Analysis of electronic and structural properties of surfaces and interfaces based on LaAlO3 and SrTiO3

0.85 MB·

by  I. I. Piyanzina

JLowTempPhysDOI10.1007/s10909-016-1483-2
Analysis of electronic and structural properties of surfaces
and interfaces based on LaAlO and SrTiO
3 3
Piyanzina I.I. · Lysogorskiy Yu.V. ·
VarlamovaI.I. · KiiamovA.G. · KoppT. ·
EyertV. · NedopekinO.V. · TayurskiiD.A.
6
1
0
2 Received:4June2015/Accepted:4January2015

n
a
Abstract Recently, it was established that a two-dimensional electron system can
J
5 ariseattheinterfacebetweentwooxideinsulatorsLaAlO3andSrTiO3.Thisparadig-
matic example exhibits metallic behaviours and magnetic properties between non-
1
magnetic and insulating oxides. Despite a huge amount of theoretical and experi-

] mentalworkathoroughunderstandinghasyettobeachieved.Weanalyzedthestruc-
l
e turaldeformationsofaLaAlO (001)slabinducedbyhydrogenadatomsandoxygen
3
- vacanciesatitssurfacebymeansofdensityfunctionaltheory.Moreover,weinvesti-
r
t gatedtheinfluenceofsurfacereconstructiononthedensityofstatesanddetermined
s
. the changeof the localdensity of states at the Fermi levelwith increasingdistance
t
a from the surface for bare LaAlO and for a conducting LaAlO /SrTiO interface.
3 3 3
m
In addition, the Al-atom displacements and distortions of the TiO -octahedra were
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- estimated.
d
n Keywords Surface·interface·LAO/STO·defects·densityfunctional·electronic
o
structure
c
[

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1 Introduction
v
9
0 Sincethediscoveryofhightemperaturesuperconductivity[1]considerableefforthas
0 beenmadeto studythebehaviorofstronglycorrelatedelectronsintransitionmetal
4 oxides. Various types of impurities, crystal structure defects, stoichiometric varia-
0
tions,externalelectricandmagneticfields,lightilluminations,uniaxialorhydrostatic
.
1
0 PiyanzinaI.I.,LysogorskiyYu.V.,VarlamovaI.I.,KiiamovA.G.,NedopekinO.V.,TayurskiiD.A.
6 InstituteofPhysics,KazanFederalUniversity,KremlyovskayaSt.18,420008Kazan,Russia
1 PiyanzinaI.I.,KoppT.
: EPVIandCenterforElectronicCorrelationsandMagnetism,Universita¨tAugsburg,Universita¨tsstraße1,
v D-86135Augsburg,Germany
i E-mail:[email protected]
X
EyertV.
r MaterialsDesignSARL,18ruedeSaisset,92120Montrouge,France
a
2 PiyanzinaI.I.etal.
pressuresareaccessibleparametersforthetransitionmetaloxidepropertiesoffilms
andheterostructures.Thestructuraltuningof
filmsresultsinavarietyoffascinatingmany-bodyphenomena.
Inparticular,theparadigmexampleofaheterostructurebetweenthenon-magnetic
and insulating oxides LaAlO (LAO) and SrTiO (STO) demonstratesrich physics
3 3
includingthecoexistenceofsuperconducting2Delectronliquid[2,3,4,5]andmag-
netism[5,6,7,8,9,10,11].
Theaimofthepresentstudyistoinvestigatetheelectronicpropertiesandstruc-
turaldistortionsofsurfacesandinterfacesbasedonLAOandSTObymeansofden-
sityfunctionaltheory.
2 Method
Weperformedab-initiocalculationswithindensityfunctionaltheory(DFT)[12,13]
and the Vienna Ab-Initio Simulation Package (VASP 5.3) [14] implemented into
MedeAsoftware[15].Exchangeandcorrelationwereincludedatthelevelofthegen-
eralizedgradientapproximation(GGA)[13].Wehaveusedtheprojector-augmented
wavemethod[16,17]withaplane-wavebasissetandacutoffenergyof400eV.The
forcetolerancewas0.05eV/A˚ andtheenergytolerancefortheself-consistencyloop
wasequalto10−5eV.TheBrillouinzonewassampledonagridof5×5×1k-points
(7×7×1for4LAO/4.5STO/4LAOheterostructure).Inordertoavoidtheinteraction
ofsurfacesandslabswiththeirperiodicimagesduringsimulation,a20A˚ vacuumre-
gionperpendiculartothesurfacewasadded.
