上海交通大学：《探索物质微观世界》课程教学资源（文献资料）obital reconstruction and covalent bonding at an oxide interface

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RESEARCH ARTICLES Orbital Reconstruction and Covalent Bonding at an Oxide Interface J.Chakhalian,12*]W.Freeland,3 H.-U.Habermeier,2 G.Cristiani,2 G.Khaliullin,2 M.van Veenendaal,3.4 B.Keimer2 AUTHORS'SUMMARY -odem microelectronics relies to a large degree on the properties of and virtually all theories ofhigh-temperature superconductivity are now based on interfaces between two different semiconductors.For example,a this orbital pattem.Our data indicate that electrons at the interface occupy a transistor is commonly formed from the electronic interactions at combination of orbitals that differs drastically from that of the bulk.With the aid such an interface,and its electrical conductivity can be controlled very ef- of numerical calculations on small atomic clusters,we trace this"orbital recon- fectively by application of an external voltage to its gate electrode.Optimizing struction"at the interface to the formation of strong covalent bonds between the operation oftransistors and other microelectronic devices requires knowledge copper and manganese atoms across the interface. of the electronic states near semiconductor interfaces,and many details 1000 have been revealed over the past half-century.Key to recent developments has been the ability to create high-quality atomically abrupt interfaces between different materials,including complex oxides with intricate or large unit cells.In bulk,these oxides show rich and varied electronic and magnetic properties produced by strong interactions among the electrons (1).Combining these materials along an interface can produce new quantum states and the opportunity to uncover unexpected phenomena. We outline steps toward visualizing and resolving the detailed behavior of electrons in specific orbitals at the interface between two complex mate- rials.We studied two materials exhibiting properties not known in ordinary semiconductors:a ferromagnetic manganese oxide and a superconducting copper oxide.The ability to manipulate electrons and their behavior at interfaces may open a path toward a new generation of electronic devices. The first step along this path is the preparation of chemically pure, atomically sharp interfaces.A sharp interface is needed both to accurately study and to constrain electronic interactions to the interface.We used pulsed laser deposition,in which a laser vaporizes bullk samples in a vacuum containing some background oxygen.A series of thin,uniform oxide layers is sequentially deposited on a single crystal forming a superlattice with sharp interfaces.Study of similar structures during the past few years has led to demonstrations of high-mobility electron systems (2) and transistor effects (3)at oxide interfaces. Understanding the electronic mechanisms responsible for these effects requires the ability to study the behavior of electrons at the interfaces, which are typically buried several nanometers below the surface,without interference from bulk electrons in each layer of the superlattice.To meet this goal,we rely on the properties of x-rays with tunable energy and polarization emitted from a synchrotron,which penetrate deeply into most Schematic showing the interface between two metal oxide compounds being materials.The x-ray photon energy was tuned to zoom in on copper and illuminated by x-rays from a synchrotron,yielding detailed information about the manganese atoms right at the interface,and the absorption of x-rays with shape,and thus occupation and degeneracy,of electronic orbitals near the interface. polarization parallel and perpendicular to the interface was used to extract information about the shape of the valence-electron clouds (ie.,"orbitals") Being able to determine these characteristics known only in the interface around these atoms(see the figure). between metal oxides should allow synthesis of materials in which the bonding In analogy to atoms in free space,the electrons in metal oxides have the across oxide interfaces can be manipulated in a predictable fashion.These choice of several types of energetically nearly equivalent (or degenerate) methods then will offer a tremendous opportunity to create dense two- electronic orbitals.The specific way in which this freedom is broken strongly dimensional electron systems with controlled interactions.It is conceivable that influences the interactions between the electrons and hence the physical such a