Groundstateoxygenholesandthemetal-insulatortransitioninthe
negativechargetransferrare-earthnickelates
ValentinaBisogni1,2*,SaraCatalano3,RobertJ.Green4,5,MartaGibert3,RaoulScherwitzl3,Yaobo
Huang1,6,VladimirN.Strocov1,PavloZubko3,7,ShadiBalandeh4,Jean-MarcTriscone3,George
Sawatzky4,5andThorstenSchmitt1#
1
ResearchDepartmentSynchrotronRadiationandNanotechnology,
PaulScherrerInstitut,CH-5232VilligenPSI,Switzerland.
2
NationalSynchrotronLightSourceII,BrookhavenNationalLaboratory,Upton,NewYork11973,
USA.
3
DépartementdePhysiquedelaMatièreCondensée,UniversityofGeneva,24QuaiErnestAnsermet,1211Genève4,Switzerland.
4
DepartmentofPhysicsandAstronomy,UniversityofBritishColumbia,Vancouver,British
Columbia,CanadaV6T1Z1.
5
QuantumMatterInstitute,UniversityofBritishColumbia,Vancouver,BritishColumbia,Canada
V6T1Z4.
6
BeijingNationalLaboratoryforCondensedMatterPhysics,andInstituteofPhysics,Chinese
AcademyofSciences,Beijing100190,China.
7
LondonCentreforNanotechnologyandDepartmentofPhysicsandAstronomy,University
CollegeLondon,17-19GordonStreet,LondonWC1H0HA,UnitedKingdom.
*e-mail:[email protected]
#e-mail:[email protected]
The metal-insulator transitions and the intriguing physical properties of rare-earth perovskite
nickelates have attracted considerable attention in recent years. Nonetheless, a complete
understanding of these materials remains elusive. Here, taking a NdNiO3 thin film as a
representative example, we utilize a combination of x-ray absorption (XAS) and resonant
inelastic x-ray scattering (RIXS) spectroscopies to resolve important aspects of the complex
electronic structure of the rare-earth nickelates. The unusual coexistence of bound and
continuumexcitationsobservedintheRIXSspectraprovidesstrongevidencefortheabundance
of oxygen 2p holes in the ground state of these materials. Using cluster calculations and
Anderson impurity model interpretation, we show that these distinct spectral signatures arise
from a Ni 3d8 configuration along with holes in the oxygen 2p valence band, confirming
suggestionsthatthesematerialsdonotobeya“conventional”positivecharge-transferpicture,
butinsteadexhibitanegativecharge-transferenergy,inlinewithrecentmodelsinterpretingthe
metaltoinsulatortransitionintermsofbonddisproportionation.
The intriguing perovskite nickelates family ReNiO3 (with Re= rare-earth) {1,2,3,4} have garnered
significantresearchinterestinrecentyears,duetotheremarkablepropertiestheyexhibit.These
includeasharpmetaltoinsulatortransition(MIT)tunablewiththeReradius{2},unusualmagnetic
order {5} and the suggestion of charge order {6} in the insulating phase. While ReNiO3 single
crystalsareveryhardtosynthesize,causingearlierexperimentstobemainlyrestrictedtopowder
samples,extremelyhighqualityepitaxialthinfilmscanbenowproduced.Asanadditionalasset,
ReNiO3inthinfilmformcanexhibitevenricherpropertiescomparedtobulk,e.g.tunabilityofthe
metal-insulatortransitionbystrain{7,8},thickness{9,10,11},orevenbyultrafastopticalexcitation
of the substrate lattice degree of freedom {12}. Additionally, there has been a lot of interest in
2
nickelate-based heterostructures, motivated on one hand by theoretical predictions of possible
superconductivity{13},andontheotherhandbyrecentobservationsofexchangebiaseffects{14}
andmodulationoftheorbitaloccupationduetostrainandinterfaceeffects{15,16,17,18}.
Theoriginoftherichphysicsbehindtheseuniquepropertiesiscomplicatedbytheusualelectron
correlationproblemoftransitionmetaloxides{19}.Asaconsequence,afullunderstandingofthe
mechanism driving the MIT in ReNiO3 remains elusive still today. Hampering the discovery of a
universally accepted description of the MIT is the more fundamental problem of understanding
theReNiO3electronicstructureandthecorrespondingNi3dorbitaloccupation.
