Table Of ContentQuantumconfinementofMottelectronsinultrathinLaNiO /LaAlO superlattices
3 3
JianLiu,1,∗ S.Okamoto,2 M.vanVeenendaal,3,4 M.Kareev,1 B.Gray,1 P.Ryan,3 J.W.Freeland,3 andJ.Chakhalian1
1Department of Physics, University of Arkansas, Fayetteville, Arkansas 72701
2MaterialsScienceandTechnologyDivision,OakRidgeNationalLaboratory,OakRidge,Tennessee37831
3Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439
4Department of Physics, Northern Illinois University, DeKalb, Illinois 60115
(Dated:January31,2011)
Weinvestigatetheelectronicreconstructionin(LaNiO ) /(LaAlO ) (n =3,5and10)superlatticesdueto
3 n 3 3
thequantumconfinement(QC)byd.c. transportandresonantsoftx-rayabsorptionspectroscopy. Inproximity
to the QC limit, a Mott-type transition from an itinerant electron behavior to a localized state is observed.
Thesystemexhibitstendencytowardscharge-orderduringthetransition. abinitioclustercalculationsarein
1 goodagreementwiththeabsorptionspectra,indicatingthattheapicalligandholedensityishighlysuppressed
1 resultinginastrongmodificationoftheelectronicstructure. Atthedimensionalcrossovercellulardynamical-
0 mean-fieldcalculationssupporttheemergenceofaMottinsulatorgroundstateintheheterostructuredultra-thin
2 slabofLaNiO3.
n
PACSnumbers:
a
J
8 Promptedbythediscoveryofhigh-T superconductivityin experimentallyrealized(LNO) /(LAO) SLs.Byusingtrans-
c n 3
2 cuprate compounds there has been a surge of activity to dis- port measurement and soft x-ray absorption spectroscopy in
cover materials with even higher transition temperature [1]. combinationwithclustercalculationsandcellulardynamical-
]
l Recent remarkable advances in synthesis of artificial layers mean-field theory calculations, we demonstrate the presence
e
- of complex oxides along with the progress in computational ofacharge-localizedgroundstateatthequantumconfinement
r methodshavere-energizedthesearchfornovelsuperconduc- limit caused by strongly enhanced correlations and broken
t
s torsoutsideofthecupratefamily[2,3]. translationalsymmetryacrosstheinterface. Insharpcontrast
.
t to conventional bandwidth-control, the observed MIT is a
a Towardsthechallenge,arecentproposalhasbeenputfor-
m ward to use heterostructuring and orbital engineering to turn continuous one with a critical region due to an emerging
correlated gap imposed by confined dimensionality. The
- hole-doped alternating unit-cell thin layers of a correlated
d system also exhibits tendency towards charge-ordering as a
metalLaNiO (LNO)andband-gapdielectricLaAlO (LAO)
n [4]. Theprop3osalutilizestheNiIII 3d7 low-spinsta3teinthe competing state during the transition. Microscopically, the
o
crossover from the localized to itinerant behaviors is driven
c bulkwithasingleunpairedelectronoccupyingthedegenerate
[ (d /d ) e orbitalwhose nodespointto theplanar by a strong competition between the reduced covalency of
x2−y2 3z2−r2 g
and apical ligands of the octahedra, respectively. Both t–J theNi-O-Albondandtherecoveryoftheligandholedensity
1
withintheLNOslab.
v andLDA+DMFTcalculations[4,5]suggestthatthequantum
1 confinement together with the electronic correlations should
58 makeitpossibletolocalizeoremptythed3z2−r2 bandleaving High-quality epitaxial [(LNO)n/(LAO)3]N SLs (n = 3,
5 theconductionelectroninthedx2−y2 band. Epitaxialstrainis 5 and 10 with N = 10, 4 and 2 respectively) were grown
. alsosuggestedasameanoftheorbitalcontroltomanipulate by laser molecular beam epitaxy with in situ monitoring by
1
0 thed3z2−r2 orbitaltoappearabovedx2−y2 andplaytheanal- Reflection High Energy Electron Diffraction, as described
1 ogousroleoftheaxialorbitalofthehigh-Tccuprates. in Ref.[11, 16]; +2.2% tensile strain is applied with TiO2-
1 Despitetheattractivesimplicityofthestructure,theexperi- terminated (001) SrTiO3 substrates [17]. Atomic force mi-
v: mental realization of LNO/LAO superlattice (SL) presents a croscopyimagingrevealsthatthesubstratesurfacemorphol-
i large degree of ambiguity in selecting a specific electronic ogyiswellmaintainedafterthegrowth;Symmetricscansand
X
ground state caused by (i) the intrinsic propensity of ortho- reciprocalspacemappingbysynchrotron-basedx-raydiffrac-
r nickelates to charge-ordering [6, 7], (ii) the orbital polar- tionindicatecoherentlygrownsinglecrystalheterostructures
a
ization due to chemical confinement imposed by interfacial withthefullystrainedstate[16].