system will exhibit properties qualitatively beyond those attainable in properties of bulk transition metal oxides.For instance,altemating occupation semiconductor heterostructures and that engineers will be able to exploit these of different orbitals on neighboring lattice sites favors feromagnetism,whereas properties in innovative electronic devices. unifomm occupation of orbitals on all lattice sites tends to generate antiferro- magnetism().Early investigations with x-rays had established that the arrange- Summary References 1.Y.Tokura,N.Nagaosa,Science 288,462 (2000). ment of valence-electron orbitals on copper atoms in bulk copper-oxide 2.A.Ohtomo,H.Y.Hwang,Nature 427,423 (2004). superconductors is particularly robust,so that there is essentially no degeneracy. 3.S.Thiel et al.,Science 313,1942 (2006). 1114 16 NOVEMBER 2007 VOL 318 SCIENCE www.sciencemag.org

Orbital Reconstruction and Covalent Bonding at an Oxide Interface J. Chakhalian,1,2* J. W. Freeland,3 H.-U. Habermeier,2 G. Cristiani,2 G. Khaliullin,2 M. van Veenendaal,3,4 B. Keimer2 AUTHORS’ SUMMARY Modern microelectronics relies to a large degree on the properties of interfaces between two different semiconductors. For example, a transistor is commonly formed from the electronic interactions at such an interface, and its electrical conductivity can be controlled very effectively by application of an external voltage to its gate electrode. Optimizing the operation of transistors and other microelectronic devices requires knowledge of the electronic states near semiconductor interfaces, and many details have been revealed over the past half-century. Key to recent developments has been the ability to create high-quality atomically abrupt interfaces between different materials, including complex oxides with intricate or large unit cells. In bulk, these oxides show rich and varied electronic and magnetic properties produced by strong interactions among the electrons (1). Combining these materials along an interface can produce new quantum states and the opportunity to uncover unexpected phenomena. We outline steps toward visualizing and resolving the detailed behavior of electrons in specific orbitals at the interface between two complex materials.We studied two materials exhibiting properties not known in ordinary semiconductors: a ferromagnetic manganese oxide and a superconducting copper oxide. The ability to manipulate electrons and their behavior at interfaces may open a path toward a new generation of electronic devices. The first step along this path is the preparation of chemically pure, atomically sharp interfaces. A sharp interface is needed both to accurately study and to constrain electronic interactions to the interface. We used pulsed laser deposition, in which a laser vaporizes bulk samples in a vacuum containing some background oxygen. A series of thin, uniform oxide layers is sequentially deposited on a single crystal forming a superlattice with sharp interfaces. Study of similar structures during the past few years has led to demonstrations of high-mobility electron systems (2) and transistor effects (3) at oxide interfaces. Understanding the electronic mechanisms responsible for these effects requires the ability to study the behavior of electrons at the interfaces, which are typically buried several nanometers below the surface, without interference from bulk electrons in each layer of the superlattice. To meet this goal, we rely on the properties of x-rays with tunable energy and polarization emitted from a synchrotron, which penetrate deeply into most materials. The x-ray photon energy was tuned to zoom in on copper and manganese atoms right at the interface, and the absorption of x-rays with polarization parallel and perpendicular to the interface was used to extract information about the shape of the valence-electron clouds (i.e., “orbitals”) around these atoms (see the figure). In analogy to atoms in free space, the electrons in metal oxides have the choice of several types of energetically nearly equivalent (or degenerate) electronic orbitals. The specific way in which this freedom is broken strongly influences the interactions between the electrons and hence the physical properties of bulk transition metal oxides. For instance, alternating occupation of different orbitals on neighboring lattice sites favors ferromagnetism, whereas uniform occupation of orbitals on all lattice sites tends to generate antiferromagnetism (1). Early investigations with x-rays had established that the arrangement of