InFigure1weillustrateinaschematicrepresentationofthesingleelectronexcitationspectrahow
differentelectronicconfigurationscanresultfromthetwopossibleregimesoftheeffectivecharge
transferenergyΔ’(calledforsimplicitychargetransferenergythroughoutthetext).Usingformal
valence rules, it is expected that the Ni atoms exhibit a 3d7 (Ni3+) character, likely in a low spin
(S=1/2) configuration. This ground state (GS) is obtained in Fig. 1 (a) for Δ’ > 0. However, highvalenceNi3+systemsarerare,andalthoughmanystudiesindeedviewReNiO3as“conventional”
positive charge-transfer compounds yet with the addition of a strong Ni-O covalency and
consequentlyaGSconfigurationofthetypeof𝛼 ∙ |3𝑑 ! + 𝛽 ∙ |3𝑑 ! 𝐿 (whereLisanO2phole){1,
20 21 22 23
, , , }, mounting evidence suggests that the ground state disobeys conventional rules.
Alternatively,anegativecharge-transfersituation{24,25},whereafinitedensitynofholesLnisselfdopedintotheO2pbandandNitakesona3d8configuration(i.e.3d8Ln),isrecentlyreceivingan
increasing interest. This scenario is represented in Figure 1 (b) for Δ’ < 0. Notably, the negative
charge-transferpictureisatthebaseofrecentchargeorbonddisproportionationmodeltheories
3
where,asfirstsuggestedbyMizokawa{26},thedisproportionedinsulatingstateischaracterizedby
alternating Ni 3d8 (n=0) and Ni 3d8L2 (n=2) sites arranged in a lattice with a breathing-mode
distortion { 27 , 28 , 29 , 30 }. These models distinctively differ from the more traditional chargedisproportionationoneswheretheNi3d7latticemovestowardsanalternationofNi3d6and3d8
sitesintheinsulatingphase{6,31,32,33,34}.
To get a full understanding of the MIT and of the unique physical properties of the rare-earth
nickelates,itiscrucialtoinvestigatethegroundstateelectronicstructureinthisclassofmaterials.
To this purpose, we stress that while both positive and negative charge-transfer interpretations
introduced above can be described as highly covalent, there are striking inherent differences
betweenthetwo.Fortheformer{16,20,21,32,34},theGS𝛼 ∙ |3𝑑 ! + 𝛽 ∙ |3𝑑 ! 𝐿 canbemodeledas
a Ni 3d7 impurity hybridizing with a full O 2p band. Here the primary low energy charge
fluctuations which couple to the GS in first order are the ones from the O 2p to the Ni 3d7
impurity,mixinginsomeNi3d8Landhigherordercharacterintothewavefunction.However,for
the negative charge-transfer case {19, 25, 26, 27, 28, 29, 30}, all Ni sites assume a 3d8 state with on
averagen=2holesinthesixoxygenscoordinatingacentralNiion.Thiscaseismoreaptlymodeled
by a Ni 3d8 impurity hybridizing with a partially filled O 2p band. The electronic structure and
consequently the character of the gap are vastly different in the two scenarios:𝛼 ∙ |3𝑑 ! + 𝛽 ∙
|3𝑑 ! 𝐿 withaO2p–Ni3dlikegapor3d8LnwithaO2p–O2plikegap[refertoFigure1(a)and(b)
respectively]. It is thus crucial to determine which scenario actually applies to the rare-earth
nickelates,inordertofurtherunderstandandpossiblytunetheirfascinatingproperties.
4
Here,wecombinetwoX-rayspectroscopies,namelyx-rayabsorption(XAS)andresonantinelastic
x-rayscattering(RIXS)attheNiL3-edge,toresolveiftheelectronicstructureofReNiO3followsa
positiveornegativecharge-transferpicture.WhiletheXASresultsaresimilartothosepreviously
reported, the first ever measurements of high resolution RIXS provide crucial insights into the
natureoftheexcitationspresent.Theunusualcoexistenceofboundandcontinuumcontributions
acrossthenarrowNiL3resonanceandthespecificnatureoftheorbitalexcitations,allowsusto
verify that the electronic ground state contains abundant O 2p holes and that the Ni sites are
indeed best described as 3d8Ln, rather than a low spin Ni 3d7, showing that the rare-earth
nickelates are indeed self-doped, negative charge-transfer materials. Further, the RIXS spectra
exhibitaclearsuppressionofthelow-energyelectron-holepaircontinuumintheinsulatingphase,
providingnotonlyafingerprintoftheopeningoftheinsulatinggapatT<TMIbutalsoexperimental
evidence of the dominant O 2p-character for the states across EF, as expected for a negative
charge-transfersystem.