bonding[8,9],(iii)polardiscontinuity[10,11],(iv)epitaxial
Toexplicatetheevolutionofthegroundstateduetoquan-
constraint on the crystal symmetry [12, 13] and (v) quantum
tumconfinement,d.c. transportmeasurementsfrom400Kto
confinement [14, 15]. Even for the undoped LNO/LAO SL
2 K were performed in the van der Pauw geometry with a
(i.e. the proposed parent cuprate-like compound), the syner-
commercialphysicalpropertiesmeasurementsystem(PPMS,
getic action of these phenomena will likely render the the-
QuantumDesign). Theobtainedtemperaturedependentcon-
oretical problem intractable to a priori predict whether the
ductivity,σ(T),oftheSLseriesisshowninFig.1(a). Atran-
cuprate-likephysicscanbeexperimentallyrealized.
sitionfrommetallictoinsulatingbehaviorcanbeclearlyseen
Here we report on emergence of a Mott-type metal- with decreasing n. The inset of Fig. 1(a) also demonstrates
insulatortransition(MIT)atthedimensionalcrossoverinthe orders of magnitude change in σ across the series taken at
2
indicatingthatthesystementerstheinsulatingphaseatn=3.
The evolution with n shown above points to an opening gap
intheexcitationspectrum.
To gain microscopic insight into the effect of confinement
on the nature of the MIT, we carried out detailed soft x-ray
absorption measurements. Since resonant x-ray absorption
is local and element specific probe, it is well suited for in-
vestigating electronic structure of buried layers. All spectra
were acquired in the bulk-sensitive fluorescence yield mode
attheNiL-edgeatthe4ID-CbeamlineoftheAdvancedPho-
ton Source [22]. Precise information on the Ni charge state
was obtained by aligning all the spectra to a NiO standard
which was measured simultaneously with the SLs. The en-
ergy resolution was set at 100 meV. The normalized spectra
FIG. 1: (a) Conductivity of (LNO) /(LAO) SLs versus tempera-
n 3
obtained at 300 K are shown in Fig. 2(a,b). For direct com-
ture.Inset:conductivityversusnat10K.Thedashedlineisaguide
for eye. (b) Resistivity as a function of T1.5 at n = 10. The red parison,theL-edgespectraofbulkLNOandSmNiO3[23]are
solid line is a guid for eye. (c) dlnσ as a function of 1/T1/4 for included.Asseen,theenergypositionandthelineshapeofthe
then=3SLshowsthelowtemperaturethreedimensionalvariable SLsareclosetothoseofthebulkRENiO (RE=rareearth)
3
rangehoppingbehavior. [24,25],indicativeofaNiIII chargestateofthe3d7low-spin
configuration. On the other hand, while the multiplet effects
are smeared in the metallic bulk LNO, the L -edge spectra
3
10 K. As seen, in analogy to the bulk LNO, the n = 10 SL of the SL show a pronounced two-peak structure (shaded in
exhibits a highly metallic behavior ascribed to the three di- green),closelyresemblingtheabsorptionspectrumofthebulk
mensionalregime. Ontheotherhand, unlikethebulkwhich SmNiO (see Fig. 2.(a)), which in the bulk is a well studied
3
isknowntofollowaT2 Fermiliquidbehavioratlowtemper- case for the charge-ordered (2NiIII→NiII+NiIV) insulating
atures[18],thelow-temperatureresistivityρatn=10shows state[6,7].Asndecreasesthismultipleteffectbecomesmore
a peculiar ∝ T3/2 dependence below 200 K (see Fig. 1(b)). pronounced,particularlyinmovingfromn=10to5.Theen-
This power law dependence has been attributed to scattering ergyseparationofthetwosplitpeaksisalsoclearlyincreasing
by bond-length fluctuations [19] and indicates the proximity as shown in Fig. 2(c). The corresponding trend at the L -
2
to a Mott-type metal-to-insulator transition (MIT) from the edge emerges as a developing low-energy shoulder at about
itinerant side due to enhanced electronic correlations in the 870.5 eV (see Fig. 2.(b)). The observed evolution is remi-
quantumconfinementregime. niscent of that reported for the bulk RENiO when crossing
3
The intermediate thickness n = 5 SL still shows a clear the metal-insulator boundary [25], unambiguously revealing
