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Orbital Ordering of the Mobile and Localized Electrons at Oxygen-Deficient LaAlO/SrTiO Interfaces 3

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Alla Chikina, Frank Lechermann, Marius-Adrian Husanu, Marco Caputo, Claudia Cancellieri, Xiaoqiang Wang, Thorsten Schmitt, Milan Radovic, and Vladimir N Strocov ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b02335 • Publication Date (Web): 11 Jul 2018 Downloaded from http://pubs.acs.org on July 12, 2018

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Orbital Ordering of the Mobile and Localized Electrons at Oxygen-Deficient LaAlO3/SrTiO3 Interfaces Alla Chikina1,*,‡, Frank Lechermann2, Marius-Adrian Husanu1,3, Marco Caputo1, Claudia Cancellieri1,4, Xiaoqiang Wang1, Thorsten Schmitt1, Milan Radovic1 & Vladimir N. Strocov1, *,‡

1

2

Swiss Light Source, Paul Scherrer Institute, Villigen CH-5232, Switzerland. Institut für Theoretische Physik, Universität Hamburg, Jungiusstrasse 9, Hamburg DE-

20355, Germany 3

National Institute of Materials Physics, Atomistilor 405A, Magurele RO-077125, Romania.

4

Empa, Swiss Federal Laboratories for Materials Science & Technology, Ueberlandstrasse

129, Duebendorf CH-8600, Switzerland.

*

Electronic mail: [email protected] and [email protected]

KEYWORDS: LaAlO3/SrTiO3 interface, oxide interfaces, Resonance photoemission, twodimensional electron gas, oxygen vacancies

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ABSTRACT Interfacing different transition-metal oxides opens a route to functionalizing their rich interplay of electron, spin, orbital and lattice degrees of freedom for electronic and spintronic devices. Electronic and magnetic properties of SrTiO3-based interfaces hosting a mobile twodimensional electron system (2DES) are strongly influenced by oxygen vacancies which form an electronic dichotomy, where strongly correlated localized electrons in the in-gap states (IGSs) coexist with non-correlated delocalized 2DES. Here, we use resonant soft-X-ray photoelectron spectroscopy to prove the eg character of the IGSs, as opposed to the t2g character of the 2DES in the paradigmatic LaAlO3/SrTiO3 interface. We furthermore separate the dxy and dxz/dxz orbital contributions based on deeper consideration of the resonant photoexcitation process in terms of orbital and momentum selectivity. Supported by a selfconsistent combination of density functional theory and dynamical mean field theory (DFT+DMFT) calculations, this experiment identifies local orbital reconstruction that goes beyond the conventional eg-vs.-t2g band ordering. A hallmark of oxygen-deficient LaAlO3/SrTiO3 is a significant hybridization of the eg and t2g orbitals. Our findings provide


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routes for tuning the electronic and magnetic properties of oxide interfaces through "defect engineering" with oxygen vacancies.

The discovery of a high mobility two-dimensional electron system (2DES) emergent at the interface between two wide band-gap insulators, LaAlO3 (LAO) and SrTiO3 (STO)1,2, opened the possibility of oxide electronics. Although the source of charge carriers at the LAO/STO interface is still under debate, it is clear that oxygen vacancies (VOs) play an important role. Each VO at the LAO/STO interface can release two electrons. One part of these electrons is injected into the mobile 2DES at the Fermi level (EF) derived from the Ti t2g orbitals. These mobile electrons are coupled to phonon modes and, at lower electron densities, form large polarons, fundamentally reducing the 2DES mobility.3-5 Another part of the electrons supplied by the VOs stays trapped at the Ti3+ ions and form in-gap states (IGSs) in the STO band gap at a binding energy (EB) near -1.3 eV.3,6,7 As the VOs are associated with a local lattice distortion,8 the localized IGS electrons are often viewed as small polarons that can even contribute to the interfacial conductivity by hopping between different Ti sites under thermal or electric field activation.9 In contrast to the t2g-derived 2DES, the IGSs are theoretically predicted to have the Ti3+ eg orbital character,10,11 although this prediction has so far escaped direct experiment verification. The dichotomic electron system, formed by the coexistence of the radically different 2DES and IGS electrons,8,11,12 significantly enriches the physical properties of oxygen-deficient LAO/STO interface. The IGSs have been directly


