsrehe-t (cid:13)(cid:14)(cid:4)(cid:22)(cid:22)(cid:21)(cid:12)(cid:21)(cid:4)(cid:5)(cid:20)(cid:11)(cid:12)(cid:7)(cid:15)40000 (cid:11)(cid:2) (cid:15) (cid:10)11 (cid:14)624000(cid:28)00 (cid:10) KK(cid:1)KK(cid:7)(cid:16) (cid:23) (cid:24) (cid:25) (cid:26) s tieshavetypicallybeeninvestigatedwiththerespecttothe3d (cid:12)(cid:18)8000 t. electronsystem,becausetheelectroncorrelationeffectinthe (cid:18)(cid:5)(cid:10) a 4d/5d electron systems is much weakerdue to the more ex- (cid:19)(cid:20)(cid:21) m (cid:18)(cid:9) tended character of the wavefunctions. Nevertheless, recent (cid:17) d- studies have revealed2 that the Sr2IrO4 layered-perovskite (cid:16)(cid:8)4000 (cid:26) n with(5d)5valenceelectronsisaMottinsulatorthatundergoes (cid:16) o themagnetictransitionintoacheckerboard-typespinarrange- (cid:11)(cid:8)(cid:15)(cid:10)(cid:27)(cid:10)(cid:1)(cid:7) c mentat a Ne´el temperatureof T = 240 K.4 Thisinsulating 0 [ N 0 1000 2000 3000 state can be understood as follows: the strong spin-orbit in- 2 teraction split the energy levels of the Ir 5d t orbitals into (cid:1)(cid:2)(cid:3)(cid:4)(cid:5)(cid:6)(cid:7)(cid:8)(cid:4)(cid:9)(cid:10)(cid:11)(cid:12)(cid:7)(cid:13)(cid:14)(cid:15) 2g v doubly-degeneratedJ =1/2andquadruply-degeneratedJ 1 = 3/2orbitals.The Jeff= 1/2orbitalsformahalf-filledbaneffd Fig. 1. AbsorptionspectraasafunctionofphotonenergyforSr2IrO4with 4 eff thicknesses of(a)10and(b)5µm.Thepeakstructurelabeled asAisthe 2 withanarrowbandwidth,whichresultsintheenhancedelec- oxygenstretchingphononmode,andthoselabeledasB,C,andDaretwo- 5 tron correlation effect that leads to a Mott insulating state. phononmodes.ThebroadstructurelabeledasE(shadedarea)isthephonon- 11. Aalsfoewmaogthneerticiriidnisuumlatooxrsid.5e,6sCsuacrhriearssBcaan2IbrOe4doapneddCbayIrcOh3emarie- a2s4s0i0stecdmm−1aigsnaonnaarbtisfoacrpttoioringi(nsaeteintgexftrofmorCdOet2aialsb)s.oTrphteiodni.pstructure around 2 calsubstitutioninthesespin-orbitcouplinginducedMottin- 1 sulators,sothattheyarepotentialhigh-temperaturesupercon- : v ductorcandidates.7,8 i To clarify electronic states and pursuit the possibility of phonon-assisted magnon absorption in the absorption spec- X trum and deduced J = 57 meV, which is consistent with the high-temperature superconductivity in Sr IrO requires the r 2 4 resonantinelasticx-rayscatteringmeasurement. a information on the superexchange interaction between two Plate-like single crystals of Sr IrO (1×1×0.2mm3) were Ir spins J. However, in contrast to the well-studied on-site 2 4 characterofIrorbitals,9,10therehavebeenfewstudiesonthe grown using the flux method.9 SrCO3, IrO2 and SrCl2 pow- dersweremixedinamolarratioof3:1:15,heatedto1300◦C, inter-siteinteractions.JackeliandKhaliullintheoreticallycal- culatedJtobe45meV,11whileJwasestimatedtobe60meV andthencooledto900◦Catacoolingrateof8◦C/h.Theab- planesoftheobtainedcrystalswerepolishedwithAl O pow- fromdetectionofthesingle-magnondispersionwithresonant 2 3 inelasticx-rayscattering.12Toestablishareliableexperimen- dersuntilthethicknessbecame5-10µm.Transmissionspec- tra were measured using a Fourier-transform infrared spec- talevaluationofJ,utilizinganothermethodishighlydesired. trometerwiththegeometryofincidentlightalongthe c-axis Infrared transmission measurement is a powerful technique for the detection of magnons.13–15 Although single magnon and polarizationalong the a-axis in the frequencyrange be- tween600and4000cm−1.Thetemperaturewascontrolledin excitationsareopticallyforbiddenunderthespatialinversion therangeof20and300KusingaHeflowcryostat. symmetry,twomagnonscoupledwithonephononcanbeex- Figure 