Results
Bulk-likeNdNiO3thinfilm.Severalhigh-qualityNdNiO3thinfilmsgrownonavarietyofsubstrates
wereinvestigated.Epitaxialfilmswerepreparedbyoff-axisradiofrequencymagnetronsputtering
{8, 9, 35 , 36 } and were fully characterized by x-ray diffraction measurements, atomic force
microscopy,transportandsoftx-rayscatteringmeasurements.Inthefollowing,wewillfocuson
30nmthickNdNiO3filmgrownon(110)-orientedNdGdO3substrateundertensilestrainconditions
(+1.6% of strain) as a representative example of bulk ReNiO3 in general. In this case, coupled
5
metal-insulator and paramagnetic-to-antiferromagnetic transitions have been found at T~150 K,
consistentwiththecorrespondingbulkcompound{1}.
Bound and Continuum excitations across Ni L3 resonance. XAS and RIXS measurements were
carriedoutbyexcitingattheNiL3-edge,correspondingtothe2p3/2-->3delectronictransitionat
around~852eV.TheXASspectrahavebeenacquiredinthepartialfluorescenceyieldmode(PFY),
byintegratingtheRIXSspectraforeachincomingphotonenergy,toinsurethebulksensitivity.
Figure 2 (a)-(c) presents an overview of XAS and RIXS data for the 30 nm thick NdNiO3 film,
measuredatboth300K(metallicphase,redcolor)andat15K(insulatingphase,bluecolor).The
NiL2,3XASshowninFig.2(a)isingoodagreementwithpreviouslypublisheddataonNdNiO3{20,
21 37 38 39
, , , }.At15K,theNi-L3regionoftheXAS(from850eVto860eV),ischaracterizedbytwo
clear structures — a sharp peak at 852.4 eV (A) and a broader peak at 854.3 eV (B) — both of
whicharepresentinotherrare-earthnickelatesaswell{1, 16, 20, 24}.At300Kbothpeaksarestill
recognizable,however,theirseparationislessevident.
Aseriesofhigh-resolutionRIXSspectrahavebeenrecordedacrosstheNi-L3resonanceinstepsof
0.1eV,asshownintheintensitycolourmapsofFig.2(b)-(c).Eachspectrumobtainedforaspecific
incidentphotonenergyhνinmeasurestheintensitiesoftheemittedphotonsasafunctionofthe
energy loss hν= hνin-hνout, where hνout is the outgoing photon energy. RIXS is able to
simultaneously probe excitations of diverse nature, e.g. lattice, magnetic, orbital and charge
excitations{40,41}.Inaddition,onecandistinguishwithRIXSbetweenlocalised,boundexcitations
anddelocalisedexcitationsinvolvingcontinua.Forlocalisedelectronicexcitations,theRIXSsignal
appearsatafixedenergylosshνwhilescanningtheincidentenergyhνinacrossacorresponding
6
resonance (Raman-like behaviour). Conversely, for delocalised electronic excitations involving
continua,theRIXSsignalhasaconstanthνoutandthereforepresentsalinearlydispersingenergy
lossasafunctionofincidentenergy(fluorescence-likebehaviour){40,41,42,43}.
FromtheRIXSmapsoftheNdNiO3thinfilminFig.2(b)-(c)onedirectlyobservesaclear,strong
Raman-like response at around 1 eV of energy loss when tuning hνin to the XAS peak A. These
atomic-like dd-orbital excitations, which are sensitive to the local ligand field symmetry, behave
similarlytothoseobservedinotheroxidematerialsliketheprototypicalNi3d8systemNiO{44}.A
fluorescence-like contribution resonates instead at the XAS peak B, contrary to the Raman-like
responsedominatedbymultipleteffectsobservedinNiOatthecorrespondingNiL3XASshoulder
{44}.Alreadybylookingatthecolourmap,thisfluorescence-likespectralsignatureisclearlyvisible
allacrosstheNiL3edgeandalwayswithalinearlydispersingbehaviour,assuggestedbythered
dotted line overimposed to the data. Interestingly, the fluorescence intensity distribution in the
NdNiO3 RIXS map has also a strong temperature dependence: while at 300 K it merges
continuously with the dd-excitations [see Fig. 2 (b)], at 15 K a dip in intensity is created
correspondingtotheincidentphotonenergyC,hνin=853eV[seeFig.2(c),dashedgreyline].