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observed by photoemission spectroscopy in bare TiO2-terminated STO,13 STO-based heterostructures14 and other oxides.15 The concerted development of the IGSs and 2DES under X-ray irradiation,16 their different nature,7 and the concomitant dichotomy of the electronic structure11,12,16 have also been discussed. Nevertheless, the present understanding of the IGS’s origin and orbital character is far from exhaustive. The orbital ordering of the electron states is a crucial for the physics of transition metal oxide systems. In the octahedral crystal field of the bulk crystal, the 3d orbitals split into three-fold degenerate t2g and two-fold degenerate eg orbitals. The interface formation, defects and manybody interactions like Coulomb repulsion may strongly change the energy and occupancy of the t2g and eg states. The orbital character and ordering of the mobile 2DES and localized IGSs play pivotal roles in their coexistence and interplay, determining the exotic properties and thus potential applications of transition metal oxide interface interfaces. For example, the recently found reversed occupation of heavy out-of-plain dxz/dyz and light in-plain dxy states leads

to

higher

carrier’s

mobility

at

γ-Al2O3/SrTiO3

heterostructure.17

Also,

superconductivity18 at the LAO/STO interface is mainly related to the dxz/dyz-derived bands. Moreover, theoretical analysis10,19 suggests that the weak ferromagnetic response of the LAO/STO interface20-23 is not an intrinsic property, but rather emerges from exchange coupling of the partially filled Ti dxy orbitals of the 2DES with half-filled Ti eg orbitals induced by VOs. This theoretical prediction has however been never experimentally proved. Here we report a direct experimental evidence for Ti eg character of IGSs due to a local orbital reconstruction near VOs. To experimentally disentangle the orbital character and ordering of the mobile and localized LAO/STO electrons, we employed resonant soft-X-ray angle-resolved photoelectron spectroscopy (SX-ARPES) to directly measure k-resolved electron dispersions. The high photon energies, hv, used in these experiments provide a probing depth sufficient to penetrate 4 ACS Paragon Plus Environment

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to the buried interface, and resonant photoemission (ResPE) across the Ti 2p core level enables elemental and chemical state selectivity24 consistent with recent study.7,16,25,34 However, here we develop an advanced model of the ResPE process that enables its interpretation in terms of k-conservation, giving a direct link to the orbital character of the electron states. Interpreted on the basis of combined density functional theory (DFT) and dynamical mean field theory (DMFT) calculations, our experimental results depict the LAO/STO electronic structure as the Ti3+/Ti4+ t2g-derived mobile 2DES electrons coexisting with the Ti3+ eg-derived localized IGS ones tunable by oxygen deficiency. RESULTS SAMPLE PREPARATION AND CHARACTERIZATION In order to tune the electronic structure through the VO-concentration, we grew the 4 unit cell (u.c.) thick LAO film on TiO2-terminated STO substrate in slightly oxygen deficient conditions (see Methods). Further ex-situ oxygen annealing quenched the VOs but the samples stayed metastable and, similar to bare STO,26 at low sample temperature gradually lost oxygen under X-ray irradiation.27,28 This oxygen loss is attributed to a double Auger process subsequent to the Ti 3p core shell ionization that ejects an oxygen atom from the surrounding Ti cage.29


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Figure 1. (a) Development of angle-integrated photoemission intensity at hv = 462.7 eV with X-ray irradiation time. The increase of VO concentration builds up the IGSs at EB ~ -1.3 eV and increases the 2DES signal at EF; (b) ARPES image at saturation, showing the dispersionless IGSs and (eye guides) dispersive dxy/dyz bands of the 2DES and the corresponding angle-integrated spectrum; The angle-integrated spectra (c) from Fig. 1(a) and (d) Ti 2p core level spectrum measured under the x-ray light irradiation are taken at intervals of 5 min under photon flux of 1.2x1013 photons/sec focused in a spot of 30x75 µm2. The intensities of the Ti3+ component and IGS increase with time and reach saturation after ~35 min. Fig. 1a displays the oxygen loss process under X-ray irradiation of hv = 462.7 eV (where both 2DES and IGS states have sufficient intensity, see below). We observe that a broad peak appears at Eb~1.3 eV and scales up on a longer time scale, reflecting formation of the IGSs. 6 ACS Paragon Plus Environment

