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Orbital Ordering of the Mobile and Localized Electrons at Oxygen-Deficient LaAlO3/SrTiO3 Interfaces ACS Nano 2018.12:7927-7935. Downloaded from pubs.acs.org by EASTERN KENTUCKY UNIV on 01/21/19. For personal use only.

Alla Chikina,*,†,# Frank Lechermann,‡ Marius-Adrian Husanu,†,§ Marco Caputo,† Claudia Cancellieri,†,∥ Xiaoqiang Wang,† Thorsten Schmitt,† Milan Radovic,† and Vladimir N. Strocov*,†,# †

Swiss Light Source, Paul Scherrer Institute, Villigen CH-5232, Switzerland Institut für Theoretische Physik, Universität Hamburg, Jungiusstrasse 9, Hamburg DE-20355, Germany § National Institute of Materials Physics, Atomistilor 405A, Magurele RO-077125, Romania ∥ Empa, Swiss Federal Laboratories for Materials Science & Technology, Ueberlandstrasse 129, Duebendorf CH-8600, Switzerland ‡

S Supporting Information *

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 noncorrelated 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 self-consistent combination of density functional theory and dynamical mean field theory 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 routes for tuning the electronic and magnetic properties of oxide interfaces through “defect engineering” with oxygen vacancies. KEYWORDS: LaAlO3/SrTiO3 interface, oxide interfaces, resonant photoemission, two-dimensional electron gas, oxygen vacancies

T

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 experimental 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 the oxygen-deficient LAO/STO interface. The IGSs have been directly observed by photoemission spectroscopy in bare TiO2-terminated STO,13 STO-based

he 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 stay 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 © 2018 American Chemical Society

Received: March 28, 2018 Accepted: July 11, 2018 Published: July 11, 2018 7927

DOI: 10.1021/acsnano.8b02335 ACS Nano 2018, 12, 7927−7935

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Cite This: ACS Nano 2018, 12, 7927−7935

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ACS Nano heterostructures,14 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 crucial for the physics of transition metal oxide systems. In the octahedral crystal field of the bulk crystal, the 3d orbitals split into 3-fold degenerate t2g and 2-fold degenerate eg orbitals. The interface formation, defects, and many-body interactions such as 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-plane dxz/dyz and light inplane dxy states leads to higher carrier mobility at the 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, hν, used in these experiments provide a probing depth sufficient to penetrate to the buried interface, and resonant photoemission (ResPE) across the Ti 2p core level enables elemental and chemical state selectivity24 consistent with recent studies.7,16,25,34 However, here we develop an advanced model of the ResPE process that enables its interpretation in terms of kconservation, 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.

Figure 1. (a) Development of angle-integrated photoemission intensity at hν = 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 (a) and (d) Ti 2p core level spectrum measured under the X-ray light irradiation are taken at intervals of 5 min under a photon flux of 1.2 × 1013 photons/s focused in a spot of 30 × 75 μm2. The intensities of the Ti3+ component and IGS increase with time and reach saturation after ∼35 min.

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. Simultaneously, the 2DES signal near EF gradually increases with irradiation. Figure 1d shows a time evolution of the Ti 2p spectra under the X-ray irradiation taken at a photon energy of hν = 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 a 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 hν = 462.7 eV is shown in Figure 1b. The image reveals the developed dispersionless IGSs and dispersive dxy/dyz bands of the 2DES forming the dichotomic electron system of the oxygendeficient 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

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 a TiO2terminated 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 Figure 1a displays the oxygen loss process under X-ray irradiation of hν = 462.7 eV (where both 2DES and IGS states 7928


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Figure 2. Theoretical electronic structure of the LAO/STO interface. (a) Relaxed supercell of the LAO/STO interface. The dotted line at 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 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 indicates hybridization of the IGS and 2DES subsystems.

Figure 3. (a) (top curve) XAS data together with (three bottom curves) angle-integrated ResPE intensity from (a) integrated within the VB, IGS, and 2DES energy intervals marked in (a). 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 reflect their different valence and orbital characters. (b) Resonant (angleintegrated) 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−Γ−X direction of the BZ at hν marked in (a). The schematics show the intensity enhancement of the dxy (dyz) states at the t2g (eg) resonances, correspondingly.