valence-electron orbitals on copper atoms in bulk copper-oxide superconductors is particularly robust, so that there is essentially no degeneracy, and virtually all theories of high-temperature superconductivity are now based on this orbital pattern. Our data indicate that electrons at the interface occupy a combination of orbitals that differs drastically from that of the bulk. With the aid of numerical calculations on small atomic clusters, we trace this “orbital reconstruction” at the interface to the formation of strong covalent bonds between copper and manganese atoms across the interface. Being able to determine these characteristics known only in the interface between metal oxides should allow synthesis of materials in which the bonding across oxide interfaces can be manipulated in a predictable fashion. These methods then will offer a tremendous opportunity to create dense twodimensional electron systems with controlled interactions. It is conceivable that such a system will exhibit properties qualitatively beyond those attainable in semiconductor heterostructures and that engineers will be able to exploit these properties in innovative electronic devices. Summary References 1. Y. Tokura, N. Nagaosa, Science 288, 462 (2000). 2. A. Ohtomo, H. Y. Hwang, Nature 427, 423 (2004). 3. S. Thiel et al., Science 313, 1942 (2006). Schematic showing the interface between two metal oxide compounds being illuminated by x-rays from a synchrotron, yielding detailed information about the shape, and thus occupation and degeneracy, of electronic orbitals near the interface. 1114 16 NOVEMBER 2007 VOL 318 SCIENCE www.sciencemag.org CREDIT : XXXXXXX RESEARCH ARTICLES on November 26, 2007 www.sciencemag.org Downloaded from

RESEARCH ARTICLES terface sensitivity is further enhanced with the FULL-LENGTH ARTICLE use of a low angle of incidence for the x-ray beam (11.2).The converse procedure was used Orbital reconstructions and covalent bonding must be considered as important factors in the to probe the electronic structure of MnO2 layers rational design of oxide heterostructures with engineered physical properties.We have on the LCMO side of the interface.Control ex- investigated the interface between high-temperature superconducting(Y,Ca)BazCusOz and periments in the bulk-sensitive fluorescence- metallic Lao.67Cao.33MnO3 by resonant x-ray spectroscopy.A charge of about-0.2 electron is yield (FY)mode were simultaneously carried transferred from Mn to Cu ions across the interface and induces a major reconstruction of the out in both cases. orbital occupation and orbital symmetry in the interfacial CuO2 layers.In particular,the Cu d3 Spectroscopy at the interface.Figure 2 orbital,which is fully occupied and electronically inactive in the bulk,is partially occupied at shows normalized absorption spectra near the the interface.Supported by exact-diagonalization calculations,these data indicate the Cu L3 edge in bulk-and interface-sensitive formation of a strong chemical bond between Cu and Mn atoms across the interface.Orbital modes.The bulk-sensitive FY data are in ex- reconstructions and associated covalent bonding are thus important factors in determining the cellent agreement with previous XAS data at the physical properties of oxide heterostructures. Cu L edge of nearly optimally doped YBCO (/6).The main narrow absorption peak around n semiconductor heterostructures,high- Probing heterostructure interfaces.The ex- 931 eV corresponds to the intra-ionic transition mobility electron systems with tunable density periments were performed at the 4-ID-C beam- 2p3d°→2p3do.The shoulder on the right- Lhave led to prominent advances in science line at the Advanced Photon Source on epitaxial hand side of the peak is attributed to the intersite and technology over the past decades.Such sys- trilayers and superlattices of the high-temperature transition 2p3d2p3dL,where L denotes 复 tems have recently been replicated in hetero- superconductor (Y.Ca)Ba CuO7 (YBCO)in a hole on the oxygen ligand.The line shape of structures of complex transition metal oxides (/) c-axis orientation,combined with ferromagnetic the main absorption peak is a signature of the 恕 leading to the observation of transistor effects(2) metallic LaCaxMnO3 (LCMO)at a doping "Zhang-Rice singlet (/7),a bound state of and the quantum Hall effect (3).Because transi- level x=/3.The quality of these multilayer charge carriers on oxygen and copper sites that tion metal oxides exhibit a notably rich phase be- structures was checked by a variety of