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Simultaneously, the 2DES signal near EF gradually increases with irradiation. Fig. 1d shows a time evolution of the Ti 2p spectra under the x-ray irradiation taken at photon energy of hv =1 keV. During the irradiation, spectral weight transfers from the Ti4+ to the Ti3+ component, reflecting development of the VOs.13 After ~35 min, the I(Ti3+)/I(Ti4+) ratio reaches the saturation value of ~0.2. All X-ray absorption spectroscopy (XAS) and ARPES measurements reported below were performed at saturation. The ARPES image measured at saturation for the Γ-X direction of the Brillouin zone (BZ) with hv = 462.7 eV is shown in Fig. 1b. The image reveals the developed dispersionless IGSs and dispersive dxy/dyz bands of the 2DES forming the dichotomic electron system of the oxygen-deficient LAO/STO interface. We identify the heavy dyz band and the light dxy one (the latter evidenced by two high-intensity points due to its hybridization with dyz), while the light dxz band is silenced with our choice of s-polarized incident X-rays.6 The bulk degeneracy of these t2g bands breaks at the interface, and the sharp localization of the dxy states near the interface pushes them below the dxz/yz ones, which are more delocalized into the STO bulk.30 THEORETICAL PICTURE We set up the basis for further analysis of the experimental results with a theoretical picture of the dichotomic electron system at the oxygen-deficient LAO/STO interface. Since the VOinduced IGSs appear as defect-like states, trapping electrons in the partially localized Ti 3d shell, many-body effects are essential for the correct characterization of the resulting electronic structure.31 Charge self-consistent DFT+DMFT32 calculations were performed for the n-type LAO/STO (001) interface with a VO lying in the TiO2 layer closest to the interface (see Fig. 2a). Variation of the OV position leads to minor quantitative differences of the spectral features (Supplementary information, note 3). Fig. 2b,c presents the calculated local 7 ACS Paragon Plus Environment

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(k-integrated) spectral function A(EB) reflecting the electron correlations. It shows a narrow 2DES peak close to EF dominated by the t2g weight and – a hallmark of the oxygen deficient LAO/STO – a broad IGS peak at EB ~ -1.5 eV dominated by eg x2-y2 weight that is absent in stoichiometric LAO/STO (see further details in the supplementary information, note 1). The peaks are in good agreement with their experimental energy position and broadening in Fig. 1c, although with a different amplitude ratio because of the ResPE matrix elements. Note that our analytical framework attributes the IGS broadening exclusively to the electron correlations as expressed by the imaginary part of the electron self-energy and leaves out the VO disorder effects also contributing to broadening of the energy levels. The agreement with the experiment demonstrates that in our case the electron correlation effects dominate the IGS lineshape. We emphasize that conventional DFT positions the IGSs too close to EF, which also stresses the importance of many-body physics for these localized electron states.31

Figure 2. Theoretical electronic structure of the LAO/STO interface: (a) relaxed supercell of LAO/STO interface. The dotted line in the bottom represents the supercell mirror plane; (b,c) Local DFT+DMFT spectral function for an interfacial VO integrated over all Ti sites, (b) Ti 3d (t2g,eg) -resolved and (c) Ti 3d m-resolved. The IGS peak at EB ∼ -1.5 eV is dominated by 8 ACS Paragon Plus Environment

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the eg x2−y2 orbitals, and the narrow 2DES peak near EF by the t2g weight. The admixture of the t2g weight in the IGSs and eg weight in the 2DES indicate hybridization of the IGS and 2DES subsystems.