To gain more insight into the origin of the observed Raman- and fluorescence-like spectral
response, we closely examine the individual RIXS spectra and we perform a fitting analysis to
extractthegeneralbehaviourofthemainspectralcomponents.AsshowninFigure3(a),theraw
RIXSspectrumisdecomposedintothreedifferentcontributions(seeSupplementaryInformation
{45}Sec.AfordetailsabouttheGaussianfit).Referringtothephotonenergyhνin=A,weidentify
fromthecorrespondingRIXSspectrum:i)localizeddd-orbitalexcitationsextendingfrom1eVto3
7
eV (black line), ii) a broad background centered around 4 eV (CT, green line), and iii) a residual
spectral weight (Fl, red line) peaking at 0.7 eV between the elastic line and the dominating ddprofile.
Remarkably,atthisphotonenergyeventhefinemultipletstructureofthedd-excitationsisingood
agreement with that of NiO {44}, suggesting immediately that NdNiO3 has an unusual Ni 3d8-like
S=1 local electronic structure similar to NiO. Figure 4 furthermore endorses this concept,
displayingthecomparisonbetweentheRIXSdataandclustermodelcalculationsperformedfora
Ni3d7GS(greyline,modelparameterstakenfrom{39})andacrystalfieldcalculationoftheddexcitationsforaNi3d8GS(blackline,modelparametersfromNiO{46};seealsoSec.Cat{45}).The
dd-excitationsobtainedfortheNi3d7casestronglydifferfromthepresentNdNiO3datanotonly
in the peak energy positions displaced to higher energies, but also in the intensity distribution
profile,thusrulingoutaNi3d7GSscenario.Thedd-excitationscalculatedfortheNiOinNi3d8
configurationinsteadpresentaremarkablygoodcorrespondencewiththepresentdata,verifying
thed8-likecharacterofNiinthiscompound:suchafindingisthefirstkeyresultofourstudyand
directly poses the question if NdNiO3 deviates from a “conventional” positive charge transfer
picturebasedonaNi3d7GSinfavourofanegativecharge-transferscenariobasedonaNi3d8GS,
asillustratedinFig.1(b).Finally,wenotethatintheinsulatingphaseanextradd-peakemerges
from the experimental data at ~ 0.75 eV, which is not captured by the Ni 3d8 calculation. We
speculatethatthiscontributioncouldbecausedbysymmetrybreakingphenomena(relatedtothe
presence of different Ni sites in the insulating phase). However, more advanced and detailed
calculationshavetobedevelopedtoreproducethisfinding.
8
Referring now to hνin=B incident photon energy, the three contributions identified above for
hνin=A—dd,CTandFl—canbestilldistinguishedinthecorrespondingRIXSspectrum.However,
comparing the two spectra in Figure 3 (a) we observe a shift in energy for the CT- and Flcontributions, contrary to the dd-excitations which are fixed in energy loss, and a strong
redistributionofspectralweightbetweenthethreespectralcomponents.
Inordertodisentanglelocalizedvs.delocalizedcharacteroftheRIXSexcitations,weextendinFig.
3(b)-(e)thefittinganalysistoalltheRIXSspectrabetween852eVand858eVincidentphoton
energiesandwetrackforeachcontributiontheintegratedintensity[Fig.3(b)-(c)]andthepeak
energy dispersion [Fig. 3 (d)-(e)] {45}. While the dd-excitations (black dots) have a Raman-like
behaviour throughout the energy range and clearly resonate at hνin=A, the other two
contributions (CT in green and Fl in red dots) resonate instead at hνin=B [see Fig. 3 (b), 15 K].
Interestingly, and contrary to the similarity in their resonant behaviour, the CT- and the Flcontributions present different peak energy dispersions as a function of the incident photon
energy [Figure 3 (d)]. The broad CT-peak (FWHM ~ 6 eV) displays a behaviour characteristic of
well-knowncharge-transferexcitationsbetweenthecentralNisiteandthesurroundingoxygens:
constant energy loss (~ 4 eV) up to the resonant energy hνin=B, and a fluorescence-like linear
dispersion at higher incident photon energies. The Fl-peak (FWHM ∼1 eV), instead, linearly
disperses vs. incident photon energy across the full Ni-L3 resonance: as introduced above, this
behaviour is a clear fingerprint for a delocalized excitation involving continua. Overall, these
findingsarecommontobothlowandhightemperaturedatasets.However,inthemetallicstate
[see Fig. 3 (c) and (e), 300 K] the intensity of both CT- and Fl-peaks is enhanced at hνin=C: the
resultingextraweightintheintegratedRIXSintensityprofile[Fig.3(c),bluedots]mimicsthefilling
9
of the valley observed in the XAS spectrum at 300 K, and explains the absence of a dip in the
intensitydistributionoftheRIXSmap[seeFig.2(b)]atthesameincidentenergy.