character- keeps the Cu plane site in the nominal valence havior in the bulk (4).these developments have ization methods (/5).In order to discriminate the state 2+as the hole density in the CuO,sheets is raised expectations that quantum states with electronic structure at the interface from surface tuned by doping.The polarization dependence of properties and functionalities qualitatively be- and bulk contributions,we have performed a sys- the FY signal also contains important informa- yond those attainable in semiconductors can be tematic series of experiments on heterostructures tion about the electronic structure near the Fermi generated at oxide interfaces. with different capping layers,taking advantage of level of YBCO.In particular,the absorption for The large variety of phases (often with radi- the element specificity and shallow probing depth photon polarization parallel to the CuO2 sheets U cally different physical properties)in transition of resonant XAS and XLD in the total electron greatly exceeds that for polarization along the c metal oxides is due to the delicate sensitivity of yield (TEY)mode (Fig.1A).For instance,the axis.This implies that holes in the conduction the charge transfer and magnetic interaction be- occupation of Cu d orbitals on the YBCO side of band of YBCO predominantly occupy the planar tween metal ions to the occupation of d orbitals the interface was studied on heterostructures with Cud orbital,which hybridizes strongly with (5).Which linear combination of the five possi- LCMO capping layers,so that no surface Cu is oxygen p orbitals in the CuO2 layers.Similar ble d orbitals is occupied on a given transition present.If the photon energy is tuned to the Cu L observations have been made in all other high- MM metal site depends,in tum,on parameters such as absorption edge,the capping layer does not in- temperature superconductors investigated thus far. electron density,ligand positions,magnetic order, fluence the detected signal apart from an overall and together they have become one of the basic and chemical bonding,which are generally dif- attenuation factor.As a result of the low electron tenets of our current understanding of this class of ferent at the interface than in the bulk.Despite its escape depth(a few nanometers),the TEY signal materials (16.18). pep pivotal role in determining the phase behavior is dominated by the CuOz layers immediately Evidence for orbital reconstruction and charge and physical properties of oxides,almost no ex- adjacent to the first interface:contributions from transfer.The interface-sensitive data shown in perimental information is available about the deeper layers are exponentially reduced.The in- Fig.2 are very different.One first notices that occupation of orbitals at oxide interfaces,and theoretical work (6-//)has thus far hardly Fig.1.(A)Schematic of 9 interface cluster addressed this issue.We report the results of soft the experimental setup Mn edge x-ray absorption spectroscopy (XAS)and soft used to obtain the XAS x-ray linear dichroism (XLD)experiments on and XLD data in TEY and h heterostructures of copper and manganese oxides FY modes.Data sensitive tailored to probe the electronic structure and or- to interfacial Cu (Mn) 0(2) bital occupation at the interface.The cuprate- atoms were taken in YBCO cap layer manganate interface is well suited as a model TEY mode with photon ◆H system for this purpose,because nearly strain-free, energies near the Cu atomically sharp heterostructures can be syn- (Mn)L absorption edge thesized (/2-14)and because the electronic on samples with LCMO properties of both materials have been studied (YBCO)capping layers. Cu edge To obtain a sizable di- extensively in the bulk chroism,we tilted the film plane with respect University of Arkansas,Fayetteville,AR 72701,USA.Max to the photon beam LCMO cap layer Planck Institute for Solid State Research,D-70569 Stuttgart,Germany.Advanced Photon Source,Argonne propagation direction.C National Laboratory,Argonne,IL 60439,USADepart- indicates the c-axis of ment of Physics,Northern Illinois University,Dekalb,IL the film;H is the applied 60115,U5A magnetic field;h and v denote the linear polarization state of the incident x-ray.(B)Atomic positions near *To whom correspondence should be addressed.E-mail: the LCMO-YBCO interface(14,29).The MnCuO cluster used for the exact-diagonalization calculations is jchakhal@uark.edu highlighted. www.sciencemag.org SCIENCE VOL 318 16 NOVEMBER 2007 1115