In our theoretical framework, the IGSs originate from an extremely strong perturbation of the STO crystal field that pushes the nominally unoccupied eg states down in energy.11,10,19 The charge transfer to the Ti ions next to the VOs is 0.92e (formal valency Ti3.08+). The results of the charge self-consistent DFT+DMFT calculations were sensitive to the U value, whose reduction by only 0.5 eV from the actual U = 3.5 eV propagated into shifting of the IGS peak by 0.4 eV above the correct EB ~ -1.5 eV. We note that the U value optimal for the interface is somewhat smaller compared to the STO(001) surface,12 which may be reasonable due to enlarged screening via the polar catastrophe avoidance mechanism at the LAO/STO interface. For the 2DES peak, the calculations show dxy-vs.-dxz/dyz orbital ordering characteristic of the bulk symmetry breaking at the interface. They overestimate however the 2DES bandwidth that is renormalized by strong electron-phonon interaction3 not included in our DFT+DMFT scheme. Supported by our charge self-consistent DFT+DMFT results, we will now discuss each of the localized IGS and mobile 2DES subsystems in more detail. EXPERIMENTAL RESULTS We start our experimental analysis with the orbital structure of the unoccupied states reflected by the Ti L-edge XAS data taken in the total-electron-yield mode. The XAS spectrum in Fig. 3a shows two peaks corresponding to transitions from the core Ti 2p1/2 and 2p3/2 states to the unoccupied 3d states – i.e. Ti L3 and L2 absorption edges. The peaks in each pair are separated by ~2.5 eV, which reflects the octahedral crystal field splitting of the 3d states into t2g and eg levels, as marked in Fig. 3a. Even after the saturating irradiation dose, 9 ACS Paragon Plus Environment

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the XAS spectral structure is dominated by the Ti4+ ions in the STO bulk with only a tiny admixture of the Ti3+ signal between the Ti4+ peaks.33 This signal is much smaller compared to the Ti 2p core level photoemission. Since the XAS probing depth is much larger compared to photoemission, this difference indicates accumulation of the Ti3+ ions in the interfacial region.34,35 In the ResPE process, sketched in Fig. 4a, the core level electrons are promoted into certain unoccupied states reflected in the XAS spectrum, and decay to a single valence band (VB) hole state via the Auger process. Although the final state in this core hole assisted photoemission process is identical to the one of direct photoemission, the photoexcitation cross section can be one or two order of magnitude higher for localized atomic shells. The intensity boost in ResPE spectra at the Ti L-edge is essential for measuring the ARPES signal from the buried LAO/STO interface3,6,7,38, which is weak not only because of the photoelectron absorption in the LAO overlayer, but also owing to electronic phase separation at the interface where the 2DES forms conducting puddles separated by extended insulating areas.27,39


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Figure 3. (a) (top curve) XAS data together with (three bottom curves) angle-integrated ResPE intensity from Fig. 3a integrated within the VB, IGS and 2DES energy intervals marked in Fig. 3a. The IGSs resonate at the unoccupied Ti3+ eg states, and the 2DES between the Ti3+ and Ti4+ eg states. The different resonant behaviors of the IGS and 2DES reflects their different valence and orbital characters. (b) Resonance (angle-integrated) photoemission intensity map for LAO/STO across the Ti L2,3 resonances, identifying the VB, IGS and 2DES states whose EB regions are marked on top. (c) Band map of the 2DES (normalized to maximum intensity) along X-Г-X direction of the BZ at hv marked in Fig. 3a. The schematics show the intensity enhancement of the dxy (dyz) states at the t2g (eg) resonances, correspondingly.

Fig. 3b shows the ResPE intensity map as a function of (EB,hv) measured through the Ti L2,3 absorption edges. The broad band of intensity extending from EB = -7.5 eV to -4.5 eV is the VB formed mainly by O 2p states hybridized with Ti 3d ones.16,38 The broad peak centered at EB ~ -1.3 eV is the VO-induced IGS, and the narrow peak at EF is the 2DES. Intensity variations of the VB, IGS and 2DES spectral structures (integrated over the corresponding EB intervals) across the Ti L2,3 edges28,38 are plotted in Fig. 3a in comparison with the XAS spectrum. We note that analogous measurements on the bare STO surface (provided in the supplementary information, note 2) show a notably different ResPE intensity behavior, where the IGS and 2DES resonate in more extended hv-regions compared to the LAO/STO and the IGS peak is shifted to higher EB. The ResPE data in Fig. 3a,b show that VB main resonant peaks coincide with the Ti4+ peaks in the XAS spectrum. This is consistent with the abundant bulk STO contribution where the O 2p states hybridize with Ti 3d ones. The two main IGS resonances at hv = 459 and 464.3 11 ACS Paragon Plus Environment

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eV coincide with energies of the unoccupied Ti3+ eg states in XAS. The t2g-derived 2DES resonates at higher hv between the Ti3+ and Ti4+ eg states and shows a more extended resonance region compared to the IGSs. As we will discuss later, the difference in the IGS vs 2DES resonant response signify the different nature of these states forming the dichotomic electron system of the LAO/STO interface.