Additionally,weexaminetheFl-excitationsmorecloselyinFigure5,wherewefocusonthelow
energylossrange(<1.5eV)ofthehigh-statisticsRIXSspectraobtainedattheNiL3pre-peakregion,
starting1eVbeforepeakA.InFig.5(a)weobservechangesinthedd-excitationsbetween300K
and 15 K (across the MIT) likely due in part to local rearrangements of the NiO6 octahedra.
Moreover, a sizeable spectral weight continuum around 0.2 - 0.5 eV [see Fig. 5 (a), inside the
ellipse-area]ispresentonlyat300Kinthemetallicphase,asbetterdisplayedbytheRIXScolour
mapsfor300Kand15K[seeFig.5(b)and(c)respectively].
Andersonimpuritymodel.TounderstandtheoriginofthevariousRIXSexcitationsandtheirlink
totheelectronicstructure,onecanemployanAndersonimpuritymodel(AIM)interpretation(see
Sec.Dat{45}).WhiletheschematicsinFig.1(a-b)showthesingleparticleremovalandaddition
excitations for different charge-transfer scenarios, RIXS actually measures charge neutral
excitations, which are well represented in a configuration interaction-based AIM. These charge
neutralexcitationsareshownschematicallyinFig.6(a)forthenegativecharge-transfercaseΔ’<0
and the positive charge-transfer case Δ’ >0 (NiO like) of a Ni 3d8 impurity. As previously
mentioned,fortheΔ’>0case,theonlychargefluctuationspossibleintheAIMarefromthefullO
2pbandtotheNiimpuritylevel.Whileconservingthetotalcharge,thesefluctuationsgiverisetoa
Ni 3d9L band corresponding to charge-transfer excitations [shown in green in Fig. 6 (a) for NiO].
However, for the Δ’ <0 case [recall Fig. 1 (b)], the presence of a self-doped, partially filled O 2p
densityofstates(DOS)extendingacrosstheFermilevelopensadditionalpathwaysfortheneutral
10
charge fluctuations. Electrons can either hop: i) from the O 2p valence band to the O 2p
conduction band leaving the Ni impurity occupation unchanged, yielding a characteristic low
energy electron-hole pair continuum of “m” excitations d8vmcm marked in red [v is a hole in the
valencebandandcanelectronintheconductionband,asshowninthesmallO2pDOSinsetof
Fig.6(a)];ii)toandfromtheNiimpuritylevel,fromtheO2pvalencebandandtowardtheO2p
conduction band, respectively, causing the impurity occupation to change by plus or minus one
and yielding a charge-transfer like continuum of excitations at higher energy. Eventually these
charge transfer excitations can be dressed by associated electron-hole pair excitations, vmcm,
resulting in d9vm+1cm or d7vmcm+1 bands of excitations (green band). We stress that the here
introduced low energy electron-hole pair excitations can be obtained only for the Δ’ <0 case,
wheretheO2pDOScrossesEF.
TheeffectsofthesetwoimpuritymodelsonRIXSaredetailedinFigs6(b).ThecaseofNiOcanbe
solvednumericallyincludingthefullcorrelationswithinthe3dshell,andweshowthecalculated
RIXSmapinFig6(b).ComparingthistotheschematicNiOconfigurationsinFig.6(a),weseethat
there are Raman-like dd-excitations below 4 eV, corresponding to reorganized Ni 3d8 orbital
occupations,andacharge-transferbandatdistinctivelyhigherenergylosseswhichisRaman-like
forlowerincidentphotonenergiesuptoca.855eV,beforedispersinglikefluorescenceforhigher
photonenergies.However,astheschematicinFig.6(a)suggests,thecharge-transferexcitations
donotextenddowntolowenergylossesfortheNiOcase(Δ’ >0).Togainfurtherinsightintothe
NdNiO3experimentaldata,theextractedCT-andFl-dispersioncurvesofFig.3(d)areoverlaidon
thecalculatedRIXSmapinFig.6(b).Indeed,theNdNiO3CT-excitations(opengreendots)showa
similar behaviour as the NiO CT ones (solid green line). The NdNiO3 Fl-excitations, instead, with
11
their distinctive fluorescence-like dispersion differ from any of the NiO excitations. Interestingly,
the identified Fl-contribution propagating down to very low energy losses is compatible instead
withtheelectron-holepaircontinuumexcitationsd8vmcmcomingfromthebroadO2pband:this
findingisthesecondkeyresultofourstudyandnaturallyoccursforthenegativecharge-transfer
case(Δ’<0)asrepresentedintheAIMschematicofFig.6(a)(redband).