3 Resultsanddiscussions
3.1 SurfacepropertiesofLAOandSTO
WeconsideredtheelectronicstructureofanLAO(001)slabthatconsistedof5.5unit
cells with identicalterminationson bothsides (La+3O−2 or Al+3O−2). The central
2
region of 1.5 unit cells was kept fixed during the optimization as in Ref. [18]. It
was shownin Ref. [19,20] that the surfacereconstructionaffectsintrinsic transport
propertiesof the slab.Since the La+3O−2 layershave a chargeof+1 and Al+3O−2
2
layers have a chargeof -1 per unit cell, the unreconstructed(1×1)LaO-terminated
(001)surfacelacksoneelectron,whereastheAlO -terminatingsurfacehasanexcess
2
ofoneelectron.WeconfirmedthattheLAOsurfacewithAlO -terminationhaslower
2
energyperareaandwewillthereforefocusonthisstructureinthediscussionbelow.
Underexperimentalconditions,thinfilmsofLAO aregrownontopoftheSTO
substrate.ThelatticeconstantaofLAOapproachesthein-planelatticeparameteraof
STO[2],thereforetheLAOlatticeparameterwasfixedatthisvalue(a=3.941A˚ from
ourGGAcalculationforbulkSTO).Itfollowsfromourfindings(seeFig.1)thatthe
bare LAO surface with AlO -terminationis a semiconductorwith a verysmall gap
2
separatingthebulkbandsandanarrowsurfacelevel.
Itisknownthatthepresenceofimpuritiesonthesurfaceaffectsthefinaltermina-
tionlayerofLAOduringsurfacegrowth[21].Weconsideredthecaseofahydrogen
3
1.0 0.6
0.8 0.5
OS,a.u.00..46 OS,a.u.00..34
D D0.2
0.2 0.1
0.0-2 -1 0 1 2 3 4 0.0 -0.4 -0.2 0.0 0.2
E-EF,eV E-EF,eV
(a) (b) (c)
Fig. 1: Top view of an optimized (2×2) bare LaAlO (001) surface with AlO -
3 2
termination (a), corresponding density of states (DOS) profile (b) and its enlarged
version near the Fermi level (c). Red spheres denote oxygen, blue spheres denote
lanthanumandyellowspheresdenotealuminum.
(H) adatomwhich is presentin almostallgrowthandannealingenvironments.The
top viewof the optimized(2×2)LaAlO surfacewith H-adatomatthe surfaceand
3
correspondingDOSareshowninFig.2.AsintheworkofKrishnaswamyetal.[18]
theH-adatombondstooxygen.Ittakesapositionbetweenthestrontiumandoxygen
andislocatedabovethesurfaceplane.ComparingFig.1b,candFig.2citfollows
thatthepresenceofaH-adatomleadstoa
fillingoftheemptystatesrightabovetheFermilevelofpureLAOand,therefore,a
suppressionofthealbeitsmallsurfaceconductivity.
7
6
5
a.u.4
OS,3
D
2
1
0-3 -2 -1 0 1 2 3 4
E-EF,eV
(a) (b) (c)
Fig. 2: Top (a) and side (b) view of an optimized (2×2) LaAlO (001) surface
3
withAlO -terminationandH-adatomswithcorrespondingdensityofstates(c).Red
2
spheres denote oxygen, blue spheres denote lanthanum and yellow spheres denote
aluminumm,andwhitespheresdenotehydrogen.
Thesecond type of surfacereconstructionis caused by an oxygen(O) vacancy.
Thistypeisoneofthemoststudied[10,22,23,24,25].Forthistypeofreconstruction
wetooka(2×2)cell(Fig.3).AsinthecaseofH-adatomsweobtainedasuppression
of surface conductivity(Fig. 3 c). We performedthe same calculationsfor an STO
slab and in contrastto LAO, the STO surfacedid notshowempty states justabove
theFermilevelandexhibitedthesameinsulatorbehaviorasthebulkSTO.
3.2 LAO/STO/LAOheterostructureproperties
TheheterostructuresinourcalculationsconsistedofacentralregionofSrTiO (with
3
fixed number of layers N =4.5) boundedon both sides with a varying number
STO
4 PiyanzinaI.I.etal.
7
6
5
a.u.4
OS,3
D
2
1
0-3 -2 -1 0 1 2 3 4
E-EF,eV
(a) (b) (c)
Fig. 3: Top (a) and side (b) view of an optimized (2×1) LaAlO (001) surface
3
withAlO -terminationandO-vacancywithcorrespondingdensityofstates(c).Red
2
spheresdenote oxygen,blue spheresdenotelanthanum,yellow spheresdenote alu-
minum,×indicatestheoxygenvacancyandarrowspointtheatomsdisplacement.
ofLaAlO layers(seeFig.4).Accordingtoexperimentalevidence[2,4],then-type
3
contactwith (TiO )0-(LaO)+ showsconductivityatthe interfaceand,therefore,we
2
havefocusedonthistypeofinterface.