FULL-LENGTH ARTICLE Orbital reconstructions and covalent bonding must be considered as important factors in the rational design of oxide heterostructures with engineered physical properties. We have investigated the interface between high-temperature superconducting (Y,Ca)Ba2Cu3O7 and metallic La0.67Ca0.33MnO3 by resonant x-ray spectroscopy. A charge of about –0.2 electron is transferred from Mn to Cu ions across the interface and induces a major reconstruction of the orbital occupation and orbital symmetry in the interfacial CuO2 layers. In particular, the Cu d3z2−r2 orbital, which is fully occupied and electronically inactive in the bulk, is partially occupied at the interface. Supported by exact-diagonalization calculations, these data indicate the formation of a strong chemical bond between Cu and Mn atoms across the interface. Orbital reconstructions and associated covalent bonding are thus important factors in determining the physical properties of oxide heterostructures. I n semiconductor heterostructures, highmobility electron systems with tunable density have led to prominent advances in science and technology over the past decades. Such systems have recently been replicated in heterostructures of complex transition metal oxides (1), leading to the observation of transistor effects (2) and the quantum Hall effect (3). Because transition metal oxides exhibit a notably rich phase behavior in the bulk (4), these developments have raised expectations that quantum states with properties and functionalities qualitatively beyond those attainable in semiconductors can be generated at oxide interfaces. The large variety of phases (often with radically different physical properties) in transition metal oxides is due to the delicate sensitivity of the charge transfer and magnetic interaction between metal ions to the occupation of d orbitals (5). Which linear combination of the five possible d orbitals is occupied on a given transition metal site depends, in turn, on parameters such as electron density, ligand positions, magnetic order, and chemical bonding, which are generally different at the interface than in the bulk. Despite its pivotal role in determining the phase behavior and physical properties of oxides, almost no experimental information is available about the occupation of orbitals at oxide interfaces, and theoretical work (6–11) has thus far hardly addressed this issue. We report the results of soft x-ray absorption spectroscopy (XAS) and soft x-ray linear dichroism (XLD) experiments on heterostructures of copper and manganese oxides tailored to probe the electronic structure and orbital occupation at the interface. The cupratemanganate interface is well suited as a model system for this purpose, because nearly strain-free, atomically sharp heterostructures can be synthesized (12–14) and because the electronic properties of both materials have been studied extensively in the bulk. Probing heterostructure interfaces. The experiments were performed at the 4-ID-C beamline at the Advanced Photon Source on epitaxial trilayers and superlattices of the high-temperature superconductor (Y,Ca)Ba2Cu3O7 (YBCO) in c-axis orientation, combined with ferromagnetic metallic La1−xCaxMnO3 (LCMO) at a doping level x = 1 =3. The quality of these multilayer structures was checked by a variety of characterization methods (15). In order to discriminate the electronic structure at the interface from surface and bulk contributions, we have performed a systematic series of experiments on heterostructures with different capping layers, taking advantage of the element specificity and shallow probing depth of resonant XAS and XLD in the total electron yield (TEY) mode (Fig. 1A). For instance, the occupation of Cu d orbitals on the YBCO side of the interface was studied on heterostructures with LCMO capping layers, so that no surface Cu is present. If the photon energy is tuned to the Cu L absorption edge, the capping layer does not influence the detected signal apart from an overall attenuation factor. As a result of the low electron escape depth (a few nanometers), the TEY signal is dominated by the CuO2 layers immediately adjacent to the first interface; contributions from deeper layers are exponentially reduced. The interface sensitivity is further enhanced with the use of a low angle of incidence for the x-ray beam (11.2°). The converse procedure was used to probe the electronic structure of MnO2 layers on the LCMO side of the interface. Control experiments in the bulk-sensitive fluorescenceyield (FY) mode were simultaneously carried out in both