Figure 4. (a) The states involved in the ResPE process. (b) Schematic dispersions of the Ti 3d states involved in the ResPE process. The transition probability maximizes when the |m> wavefunctions in the CB and with energy Em and lifetime Гm. These states, owing to an extremely strong perturbation of the ground-state atomic potential by the presence of the core hole, form a multiplet. The subsequent decay of the |m> states into the wave functions with the states runs through a continuum of the off-symmetry km-points where their strict orthogonality to the


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states. Now we will demonstrate how the orbital and k-selectivity of ResPE enables probing the valence and orbital character of electron states in LAO/STO. DISCUSSION IGS SUBSYSTEM: VALENCE AND ORBITAL CHARACTER First, we turn to the IGSs. As we have seen in Figs. 3a,b, their photoemission response resonates when hv is tuned to the unoccupied Ti3+ eg states. First of all, this fact most directly shows that the IGSs are localized at the Ti3+ ions. The above considerations about the ResPE process take our analysis yet further. As k is irrelevant for the disordered IGSs, their resonance at the unoccupied eg orbitals indicates that these states have the same eg character. This idea is corroborated by calculations in Fig. 2c that indeed show the same eg x2-y2 orbital character of corresponding occupied and unoccupied states. Our ResPE data provide therefore the experimental verification of the theoretical prediction10,11,19 that the IGSs are localized at the Ti3+ ions and originate from the nominally unoccupied eg states pushed down in energy by the strong VO-induced perturbation of the STO crystal field. 2DEG SUBSYSTEM: K-CONVERSERVATION IN RESPE Now we will focus on the 2DES resonant behavior. At odds with the above idea of the identical orbital character of the |m> and and states connected with the state of the ResPE process. The next available unoccupied states will rather be the eg states located at higher energy, as sketched in Fig. 4b. Indeed, our ResPE data at both L3 and L2 edges show the dxz/dyz resonant enhancement in the hv-region corresponding to eg states, with the k-conservation in the ResPE process prevailing over the orbital similarity. We note that this resonance extends from the Ti3+ to Ti4+ eg unoccupied energy levels because the dxz/dyz states are delocalized in the interfacial QW where valency of the Ti atoms varies from 3+ at the interface to 4+ in the bulk. Our kconservation mechanism explains the delay of the t2g resonance to the unoccupied e2g region. We will now turn to the dxy states. As they are localized essentially within one interface layer44, their kz momentum is not well defined, and the k-conservation applies only to the parallel momentum k//. The unoccupied dxy states cannot couple to the occupied dxy ones because they are localized at different atomic planes. Consequently, the |m> states available for ResPE from the dxy states start from the higher-n QW states of the unoccupied t2g dxz/yz manifold just above EF. This is what we see in our ResPE data, where the dxy resonant enhancement starts from the unoccupied t2g region before the e2g one.