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 Figure 2a). Variation of the OV position leads to minor quantitative differences of the spectral features (Supporting Information, note 3). Figure 2b,c present

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 VO-induced IGSs appear as defect-like states, trapping electrons in the partially localized Ti 3d shell, many-body effects are essential for the correct characterization 7929


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Figure 4. (a) 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⟩ wave functions in the CB and ⟨f | ones in the VB are coupled by k-conservation.

the calculated local (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 Supporting Information, note 1). The peaks are in good agreement with their experimental energy position and broadening in Figure 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 line shape. 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 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 a 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 Figure 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 Figure 3a. Even after the saturating irradiation dose, 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 Figure 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 1 or 2 orders 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 interface,3,6,7,38 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 Figure 3b shows the ResPE intensity map as a function of (EB, hν) 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 Figure 3a in comparison with the XAS spectrum. We note that analogous measurements on the bare STO surface (provided in the Supporting Information, note 2) show a notably different ResPE intensity behavior, where the IGS and 2DES resonate in more extended hν-regions compared to the LAO/STO and the IGS peak is shifted to higher EB. The ResPE data in Figure 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 7930


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ACS Nano resonances at hν = 459 and 464.3 eV coincide with energies of the unoccupied Ti3+ eg states in XAS. The t2g-derived 2DES resonates at higher hν 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 signifies the different nature of these states forming the dichotomic electron system of the LAO/ STO interface. The ResPE phenomenon, sketched in Figure 4a, is a secondorder process that can be described by the Kramers− Heisenberg formula:36,37

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 Figure 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 VOinduced perturbation of the STO crystal field. 2DES 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 ⟨f| states involved in the ResPE process, we find that at both the L3 and L2 edges the t2g-derived 2DES shows its strongest resonance between the Ti3+ and Ti4+ eg unoccupied states and only weaker resonant enhancement between the Ti3+ and Ti4+ t2g ones. We connect this peculiarity with the relaxed kconservation between the dispersive |m⟩ and ⟨f| states involved in the ResPE process. This idea is conveyed in Figure 4, which sketches the 2DES dispersions over the partially occupied t2g and unoccupied eg states (excluding the IGS). Where do we find the |m⟩ states connected with the ⟨f| ones by the momentum conservation km = kf ? First, we will analyze the ResPE response of the dxz/dyz states. Their manifold6,30 is formed in the LAO/STO interfacial quantum well (QW) due to confinement42,43 of the dxz/dyz-derived Bloch waves from the STO bulk. The partially occupied sector of this manifold is formed by the n = 1 QW state, which confines the Bloch wave with the out-ofplane momentum kz ≈ 0. The unoccupied part of the manifold (not shown in Figure 4 for brevity) is formed, in turn, by the higher-n QW states originating from the Bloch waves with larger kz. Therefore, conservation of the full three-dimensional k does not allow coupling of the occupied dxz/dyz states to the unoccupied ones in the |m⟩ state of the ResPE process. The next available unoccupied states will rather be the eg states located at higher energy, as sketched in Figure 4b. Indeed, our ResPE data at both L3 and L2 edges show the dxz/dyz resonant enhancement in the hν-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 k-conservation 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 layer,44 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. This picture of the orbital and k-selectivity of the ResPE process is confirmed by the sequence of ARPES images in Figure 3b that were acquired along the Γ−X direction of the BZ at several hν 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

2

Iα 2π ∑ ⟨f |TD|i⟩ + f

∑m

⟨f |TA|m⟩⟨m|TD|i⟩ Ei − Em +

i Γm 2

δ(Ef − Ei)

where the first term describes the direct photoemission process, in which the final state ⟨f|, containing a hole in the VB and an outgoing photoelectron, is coupled to the initial state |i⟩ (containing an incoming photon) through the dipole operator TD. The second term describes a core-hole-assisted photoemission process, where the same initial and final states are coupled through a summation over all the possible intermediate states |m⟩ with energy Em and lifetime Γm. 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 ⟨f | state is governed by the Auger operator TA. The corehole-assisted process can have 1 or 2 orders of magnitude higher cross section with respect to the direct photoemission term. Note that the identity of the direct and resonance photoemission final states gives rise to interference effects in the overall intensity, although usually the prevalence of the Auger term makes these effects less important. The above formula for the ResPE process shows that the largest overlap of the |m⟩ wave functions with the ⟨f | ones in the Auger termand thus the strongest ResPE responsecan in general be expected when the intermediate states with the electron in the conduction band (CB) and the final states have their wave functions not only localized at the same atom but also sharing similarity in terms of their orbital and k-character, resulting in the orbital and k-selectivity of ResPE process. We note that delocalization of the electron system relaxes the orbital selectivity compared to the atomic picture, because the summation over the |m⟩ states runs through a continuum of the off-symmetry km-points where their strict orthogonality to the ⟨f | states breaks down. The k-conservation in the ResPE process is analogous to the widely discussed k-conservation in resonant inelastic X-ray scattering (RIXS) from wide-band electron systems.40,41 With the ResPE and RIXS processes described within the same Kramers−Heisenberg formalism, their k-selectivity is relaxed due to the finite lifetime of the |m⟩ states. Now we will demonstrate how the orbital and kselectivity 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 Figure 3a,b, their photoemission response resonates when hν 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 7931