Fig.4:Structureof4LAO/4.5STO/4LAOsuperlattice.
7 7 7
6 6 6
5 5 5
a.u.4 a.u.4 a.u.4
OS,3 OS,3 OS,3
D2 D2 D2
1 1 1
0-3 -2 -1 0 1 2 3 0-3 -2 -1 0 1 2 3 0-3 -2 -1 0 1 2 3
E-EF,eV E-EF,eV E-EF,eV
(a) (b) (c)
Fig.5:DensityofstatesplotsforLAO/4.5STO/LAOsuperlatticeinterfacewith(a)
2LAOlayers,(b)3LAOlayers,(c)4LAOlayers.
We calculatedthedensityofstatesprofilesforavaryingnumberofLAO layers
(Fig.5a-c).Itwasfoundthatthebandgapdecreaseswithanincreaseinthenumberof
LAOlayers.AtfourLAOlayersthebandgapoftheLAO/STO/LAOheterostructure
vanishes (in agreement with [26]). Layer resolved analysis of the density of states
andstructuraldeformationwasperformedforthissystem(Fig.6).
Fig. 6 a presents the spatially resolved DOS at the Fermi level n(E ) of a 4
F
LAO/4.5STO/4LAOheterostructure.Themaximumintensitycorrespondstosurface
AlO andtheTiO layersontheinterfaceandagreeswellwithpreviousfindings[27,
2 2
10,2]. The value of n(E ) doesnotvanish in the middle of the STO slab due to its
F
5
a.u.0.020
level,0.015
mi
Fer0.010
the0.005
at
OS0.000
D 0 5 10 15 20 25
LayerNo. (a)
Q a
Q
0,16 Q z’ (b) 0,16
Q z’’
¯0,02
ment, ¯0,12 mean value,0,01 (c) 0,12
Displace00,,0048 Distortion from the -00,,0001 00,,0048
-0,02
LaO LaO LaO LaO SrO SrO SrO SrO LaO LaO LaO LaO
0,00 0,00
AlO2AlO2AlO2AlO2TiO2 TiO2 T iO2TiO2 TiO2AlO2AlO2AlO2AlO2
Fig. 6: Projection onto the different layers of 4 LAO/4.5 STO/4 LAO superlattice:
density of state at the Fermi level calculated in a small energy window (40 meV)
(a), displacement of Al atoms with respect to oxygen plane (b) and symmetrized
displacementsQ(normalcoordinates)ofTiO octahedralsystems [28](c).
6
smallthicknessduringsimulations.OrbitaldecompositionofDOSindicatesthatthe
electronic states at the Fermi level consist entirely of Ti 3d states, which agrees
xy
with previous work [10,11,27]. As in the case of a bare LAO surface with AlO
2
termination,thereisacontributionton(E )fromthesurfacelayers.
F
As a result of the DFT structural optimization we obtained distortions of the
LAO/STOheterostructure,seeFig.4.Tiatomsmovedtowardsthecenteroftheslab,
whereas La atoms shifted towards the surface. Fig. 6 b shows the displacement of
Alatomsoutoftheoxygenplanes.ThebiggestshiftwasobtainedforAlatomsnear
theLAO/STOinterface.Thesymmetrizeddisplacements,Q,(normalcoordinates)of
TiO octahedralsystemarepresentedinFig.6c.Q indicatesthecollectiveatomdis-
6 i
placements,which,underthesymmetryoperation,transformaccordingtooneofits
irreduciblerepresentations[28].ThesmallestmagnitudewasfoundforQ breathing
a
mode,whichcorrespondstoatotallysymmetricshiftingofOatomsoutofthecen-
ter Ti atom. Further non-zero normal coordinates, Q, were associated with oxygen
movementsinz-directionperpendiculartotheinterface,andtheyledtothesplitting
oft orbital.
2g
6 PiyanzinaI.I.etal.
4 Conclusion
Inthepresentwork,bymeansofDFTcalculationsonthe GGAleveloftheory,we
haveinvestigatedthestructuralandelectronicpropertiesofthesurfacesandinterfaces
basedonthetwoinsulators,LAOandSTO.WefoundthatthebaresurfaceofLAO
with AlO -termination has conductive surface layers, whereas the STO surface is
2
insulating.Twoconsideredtypesofsurfacedefects(H-adatomandO-vacancy)ofthe
AlO -terminated(001)LAO slab lead to a suppressionof surface conductivity.For
2
theLAO/STO/LAOinterface,aninsulator-metaltransitionwasfoundbyincreasing
thenumberofLAOlayersfromthreetofour.Thelargestcontributiontotheinterface
conductivityisfromtheTiO layers(Tid state),decreasingwithdistancefromthe
2 xy
interface.We also obtainedsurfaceconductivity,which we expectto be suppressed
bydefectsasforthecaseoftheLAOslab.