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This picture of the orbital and k-selectivity of the ResPE process is confirmed by the sequence of ARPES images in Fig. 3b that were acquired along the Γ-X direction of the BZ at several hv across the Ti L3,2 edges. Indeed, for both edges the dxy resonant response prevails over the dxz/dyz one over the t2g region in the unoccupied states, and the dxz/dyz response prevails over the eg region. The observation is consistent with our theoretical results in Fig. 2, which show the out-of-plane dz2 character of these eg states which, due to off-symmetry kmpoints, have large overlap with the out-of-plane dxz/dyz states. We note that the interfaceinduced breaking of degeneracy of the bulk t2g states is crucial for this orbital selectivity of the ResPE process. An interesting phenomenon clearly beyond the above k-conservation mechanism is that the dxy-to-dxz/dyz resonant intensity ratio differs between the L3 and L2 edges. With the same intermediate states, the only difference between these two cases resides in the different orbital momenta of the involved Ti 2p3/2 or 2p1/2 core levels. Our experimental findings can stimulate further theoretical efforts on orbital selectivity phenomena in the ResPE process that will combine many-body electronic structure, resonant photoexcitation and interfacial effects. HYBRIDIZATION BETWEEN THE 2DEG AND IGS SUBSYSTEMS Upon closer inspection of the ResPE data at the L3 edge, in Fig. 3b, we find that the IGS peak appears already at the Ti3+ t2g resonances below the main Ti3+ eg ones and is stronger at the L3 and weaker at the L2 edge. This observation suggests a t2g admixture in the IGSs due to their hybridization with the 2DES. Moreover, the IGS peak slightly shifts in EB as a function of excitation energy from EB=-1.28 eV at the Ti3+ t2g edge to EB=-1.38 eV at the Ti3+ eg edge. This shift might in principle be attributed to two types of VOs in different atomic environments. However, our charge self-consistent DFT+DMFT calculations, including only one VO-configuration, explain the shift by the IGS hybridization with the 2DES: the IGS 16 ACS Paragon Plus Environment

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spectral weight in Fig. 2b includes a smaller t2g component slightly shifted to the high-EB side of the IGS peak and a larger eg one shifted to the low-EB side, as we observe in the experiment. Closer consideration of our calculations for LAO/STO in Fig. 2b suggests not only a t2g admixture in the IGSs but also a significant eg admixture in the 2DES near EF. We note that calculations for the bare TiO2-terminated STO(001) surface12 predict negligible hybridization between the IGSs and 2DES, leading to almost pure eg and t2g character. Indeed, the ResPE measurements of the bare TiO2-terminated STO surface (see the supplementary information, note 2) show only negligible Ti3+ t2g resonance of the IGS peak denying any significant eg–t2g hybridization. The discovered t2g-eg hybridization has important consequences for a number of physical properties of the oxygen-deficient LAO/STO interfaces. Usually a two-band model including the dxy and dxz/yz t2g bands is employed to describe their transport properties. Our results show that the eg orbitals should be an integral part of this model. The t2g-eg hybridization should also affect the magnetic properties of the LAO/STO interfaces, because it strengthens exchange coupling between the eg-like nearly-localized Ti3+ magnetic moments. This effect extends to the puzzling coexistence of ferromagnetism and superconductivity in LAO/STO, because the hybridization may give rise to a coherent mechanism between the Ti3+ moments and itinerant t2g-like carriers that would suppress the usual destruction of superconductivity18 by scattering from magnetic moments, an mechanism alternative to phase separation.22 SUMMARY


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Figure 5. Sketch of band ordering and charge transfer at the LAO/STO interface. The interface breaks the in-plane and out-of-plane orbital degeneracy, and the local distortion and Coulomb interaction near the VOs drive a charge transfer, shifting the eg-derived IGS below the t2g-derived 2DES. This atomic scheme (excluding the orbital hybridization effects) is general for all oxygen-deficient STO-based interfaces. In summary, we can draw the following physical picture of orbital ordering at the LAO/STO interface schematically shown on Fig. 5. In addition to the the t2g < eg orbital ordering established for the stoichiometric LAO/STO interfaces, we show that oxygen deficiency causes the orbital reconstruction in proximity to the VOs, which leads to partial occupation of localized eg orbitals. Our results show a significant t2g-eg hybridization, which has important consequences45 for a number of physical phenomena such as electron-phonon interactions and ferromagnetism at LAO/STO interfaces. These results, supported by DFT+DMFT electronic structure calculations, are experimentally confirmed using a spectroscopic methodology that exploits an insightful description of the resonance photoexcitation process in terms of orbital and k-selectivity. Our results put forward solid physical grounds for the


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technological use of "defect engineering" through the manipulation of VOs in order to tune physical properties in the whole class of oxide heterostructure systems.