Discussion
Themainfindingsofthepresenteddataanalysisandinterpretationareasfollows:i)localisedddexcitations sharply resonate at the XAS peak A and with a lineshape consistent with a Ni 3d8
configurationsimilarlytoNiO;ii)delocalisedFl-excitationsmostlyresonateattheXASpeakBand
areinterpretedaselectron-holepairexcitationsacrosstheO2pbandcutbyEF[Fig.1(b),green
contours];iii)spectralweightreductionoftheelectron-holepairexcitationsclosetozeroenergy
loss[seeFig.5(b-c)]suggeststheopeningofanO-Oinsulatinggap[refertoschemeinFig.1(b)]at
lowtemperature,inlinewithpreviouslyreportedopticalconductivitystudies{47,48}andsimilarly
as in more recent ARPES data {49} revealing a spectral weight transfer from near EF to higher
bindingenergiesacrosstheMIT.
ThiscollectionofresultsclearlyidentifiesNdNiO3asanegativeΔ’ charge-transfermaterial,witha
localNi3d8configuration,apredominantO2pcharacteracrosstheFermilevelandaconsequent
GSofmainly3d8Ln.ThispictureiscompatiblewiththescenarioproposedbyMizokawa{26},also
discussed as “bond disproportionation model” in recent theoretical approaches {27, 28, 29, 30,50},
whichseesanexpanded3d8Nisite(n=0,S=1)andacollapsed3d8L2Nisite(n=2,S=0)alternatingin
theinsulatingphasewiththefollowingspinorder↑ 0 ↓ 0 andanhomogeneous3d8L(n=1)GSin
12
themetallicphase.Asunderlinedinpreviousworks{28,29},thismodelisinagreementwithseveral
breakthroughexperimentalfindings:i)(1/201/2)antiferromagneticBraggpeakintheinsulating
phase {5}; ii) charge ordering {6}, which in this model is distributed among both Ni and O sites
insteadoftheonlyNisites;iii)absenceoforbitalorder{34};iv)evidenceofstrongNi-Ocovalence
intheGS{4,20,21}.
Furthermore, the different resonant behaviour extracted in this study for the localised and
delocalised RIXS excitations suggest that the two distinct XAS peaks marked at low temperature
mostlyresultfromthetwodifferentcomponentsoftheGS,beingXASpeakAmostlyassociated
with a Ni 3d8 configuration, and XAS peak B with the delocalised ground state Ni 3d8L2
configuration. This is in line with the energy dependence of the (1/2 0 1/2) peak resonating at
hνin=A{31,37},hereassignedtothemagneticallyactiveS=1site.
Inconclusion,bycombiningNi-L3XASandRIXSmeasurementswestudiedtheelectronicground
state properties of ReNiO3, in order to discriminate the electronic structure between a negative
charge-transferandapositivecharge-transferscenario.Byanalysingthefirsteverhigh-resolution
NiL3RIXSdataobtainedforReNiO3,weidentifiedthecoexistenceofbound,localisedexcitations
and strong continuum excitations in both the XAS and the RIXS spectra, in contrast to earlier
absorptionstudieswhichassumedprimarilycharge-transfermultipleteffectsintheXAS.Further,
wedisentangledthecontinuumfeaturesintheRIXSspectraintocharge-transferandfluorescence
excitations,showingthelattertoariseduetothepresenceofagroundstatecontainingholesin
theoxygen2pband.Electron-holepairexcitationsfromoxygen2pstatesacrosstheFermilevel
havebeenidentifieddowntozeroenergyloss,mimickingtheopeningofagapforT<TMI.Allthese
13
experimental observations provide a clear indication of an O 2p hole-rich ground state with Ni
3d8Lnelectronicconfigurationasthemaincomponent,asexpectedforanegativecharge-transfer
system. This GS configuration lends support to the treatment of the ReNiO3 as a S=1 Kondo or
AndersonlatticeproblemwithaNi3d8Ln(n=1)metallicgroundstate,andrealizingtheMITbya
bonddisproportionationleadingtotwoNisiteenvironments:Ni3d8(n=0,S=1)andNi3d8L2(n=2,
S=0),differinginthehybridizationwiththeO2pholestatesyetleavingthechargeatthenickel
sitesalmostequal.Whilethisresultisvitalfortheunderstandingoftherareearthnickelatefamily
perse,thecombinedXASandRIXSapproachdemonstratedhereopenstheopportunitytoclassify
the electronic structure for other cases of very small or negative charge-transfer gaps Δ’, which
could be common to other materials with unconventionally high formal oxidation states (as for
example the sulfides, selenides and tellurides, and many high oxidation state compounds in
general).
