Observation of Phonon-Assisted Magnon Absorption in Spin-Orbit Coupling Induced Mott Insulator Sr$_{2}$IrO$_{4}$

0.11 MB·English

by  Yasuyuki Hirata

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
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