1 shows absorption spectra of Sr IrO with thick- citedbylight,whichisdetectedasaphonon-assistedmagnon 2 4 nesses of (a) 10 and (b) 5 µm. The background absorption absorption. J values have been successfully estimated from increaseslinearlywiththephotonenergyandoriginatesfrom thepeakenergyofphonon-assistedmagnonabsorptioninthe absorptionspectrumforLa NiO andLa CuO .13–15 the low-energy tail structure of the Mott excitation centered 2 4 2 4 around 0.54 eV,10 and decreases monotonically on cooling. In this paper, we reporton magnon excitation in the anti- The spectraforthe 10 µm sample [Fig. 1(a)],hasan artifact ferromagneticMott insulatorSr IrO , as revealed by optical 2 4 originating from CO absorption at around 2400 cm−1, and transmission measurements. We have successfully observed 2 J.Phys.Soc.Jpn. DRAFT peakstructuresat698(A),972(B),1042(C),and1415cm−1 reported J =121meVforLa CuO .15Thisindicatesthatthe 2 4 (D)arediscernibleatallmeasuredtemperatures.Inaddition, energyscaleofMottphysicsiniridiumoxideswouldberather a broad peak appears at 1870 cm−1 (E) only in the spectra measured below 180 K. In the spectra for the 5 µm sample large, even in a 5d system;7 if it is simply assumed that the [Fig.1(b)],theB,C,andDmodesareconcealedbytheoscil- superconductingtransition temperature T is proportionalto c latingfeaturesduetotheinterferenceofbackwardreflections; J, then the presentresultsuggeststhatdopediridiumoxides however,theAandEpeakstructurescanberecognizedmore canbepotentialsuperconductorswithT comparabletohigh- c clearly in the thinner sample. The origin of these features T copperoxidesuperconductors. c can be elucidated with reference to phonon frequencies de- Inconclusion,infraredtransmissionspectroscopywasper- terminedfromreflectivitymeasurements.10Sr2IrO4hasthree formedtoinvestigateSr2IrO4asaspin-orbitcouplinginduced in-planeopticalphononsthatinvolveoxygenvibrations;two Mottinsulator.Aphonon-assistedmagnonabsorptionwitha bendingmodesat284and356cm−1,andonestretchingmode peak energy of 1870 cm−1 is developed below 180 K. The at 663 cm−1.10 Peak A can thereforebe reasonablyassigned nearest-neighborsuperexchangeinteractionisestimatedtobe to the oxygenstretching mode.Peaks B, C, and D are inter- J = 57 meV, which is consistent with results obtained from pretedastwo-phononmodes;284and663cm−1 phononsfor resonantinelasticx-rayscatteringandRamanscatteringmea- peakB,356and663cm−1 phononsforpeakC,andtwo663 surements. The J value is approximately half that of J for cm−1phononsforpeakD. La CuO , which suggests that a novel phenomenon is real- 2 4 ThebroadEstructurecenteredat1870cm−1cannotbeex- ized at rather high temperature, even in 5d transition metal plainedbyeitherone-phononortwo-phononexcitations.This oxides. mode is developed only below 180 K, which is lower than We are gratefulto Y. Ueda,M. Isobeand S. Fujiyamafor TN = 240 K, which suggests a magnetic origin. Moreover, helpfuldiscussionsandexperimentalsupport.Thisworkwas the peak width of ca. 200 cm−1 and the peak height of ca. supportedbySpecialCoordinationFundsforPromotingSci- 500 cm−1 are comparable with those of the phonon-assisted enceandTechnology,PromotionofEnvironmentalImprove- magnon absorption in La2CuO4, where the peak width is ment for Independence of Young Researchers, and a Grant- ca. 300 cm−1 and the peak height is ca. 200 cm−1.13 The in-AidforScientificResearch(B)(No.20740211). high-energy tail structure observed in the phonon-assisted magnonabsorptionofLa CuO isnotexplicitlyrecognizedin 2 4 Sr IrO .Thisismostlikelybecausethetailstructurelapsover 2 4 thelargebackgroundelectronicabsorptions;wehereassumed 1) J.G.BednorzandK.A.Mu¨ller,Z.Phys.B64,189(1986). asimpleextrapolationforthetailstructureandindicated the 2) B.J.Kim,H.Jin,S.J.Moon,J.