Methods
Experimental details. X-ray absorption (XAS) and high-resolution Resonant Inelastic X-ray
Scattering(RIXS)measurementshavebeenperformedattheADRESSbeamlineoftheSwissLight
Source {51}, Paul Scherrer Institute. The sample has been oriented in grazing incidence, with the
incomingphotonbeamimpingingat15degreeswithrespecttothesamplesurface.Thescattering
planefortheRIXSmeasurementswascoincidingwiththecrystallographicac-plane(orbc-plane,
equivalently). All the data displayed here have been measured for incoming photons polarized
paralleltothea(b)axis(alsoreferredtoasσpolarization,perpendiculartothescatteringplane).
FortheRIXSmeasurementsweusedtheSAXESspectrometer{52}preparedwithascatteringangle
14
of130degreesandatotalenergyresolutionof110meV.Thespectrometerwassetinthehighefficiency configuration, using the 1500 lines/mm VLS spherical grating. This set-up allowed
acquiringaround600-800photonsatthemaximaoftheprominentspectralstructuresalreadyin1
min.Therecordedscatteredphotonswerenotfilteredbytheoutgoingpolarization.
Acknowledgements:ThisworkwasperformedattheADRESSbeamlineoftheSwissLightSource
at the Paul Scherrer Institut, Switzerland. Part of this research has been funded by the Swiss
NationalScienceFoundationthroughitsNationalCentreofCompetenceinResearchMaNEPand
theSinergianetworkMottPhysicsBeyondtheHeisenberg(MPBH)model.Theresearchleadingto
these results has received funding from the European Community's Seventh Framework
Programme(FP7/2007-2013)underGrantAgreementNr.290605(COFUND:PSI-FELLOW)andERC
GrantAgreementNr.319286(Q-MAC).Moreover,thisworkhasreceivedsupportthroughfunding
from the Canadian funding agencies NSERC, CRC, and the Max Planck/UBC center for Quantum
Materials.
AuthorsContribution:V.B.,S.C.,M.G.,R.S.,G.S.,J.-M.T.andT.S.plannedtheproject.S.C,M.G.,R.
S.,P.Z.andJ.-M.T.grewandcharacterizedthethinfilmsamples.V.B.,S.C.,R.S.,Y.H.,V.S.andT.S.
carried out the experiment. V.B. and T.S. analysed the data. R.G., S.B. and G.S. carried out the
theoretical calculations. V.B., R.G., G.S. and T.S. wrote the paper with contributions from all
authors.
Author Information: The authors declare no competing financial interests. Correspondence and
requestsshouldbeaddressedtoV.B.([email protected])andT.S.([email protected]).
15
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FigureLegends:
Figure1|Singleelectronexcitationspectraintermsofchargeremovalandchargeadditionfor
differentregimesofeffectivecharge-transferΔ’.Thissketchintroducesconventionalparameters
used to describe charge dynamics in transition metal oxide (TMO) compounds with formal 3d7
filling:1)chargetransferenergyΔ:energycostfortransferringanelectron/holefromtheLband
totheNi3dband(withrespecttothebandcenterofmass);2)HubbardU:energycostneededto
remove an electron from the occupied 3d band and to add it to the unoccupied 3d band; 3)
effective charge-transfer energy Δ’: key-parameter to distinguish between the two different
regimes discussed here. Δ’ is defined by the equation in the figure, starting from Δ. This figure
identifiesthegroundstate(GS)andthegapcharacterobtainedinthetwoΔ’regimes:a,Δ’>0.In
conventional positive charge transfer compounds the lowest energy removal states are ligandbased, and the lowest energy addition states are transition metal based, leading to a charge
transferderivedenergygap(O2p–Ni3dlike)anda3d7GS.b,Δ’<0.Innegativechargetransfer
compounds,oneholeperNiisdopedintotheligandband,givingadensityofligandholesn,Ln.
The GS is here 3d8Ln. The red-dashed contour bands are a cartoon-like demonstration of the
openingofthegapinthemainlyO2pcontinuumresultinginthemetaltoinsulatortransition.In
thiscase,thelowestenergyremovalandadditionstatesarebothligand-based,leadingtoanO2p
–O2plikegap.Undernocircumstancescanthistypeofgapresultfromconfigurationa,unless
active doping is considered. Note that this figure neglects Ni – O hybridization, to provide clear
distinctionbetweentheregimes.