Acknowledgements Thereported studywas supported bythe Supercomputing Center ofLomonosov
Moscow StateUniversity withsupportoftheRussianGovernment ProgramofCompetitive Growthof
KazanFederalUniversityandoftheDFGSonderforschungsbereichTRR80.TheworkofA.G.Kiiamov
wasfundedbythesubsidyallocated toKazanFederalUniversityforthestateassignmentinthesphere
ofscientificactivities. TheauthorsacknowledgehelpfuldiscussionwithD.Juraschek,A.Petrova,Kate
ReidyandJessicaWeitbretch.
References
1. J.G.Bednorz,K.A.Mu¨ller,Z.Phys.B64,189(1986).
2. A.OhtomoandH.Hwang,Nature427,423(2004).
3. N.Reyren, S.Thiel, A.D.Caviglia, L.F.Kourkoutis, G.Hammerl, C. Richter, C. W.Schneider,
T.Kopp,A.-S.Ru¨etschi,D.Jaccard,M.Gabay,D.A.Muller,J.-M.Triscone,andJ.Mannhart,Science
317,1196(2007).
4. S.Thiel,G.Hammerl,A.Schmehl,C.Schneider,J.Mannhart,Science313,1942(2006).
5. Y.YuandA.Zunger,NatureCommun.5,5118(2014).
6. A.Brinkman,M.Huijben,M.VanZalk,J.Huijben,U.Zeitler,J.Maan,W.VanderWiel,G.Rijnders,
D.Blank,andH.Hilgenkamp,NatureMat.6,493(2007).
7. L.Li,C.Richter,J.Mannhart,andR.Ashoori,NaturePhys.7,762(2011).
8. Ariando,X.Wang,G.Baskaran,Z.Q.Liu,J.Huijben,J.B.Yi,A.Annadi,A.R.Barman,A.Rusydi,
S.Dhar,Y.P.Feng,J.Ding,H.Hilgenkamp,andT.Venkatesan,NatureCommun.2,188(2011).
9. B.Kalisky,J.A.Bert,B.B.Klopfer,C.Bell,H.K.Sato,M.Hosoda,Y.Hikita,H.Y.Hwang,andK.
A.Moler,NatureCommun.3,922(2012).
10. N.Pavlenko,T.Kopp,E.Tsymbal,J.Mannhart,andG.Sawatzky,Phys.Rev.B86,064431(2012).
11. N.Pavlenko,T.Kopp,J.Mannhart,Phys.Rev.B88201104(2013).
12. P.HohenbergandW.Kohn,Phys.Rev.136,B864(1964).
13. J.P.Perdew,K.Burke,andM.Ernzerhof,Phys.Rev.Lett.77,3865(1996);W.KohnandL.J.Sham,
Phys.Rev.A140,1133(1965)
14. G.KresseandJ.Furthmu¨ller,Phys.Rev.B54,169(1996).
15. MedeA(cid:13)R-2.17,MaterialsDesign,Inc.,AngelFire,NM,USA,2015.
16. P.E.Blo¨chl,Phys.Rev.B50,17953(1994).
17. G.KresseandD.Joubert,Phys.Rev.B59,1758(1999).
18. K.Krishnaswamy,C.Dreyer,A.Janotti,andC.VandeWalle,Phys.Rev.B90,235436(2014).
19. A.Janotti,L.Bjaalie,L.Gordon,andC.VandeWalle,Phys.Rev.B86,241108(2012).
20. Y.Xie,Y.Hikita,C.Bell,H.Y.Hwang,NatureCommun.2,494(2011).
21. J.Yao,P.Merrill,S.Perry,D.Marton,andJ.Rabalais,J.Chem.Phys.108,1645(1998).
22. A.Kalabukhov,R.Gunnarsson,J.Bo¨rjesson,E.Olsson,T.Claeson,andD.Winkler,Phys.Rev.B75,
121404(2007).
7
23. V.Vonk,J.Huijben, D.Kukuruznyak, A.Stierle, H.Hilgenkamp, A.Brinkman, andS.Harkema,
Phys.Rev.B85,045401(2012).
24. Y.Li,S.N.Phattalung,S.Limpijumnong,J.Kim,andJ.Yu,Phys.ReviewB84,245307(2011).
25. L.Zhang,X.F.Zhou,H.T.Wang,J.J.Xu,J.Li,E.Wang,andS.H.Wei,Phys.Rev.B82,125412
(2010).
26. F.Cossu,U.Schwingenschlo¨gl,andV.Eyert,Phys.Rev.B88,045119(2013)
27. R.PentchevaandW.E.Pickett,Phys.Rev.B74,035112(2006).
28. I.B.Bersuker,TheJahn-TellerEffect,CambridgeUniversityPress,(2006).
.