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SUMMARY In summary, we can draw the following physical picture of orbital ordering at the LAO/STO interface, schematically shown in Figure 5. In addition to the t2g < eg orbital ordering

response prevails over the eg region. The observation is consistent with our theoretical results in Figure 2, which show the out-of-plane dz2 character of these eg states, which, due to off-symmetry km-points, have large overlap with the out-ofplane dxz/dyz states. We note that the interface-induced 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 kconservation 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 2DES and IGS Subsystems. Upon closer inspection of the ResPE data at the L3 edge, in Figure 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 spectral weight in Figure 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 Figure 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 Supporting 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 oxygendeficient 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 t 2g−e g 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, a mechanism alternative to phase separation.22

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 egderived IGS below the t2g-derived 2DES. This atomic scheme (excluding the orbital hybridization effects) is general for all oxygen-deficient STO-based interfaces.

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 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 a vacuum at 500 °C. The LAO films were grown at 720 °C in an O2 pressure of 8 × 10−5 mbar, which is reduced compared to the standard growth procedure. After deposition, samples were cooled at the same oxygen pressure and postannealed ex situ in atmospheric pressure of O2 at 500 °C. 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 Xray 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 an 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 VOs,3 indicating that their formation strongly depends on the growth conditions. ARPES Experiment. SX-ARPES is ideally suited for the investigation of the electronic structure of buried interfaces such as LAO/STO due to its enhanced probing depth with respect to 7932


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ACS Nano conventional VUV ARPES, as well as its element specificity.24 All measurements were performed at the SX-ARPES endstation46 of the Advanced Resonant Spectroscopies (ADRESS) beamline47 of the 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 on 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 framework48 builds upon the mixed-basis pseudopotential approach for the DFT part and the continuous-time quantum-Monte Carlo method,49,50 as implemented in the TRIQS package,50−52 for the DMFT impurity problem. We utilize the generalized-gradient approximation in PBE form53 within the Kohn−Sham cycle. The calculations employ a relaxed superlattice consisting of six TiO2 layers and four AlO2 layers with a (2 × 2) interlayer resolution (see Figure 2a). To simulate the oxygen deficiency, a VO is placed in the TiO2 layer right at the interface; that is, one of eight in-plane oxygen atoms is missing. This amounts to an in-plane V O 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 on the STO surface.12

formed at the JURECA Cluster of the Juelich Supercomputing Centre (JSC) under project no. hhh08.