Figure2|OverviewofXASandRIXSmeasurementsfora30nmNdNiO3filmonNdGdO3.a,Ni
L3,2XASmeasuredinPFYmodeat300K(metallicphase)inredandat15K(insulatingphase)in
blue.RefertoSec.Bof{45}forXASintotalelectronyieldmode.b(c),RIXSintensitymapmeasured
acrosstheNiL3edgeat300K(15K).ThewhitesolidlinedisplaystheXASmeasuredatthesame
temperature.ThelettersA,B,Cmarkthethreedifferentincidentenergiesmentionedinthetext,
whiledd,CTandFlrefertoRIXSexcitationsofdifferentcharacter,alsodiscussedinthetext.The
grey dashed line indicates the incident energy giving the most pronounced changes in the RIXS
mapandintheXASacrosstheMIT.Thereddottedlineprovidesaguidetotheeyeforthelinearly
energydispersingFlfeature.
Figure3|RIXSdataanalysisacrosstheMIT.a,RIXSlinespectra(greyopen-dotline)measuredat
hνin=Aandhνin=Bat15K.ThethinsolidlinesrefertoGaussianfitsofdd-excitations(black),theCT
excitation(green)andtheFlexcitation(red).Thebluedashedlinereferstothesumofthethree
fittingcontributions,plustheelasticlineat0eV.b(c),Integratedintensityofthefitdd-,CT-and
Fl-excitationstogetherwiththetotalintegratedintensityat15K(300K).Thegreyarrowmarksthe
incident energy giving the most pronounced changes in the RIXS integrated intensity across the
MIT. d (e), Peak energy dispersion for the CT- and Fl-excitation at 15 K (300K). The same colour
codeasin(a)isusedthroughoutthefigure.
Figure 4 | Comparison between experimental and calculated dd-excitations.Data:RIXSspectra
atT=15K(bluecircles)andatT=300K(redcircles),hνin=A,LVpolarization.Calculation:crystalfield
RIXScalculationforaNi3d8GSbasedonNiOparameters{46}(blackline);clustercalculationfora
Ni3d7GSusingtheparametersinRef.{39}(greyline).Pleasenotethattheelasticlineshavebeen
removed from the calculated spectra to better display the differences at the low energies. The
21
experimental dd-line shape is well reproduced by the Ni 3d8 cluster calculation, supporting the
negative charge transfer scenario for ReNiO3. The good matching between data and calculation
showsthat:1)thereisnoinversionofcrystalfieldinReNiO3comparedtoNiO,inagreementwith
Ref.{25}fordominatingCoulombinteraction;2)beingtheNi-OdistanceshorterinReNiO3thanin
NiO,thenegativechargetransferenergymayleadtoareductionoftheexpectedt2g-egsplitting.
Figure 5 | Low energy electron-hole pair continuum in the RIXS spectra. a, RIXS line spectra
measuredforincidentenergiesgoingfromhνin=A-1eVuptohνin=A-0.2eVinstepsof0.1eVat15
K (blue) and 300 K (red). Each spectrum has been acquired for 5 minutes. The grey ellipse
highlightstheenergylossregionwheretheelectron-holepaircontinuumismoreprominent.b(c),
Magnificationofthelowenergylossregion(<0.9eV)oftheRIXSmapwithalogarithmicintensity
scale at 300 K (15 K). The spectra have been normalized to the dd-area in order to have
comparable background signal in the low energy loss region. The red ellipses underline the
electron-hole pair continuum present at 300 K (b) and the intensity gap in the same energy
windowat15K(c).
Figure6|AndersonimpuritymodelinterpretationoftheRIXSandXASspectra.a,AIMschematic
forchargeneutralRIXSexcitations.TheconfigurationsoftheAIMareverydifferentforΔ’ <0(left,
NdNiO3) and Δ’> 0 (right, NiO) 3d8 compounds. CT-like excitations are obtained in both cases
(green bands), while Fl-like excitations (red band) are found only for Δ’ < 0 and correspond to
electron-hole pair excitations as shown in the O 2p DOS inset. b, Calculated RIXS map for the
positive charge-transfer compound NiO, using the AIM. The NdNiO3 CT- and Fl-excitations
dispersion curves from Fig. 3 (d) are overlaid for comparison (green and red open dots,
22
respectively), as well as the NiO CT-excitation dispersion (green solid line). Horizontal coloured
lines make the connection between the RIXS excitations and the assigned interpretation in the
AIMschematicofFigurea.
23
FIGURE1
FIGURE2
25
FIGURE3
FIGURE4
26
FIGURE5
FIGURE6
27