metallicbehaviorathightemperaturesbutreachesaconduc- the carrier localization in the presence of a developing cor-
tivity maximum at T = 120 K. While the metallicity of related gap in the quantum confined LNO slabs and a latent
max
LNOfilmsunderlargetensilestrainisknowntoceasewhen tendency to charge ordering during the three-to-two dimen-
the thickness is ≤ 3nm [20], it is remarkable that the high sional crossover. In accordance with the observations from
temperature metallic behavior is well maintained in such a d.c. transport,theemergenceofthemultipletevenintherela-
thinslab(< 2nm). Thisresultunambiguouslyhighlightsthe tivelythickLNOslab(n=10)lendssolidsupporttostrongly
exquisite role of the confinement structure with interfaces in confinement-enhanced correlations. Meanwhile, when sub-
defining the transport properties of SLs as compared to the ject to this enhancement, the inclination to charge ordering
bulk LNO and single layer films. Notice, the conductivity σ corroboratesitsroleasaprimarycompetinggroundstatenot
here is also well below the Ioffe-Regel limit [21] (∼ 2000 onlyinthebulk[6]butalsointhepredictedheterostructured
S/cm), manifestingthemodificationofthecorrelationnature LNO[4].
oftheobservedbehaviorsandtheminorroleofdisorder. Be- To shed additional light on the observed spectroscopic re-
low T , σ(T )−σ(2 K) (∼381.6 S/cm) is as large as sults, weperformedabinitioclustercalculationsonaNiO
max max 6
σ(2 K) and almost twice of σ(T )−σ(400 K). Neither cluster. The model Hamiltonian includes on-site energies,
max
hoppingconductivitynorquantumcorrectionsisfoundtobe crystalfield,Coulombinteractionincludingthefullmultiplet
abletoaccountforthisdσ/dT > 0region. Thistransitional structureandspin-orbitcoupling[26].Themajorityofthepa-
behaviorexplicitlyindicatesthatalltheinvolvedinteractions rameters are equivalent to those in Ref.[26]. The monopole
and relevant energy scales, like electron correlations and the partoftheCoulombinteractionissetat6eV.Themajorcon-
bandwidth, are of a similar strength and strongly competing figurationsusedinthecalculationared2 d andd2 L ,
eg↑ eg↓ eg↑ eg↓
witheachother. Thus,n=5fallsintoacriticalregionofthe whered andL standrespectivelyfora3dholeandalig-
egσ eg↓
confinement-controlledMIT.Uponfurtherdecreasingnto3, andholewithe symmetryandspinσ =↑,↓. Notethattriva-
g
dσ/dT >0atalltemperaturesandσ(T)isfoundtofollowa lent nickel compounds, such as LNO, are significantly more
variablerangehoppingbehaviorbelow100K(seeFig.1(c)), covalent than divalent systems as the charge-transfer energy
3
spectra and the calculation, we can speculate on a possible
physicaloriginofthen−dependenceofthecorrelatedgapen-
ergy. Theincreasingmagnitudeof∆ withdecreasingnum-
7
ber of LNO unit cells can be connected to the effect of the
confinement which breaks the translation symmetry with in-
terfacesandcausestheformationofasignificantlylesscova-
lentNi-O-Alchemicalbond. Theabsenceofd-electronstates
on Al and its much lower electronegativity compared to Ni
stronglysuppressestheligandholedensityontheapicaloxy-
genO [8,9]. Consequently,becauseoftheproximitytothe
a
more positively charged Al ion, the Madelung potential on
the interfacial O is further raised with respect to the planar
a
O ;thisleadstofurthersuppressionontheapicalholedensity
p
andchargetransferprocess. Asaresult,theinterfacialNi-O
a
bondlengthbecomesshorter[4],theligandholeistransferred
into the NiO plane and the d8L configuration will acquire
2
moreofthed character. Thiseffectisincloseanalogy
x2−y2
tothephysicsofcuprateswheretheMadelungenergyofthe
apical oxygen and the Cu-O bond length are believed to be
a
importantparameterstocontroltheplanarholedensityandto
stabilizetheZhang-Ricestatesuponhole-doping[27].