METHODS SAMPLE PREPARATION The 4-u.c. thick LAO films were grown by Pulsed Laser Deposition (PLD) on stoichiometric TiO2-terminated STO(001) substrates. Prior to the growth, the samples were flashed in vacuum at 500 oC. The LAO films were grown at 720o C in O2 pressure of 8×10−5 mbar, which is reduced compared to the standard growth procedure. After deposition, samples were cooled down at the same oxygen pressure and post-annealed ex-situ in atmospheric pressure of O2 at 500 oC. In the low temperature conditions of our SX-ARPES experiment, these samples were gradually developing oxygen deficiency under X-ray irradiation. We note that increasing the sample temperature above ~150 K annihilated the VOs formed by X-ray irradiation, presumably due to thermally-activated diffusion of O atoms from the STO bulk, as evidenced by the quenching of the IGS signal in parallel with the reduction of the 2DES one. The VOs were also annihilated under exposure of the samples to X-rays in O2 pressure of ~10-7 mbar. For the standard LAO/STO samples grown at larger oxygen pressure, the irradiation does not develop any significant concentration of VOs3, indicating that their formation strongly depends on the growth conditions.

ARPES EXPERIMENT SX-ARPES is ideally suited for the investigation of electronic structure of buried interfaces such as LAO/STO due to its enhanced probing depth with respect to conventional VUV ARPES, as well as its element specificity.24 All measurements were performed at the SXARPES endstation47 of the Advanced Resonant Spectroscopies (ADRESS) beamline48 of the 19 ACS Paragon Plus Environment

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Swiss Light Source, Paul Scherrer Institute, Switzerland. The ResPE data were collected using s-polarized incident X-rays, selecting the Ti t2g dxy- and dyz-derived 2DES states,3,24 and the Ti 2p core level data using p-polarized X-rays. The combined energy resolution was of the order of 50 meV. To suppress thermal effects that smear the coherent spectral weight, the sample temperature was kept at 12 K.

THEORETICAL METHODS Our charge self-consistent DFT+DMFT framework49 builds upon the mixed-basis pseudopotential approach for the DFT part and the continuous-time quantum-Monte-Carlo method,50,51 as implemented in the TRIQS package,51-53 for the DMFT impurity problem. We utilize the generalized-gradient approximation in PBE form54 within the Kohn-Sham cycle. The calculations employ a relaxed superlattice consisting of 6 TiO2 layers and 4 AlO2 layers with a (2x2) interlayer resolution (see Fig. 2a). To simulate the oxygen deficiency, a VO is placed in the TiO2 layer right at the interface, i.e. one of eight in-plane oxygen atoms is missing. This amounts to an in-plane VO concentration of 0.125. Local Coulomb interactions with the Hubbard's U = 3.5 eV and Hund's JH = 0.5 eV are used for the three dominant projected-local orbitals on each Ti site, in line with previous studies for the STO surface.12

ASSOCIATED CONTENT

The supporting information is available free of charge on the ACS Publication website. DFT+DMFT calculations for the stoichiometric LAO-STO interface The ResPE map measured for LAO-STO and bare STO samples DFT+DMFT calculations for the LAO-STO interface with VO in the SrO layer just below that interface TiO2 layer


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AUTHOR INFORMATION Corresponding Author * Electronic mail: [email protected] and [email protected] Author Contributions ‡

These authors contributed equally. A.C., M.-A.H., M.C. and V.N.S. performed the

experiment supported by X.W. and T.S. A.C. and M.-A.H. prepared the samples supported by M.R. and C.C. A.C. processed the data. F.L. performed the DFT+DMFT calculations. V.N.S. conceived the research and ResPE model. All authors discussed the results and interpretations, as well as the manuscript written by A.C., V.N.S. and F.L. ACKNOWLEDGMENT We thank C. Laubschat for promoting discussions. A.C. and M.C. acknowledge funding from the Swiss National Science Foundation under the grant No. 200021_165529, F.L. from German Science Foundation (DFG) under grant No. LE 2446/4-1, and M.-A.H. from the Swiss Excellence Scholarship under the grant ESKAS-no.2015.0257. Computations were performed at the JURECA Cluster of the Juelich Supercomputing Centre (JSC) under project No. hhh08.


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