REFERENCES (1) Ohtomo, A.; Hwang, H. Y. A High-Mobility Electron Gas at the LaAlO3/SrTiO3 Heterointerface. Nature 2004, 427, 423−426. (2) Hwang, H. Y.; Iwasa, Y.; Kawasaki, M.; Keimer, B.; Nagaosa, N.; Tokura, Y. Emergent Phenomena at Oxide Interfaces. Nat. Mater. 2012, 11, 103−113. (3) Cancellieri, C.; Mishchenko, A. S.; Aschauer, U.; Filippetti, A.; Faber, C.; Barišić, O. S.; Rogalev, V. A.; Schmitt, T.; Nagaosa, N.; Strocov, V. N. Polaronic Metal State at the LaAlO3/SrTiO3 Interface. Nat. Commun. 2016, 7, 10386. (4) Wang, Z.; McKeown Walker, S.; Tamai, A.; Wang, Y.; Ristic, Z.; Bruno, F. Y.; de la Torre, A.; Riccò, S.; Plumb, N. C.; Shi, M.; Hlawenka, P.; Sánchez-Barriga, J.; Varykhalov, A.; Kim, T. K.; Hoesch, M.; King, P. D. C.; Meevasana, W.; Diebold, U.; Mesot, J.; Moritz, B.; et al. Tailoring the Nature and Strength of ElectronPhonon Interactions in the SrTiO3(001) 2D Electron Liquid. Nat. Mater. 2016, 15, 835−839. (5) Chen, C.; Avila, J.; Frantzeskakis, E.; Levy, A.; Asensio, M. C. Observation of a Two-Dimensional Liquid of Fröhlich Polarons at the Bare SrTiO3 Surface. Nat. Commun. 2015, 6, 8585. (6) Cancellieri, C.; Reinle-Schmitt, M. L.; Kobayashi, M.; Strocov, V. N.; Willmott, P. R.; Fontaine, D.; Ghosez, P.; Filippetti, A.; Delugas, P.; Fiorentini, V. Doping-Dependent Band Structure of LaAlO3/ SrTiO3 Interfaces by Soft X-Ray Polarization-Controlled Resonant Angle-Resolved Photoemission. Phys. Rev. B: Condens. Matter Mater. Phys. 2014, 89, 121412. (7) Berner, G.; Sing, M.; Fujiwara, H.; Yasui, A.; Saitoh, Y.; Yamasaki, A.; Nishitani, Y.; Sekiyama, A.; Pavlenko, N.; Kopp, T.; Richter, C.; Mannhart, J.; Suga, S.; Claessen, R. Direct K-Space Mapping of the Electronic Structure in an Oxide-Oxide Interface. Phys. Rev. Lett. 2013, 110, 247601. (8) Zhou, K.-J.; Radovic, M.; Schlappa, J.; Strocov, V.; Frison, R.; Mesot, J.; Patthey, L.; Schmitt, T. Localized and Delocalized Ti 3d Carriers in LaAlO3/SrTiO3 Superlattices Revealed by Resonant Inelastic X-Ray Scattering. Phys. Rev. B: Condens. Matter Mater. Phys. 2011, 83, 201402. (9) Hao, X.; Wang, Z.; Schmid, M.; Diebold, U.; Franchini, C. Coexistence of Trapped and Free Excess Electrons in SrTiO3. Phys. Rev. B: Condens. Matter Mater. Phys. 2015, 91, 085204. (10) Behrmann, M.; Lechermann, F. Interface Exchange Processes in LaAlO3/SrTiO3 Induced by Oxygen Vacancies. Phys. Rev. B: Condens. Matter Mater. Phys. 2015, 92, 125148. (11) Pavlenko, N.; Kopp, T.; Tsymbal, E. Y.; Sawatzky, G. A.; Mannhart, J. Magnetic and Superconducting Phases at the LaAlO3/ SrTiO3 interface: The Role of Interfacial Ti 3d Electrons. Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 85, 020407. (12) Lechermann, F.; Jeschke, H. O.; Kim, A. J.; Backes, S.; Valentí, R. Electron Dichotomy on the SrTiO3 Defect Surface Augmented by Many-Body Effects. Phys. Rev. B: Condens. Matter Mater. Phys. 2016, 93, 121103. (13) Dudy, L.; Sing, M.; Scheiderer, P.; Denlinger, J. D.; Schütz, P.; Gabel, J.; Buchwald, M.; Schlueter, C.; Lee, T.-L.; Claessen, R. In Situ Control of Separate Electronic Phases on SrTiO3 Surfaces by Oxygen Dosing. Adv. Mater. 2016, 28, 7443−7449. (14) Schütz, P.; Christensen, D. V.; Borisov, V.; Pfaff, F.; Scheiderer, P.; Dudy, L.; Zapf, M.; Gabel, J.; Chen, Y. Z.; Pryds, N.; Rogalev, A.; Strocov, V. N.; Schlueter, C.; Lee, T.-L.; Jeschke, H. O.; Valentí, R.; Sing, M.; Claessen, R. Microscopic Origin of the Mobility Enhancement at a Spinel/Perovskite Oxide Heterointerface Revealed by Photoemission Spectroscopy. Phys. Rev. B: Condens. Matter Mater. Phys. 2017, 96, 161409. (15) Backes, S.; Rödel, T. C.; Fortuna, F.; Frantzeskakis, E.; Le Fèvre, P.; Bertran, F.; Kobayashi, M.; Yukawa, R.; Mitsuhashi, T.; Kitamura, M.; Horiba, K.; Kumigashira, H.; Saint-Martin, R.; Fouchet, A.; Berini, B.; Dumont, Y.; Kim, A. J.; Lechermann, F.; Jeschke, H. O.; Rozenberg, M. J.; et al. Hubbard Band versus Oxygen Vacancy States

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.8b02335. DFT+DMFT calculations for the stoichiometric LAO− STO interface; 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 the interface TiO2 layer (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Alla Chikina: 0000-0003-3635-6503 Claudia Cancellieri: 0000-0003-4124-4362 Author Contributions

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, and the manuscript was written by A.C., V.N.S., and F.L. Author Contributions #

A. Chikina and V. N. Strocov contributed equally.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS We thank C. Laubschat for promoting discussions. A.C. and M.C. acknowledge funding from the Swiss National Science Foundation under 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 grant ESKAS-no.2015.0257. Computations were per7933


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