Asdiscussedbeforetheclustercalculationscanbesuccess-
fully applied to provide the insight how the local electronic
structure is modified in proximity to the interface. However,
the observed global MIT resulting from the competition be-
tween the short-range electron correlation and the reduced
bandwidth subject to quantum confinement is better treated
within the layer extension of cellular dynamical-mean-field
theory (layer CDMFT) [28] with the Lanczos exact diago-
nalization impurity solver [30]. Note that CDMFT was re-
centlyappliedtoexplaintheopeningofacorrelation-induced
pseudogap in SrVO thin films [29]. The layer CDMFT we
3
FIG.2: X-rayabsorptionspectraof(LNO)n/(LAO)3SLsattheL3- employed here goes beyond that previous study by includ-
edge (a) and L -edge (b) for n = 3,5, and 10 together with the
2 ingshort-rangecorrelationswithineachmonolayertocapture
theoretical calculations done for a NiO cluster for a charge trans-
6 thephysicsofpseudogap. Tomaketheproblemtractableand
ferenergyof0.8(red)and1.6(blue)eV.SpectraofbulkLNOand
yetrelevantfortheSLsweconsiderthemulti-layeredsingle-
SmNiO [23]areincludedforcomparison.Theshaded(green)areas
3
indicatethemostsignificantchangesduringtheevolution. (c)The bandHubbardmodelforNi3dandsimulatethehigh-energy
observed multiplet splitting energy (square) and the corresponding Al 3s states as vacuum. The short-range correlation is anti-
calculated charge transfer energy (circle) versus n. Solid lines are ferromagnetic as recently suggested in a similar system with
fitstopower-law. theplanard orbitalconfiguration[5]. Toneutralizethe
x2−y2
CDMFT’s overemphasis on the in-plane correlations, we in-
troduceasmallanisotropybetweenthein-planet andout-
in
∆n = E(dn+1L) − E(dn) decreases due to the change in of-planetouttransfersastin =t(1−δ)andtout =t(1+2δ)
electroncountwith∆7 ∼= ∆8−U,leadingtostrongmixing so that tout competes with the in-plane correlations. The
between the d8L and d7 configurations. The degree of Ni-O bandwidth of the bulk non-interacting system is unchanged,
covalency here is varied by changing the charge-transfer en- W =12t.
ergy∆7.Figure2(a)showstheresultofcalculationswhen∆7 SincebulkLNOisametalweconsidermoderatevaluesof
was varied from 0.8 eV to 1.6 eV. The calculation strongly U/t (U = 6t). Fig. 3 (a) presents the n-dependent evolu-
implies that when the local Ni-O covalency is reduced with tionofthespectralfunction. Asclearlyseen, thecorrelation
increasing ∆7, the observed evolution of the L3-edge mul- gap in the two dimensional limit is filled up and becomes a
tiplet splitting with n is well reproduced. To quantify the pseudogap with increasing n, lending theoretical support to
n-dependence, the value of ∆7 for each SL is obtained by the notion that the experimentally observed MIT arises from
matching the corresponding size of the splitting. As can be the dimensional evolution of a quantum confined correlated
seeninFig.2(c), then-dependenceof∆7 clearlytracksthat carriers transforming from an insulator to a metal through a
oftheenergyseparationofthesplitpeaksattheL3-edge. ‘bad’metalphase. Notice,thesharpquasiparticlepeakisab-
With this excellent agreement between the experimental sent at ω = 0 even in the thickest slab because out-of-plane
4
The authors acknowledge fruitful discussions with R.
Pentcheva,D.Khomskii,A.MillisandG.A.Sawatzky. J.C.
wassupportedbyDOD-AROunderthegrantNo.0402-17291
and NSF grant No. DMR-0747808. Work at the Advanced
Photon Source, Argonne is supported by the U.S. Depart-
mentofEnergy,OfficeofScienceundergrantNo. DEAC02-
06CH11357. MvV was supported by the U.S. Department
ofEnergy, OfficeofBasicEnergySciences, DivisionofMa-
terials Sciences and Engineering under contract DE-FG02-
03ER46097. S.O. was supported by the Materials Sciences
and Engineering Division, Office of Basic Energy Sciences,
theUSDOE.
FIG. 3: Layer CDMFT results for the multilayer Hubbard model
withvariousthicknessn. Thetransferanisotropyδissetat0.2. (a)
AveragespectralfunctionforU/t = 6. (b)Sheetconductanceσ(cid:50)
versusnforU/t=5.5,6and6.5.
∗ Electronicaddress:[email protected]
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4
state of charge-ordering. The strong multiplet splitting ap- NiL -edgespectra.
3
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layer-resolvedCDMFTcalculations.