-Y.Kim,B.-G.Park,C.S.Leem,J.Yu, T.W.Noh,C.Kim,S.-J.Oh,J.-H.Park, V.Durairaj, G.Cao, andE. magnoncontributionsbytheshadedareainFig.1.Consider- Rotenberg,Phys.Rev.Lett.101,076402(2008). ingtheseissues,weconcludethatthebroadEstructureisthe 3) M.Itoh,T.Shimura,Y.Inaguma,andY.Morii,J.SolidStateChem.118, phonon-assistedmagnonabsorption,wheretwomagnonsand 206(1995). onephononareexcited. 4) M.K.Crawford,M.A.Subramanian, R.L.Harlow,J.A.Fernandez- According to the theoretical study by Lorenzana and Baca,Z.R.WangandD.C.Johnston,Phys.Rev.B49,9198(1994). 5) H.Okabe,M.Isobe,E.Takayama-Muromachi,A.Koda,S.Takeshita, Sawatzky,ifthenearest-neighborS =1/2HeisenbergHamil- M.Hiraishi,M.Miyazaki,R.Kadono,Y.Miyake,andJ.Akimitsu,Phys. tonian is considered, then the peak energy of the phonon- Rev.B83,155118(2011). assisted magnon absorption is represented as Einfrared = 6) K.Ohgushi,T.Yagi,H.Gotou,Y.Kiuchi,andY.Ueda,PhysicaB404, 2.73J+ω ,whereω istheenergyoftheassistingphonon.15 3261(2009). ph ph Ifwesupposethatthe663cm−1 oxygenstretchingmodein- 7) F.WangandT.Senthil,Phys.Rev.Lett.106,136402(2011). 8) J.S.Lee, Y.Krockenberger, K.S.Takahashi, M.Kawasaki, and Y. volvestheobservedphonon-assistedmagnonmode,asinthe Tokura,Phys.Rev.B85,035101(2012). case of La CuO , then J is calculated to be 57 meV.16 This 2 4 9) B.J.Kim,H.Ohsumi,T.Komesu,S.Sakai,T.Morita,H.Takagi,and isfairlyclosetothe estimationfromresonantinelastic x-ray T.Arima,Science323,1329(2009). scattering (J = 60 meV), although direct comparison is not 10) S.J.Moon,H.Jin,W.S.Choi,J.S.Lee,S.S.A.Seo,J.Yu,G.Cao,T. appropriate because not only the nearest-neighborexchange W.Noh,andY.S.Lee,Phys.Rev.B801,195110(2009). 11) G.JackeliandG.Khaliullin,Phys.Rev.Lett.102,017205(2009). interactionJ,butalsothenext-nearest-neighborexchangein- 12) J.Kim, D.Casa, M.H.Upton, T.Gog, Y.-J.Kim, J.F.Mitchell, M. teraction J′, and the third-nearest-neighbor exchange inter- vanVeenendaal,M.Daghofer,J.vandenBrink,G.Khaliullin,andB. action J′′, are considered in the latter estimation.12 Cetin et J.Kim,Phys.Rev.Lett.108,177003(2012). al. reported a broad mode around 1800 cm−1 in the Raman 13) J.D.Perkins,J.M.Graybeal,M.A.Kastner,R.J.Birgeneau,J.P.Falck, spectra.ThemodeisdevelopedbelowT andis assignedto andM.Greven,Phys.Rev.Lett.71,1621(1993). N a two-magnonexcitation.17 In the frameworkofthe nearest- 14) J.D.Perkins,D.S.Kleinberg,M.A.Kastner,R.J.Birgeneau,Y.Endoh, K.Yamada,andS.Hosoya,Phys.Rev.B52,R9863(1995). neighborS = 1/2 Heisenberg model, the peak energy of the 15) J.LorenzanaandG.A.Sawatzky,Phys.Rev.Lett.74,1867(1995);J. two-magnonmodeisrepresentedas ERaman =3.38J;18there- LorenzanaandG.A.Sawatzky,Phys.Rev.B52,9576(1995). fore,theexchangeinteractionisestimatedtobe J =66meV, 16) Iftheassistingphononis284or356cm−1mode,thentheestimatedJ whichisconsistentwithourpresentresult.Ontheotherhand, valuebecomes74or71meV,respectively. 17) M.F.Cetin,P.Lemmens,V.Gnezdilov,D.Wulferding,D.Menzel,T. the results of the resonant magnetic x-ray diffuse scattering Takayama,K.Ohashi,andH.Takagi,Phys.Rev.B85,195148(2012). give J ∼ 0.1 eV, which is larger than our conclusion.19 The 18) C.M.CanaliandS.M.Girvin,Phys.Rev.B45,7127(1992). reasonofthisdiscrepancyisnotclearatpresent. 19) S.Fujiyama,H.Ohsumi,T.Komesu,J.Matsuno,B.J.Kim,M.Takata, J = 57 meV for Sr IrO is approximatelyhalf that of the T.Arima,andH.Takagi,Phys.Rev.Lett.108,247212(2012). 2 4 2