cases. Spectroscopy at the interface. Figure 2 shows normalized absorption spectra near the Cu L3 edge in bulk- and interface-sensitive modes. The bulk-sensitive FY data are in excellent agreement with previous XAS data at the Cu L edge of nearly optimally doped YBCO (16). The main narrow absorption peak around 931 eV corresponds to the intra-ionic transition 2p6 3d9 → 2p5 3d10. The shoulder on the righthand side of the peak is attributed to the intersite transition 2p6 3d9 L→ 2p5 3d10L, where L denotes a hole on the oxygen ligand. The line shape of the main absorption peak is a signature of the “Zhang-Rice singlet” (17), a bound state of charge carriers on oxygen and copper sites that keeps the Cu plane site in the nominal valence state 2+ as the hole density in the CuO2 sheets is tuned by doping. The polarization dependence of the FY signal also contains important information about the electronic structure near the Fermi level of YBCO. In particular, the absorption for photon polarization parallel to the CuO2 sheets greatly exceeds that for polarization along the c axis. This implies that holes in the conduction band of YBCO predominantly occupy the planar Cu dx2−y2 orbital, which hybridizes strongly with oxygen p orbitals in the CuO2 layers. Similar observations have been made in all other hightemperature superconductors investigated thus far, and together they have become one of the basic tenets of our current understanding of this class of materials (16, 18). Evidence for orbital reconstruction and charge transfer. The interface-sensitive data shown in Fig. 2 are very different. One first notices that 1 University of Arkansas, Fayetteville, AR 72701, USA. 2 Max Planck Institute for Solid State Research, D-70569 Stuttgart, Germany. 3 Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439, USA. 4 Department of Physics, Northern Illinois University, Dekalb, IL 60115, USA. *To whom correspondence should be addressed. E-mail: jchakhal@uark.edu Fig. 1. (A) Schematic of the experimental setup used to obtain the XAS and XLD data in TEY and FY modes. Data sensitive to interfacial Cu (Mn) atoms were taken in TEY mode with photon energies near the Cu (Mn) L absorption edge, on samples with LCMO (YBCO) capping layers. To obtain a sizable dichroism, we tilted the film plane with respect to the photon beam propagation direction. C indicates the c-axis of the film; H is the applied magnetic field; h and v denote the linear polarization state of the incident x-ray. (B) Atomic positions near the LCMO-YBCO interface (14, 29). The MnCuO10 cluster used for the exact-diagonalization calculations is highlighted. www.sciencemag.org SCIENCE VOL 318 16 NOVEMBER 2007 1115 RESEARCH ARTICLES on November 26, 2007 www.sciencemag.org Downloaded from

RESEARCH ARTICLES the interfacial absorption peak is shifted to In order to uncover the origin of the un- corresponding data in the literature.The spectra lower energy with respect to the bulk by ~0.4 eV expected shift of the absorption peak and to are much broader than those taken near the Cu L and that the high-energy shoulder is no longer obtain further information about the electronic edge,because all of the five Mn d orbitals are present.The shift of the peak is evidence of a states at the interface,we have varied the photon partially occupied,giving rise to a complicated change in valence state of Cu ions near the inter- polarization in the interface-sensitive detection multiplet splitting of the absorption peak.The face.This indicates that charge is transferred mode (Fig.2).In marked contrast to the bulk- peak intensity is independent of photon polariza- across the interface and that a charged double sensitive data,the strengths of the absorption tion within the experimental error.This finding layer is formed,as generally expected for hetero- signals for polarization perpendicular and parallel has been taken as evidence of an orbitally dis- structures of materials with different work func- to the layers are almost equal.This is a mani- ordered state with equal occupation of Mnd tions.In agreement with specific predictions for festation of an"orbital reconstruction."Whereas andd3 orbitals in bulk metallic LCMO.In the system at hand (11),the charge-transfer di- the holes are constrained to the Cudorbital the interface-sensitive detection mode.neither the rection is such that the hole density in YBCO is in the bulk,at least some of them occupy the peak position nor its polarization dependence are reduced at the interface.Because of the strong d3 orbitals at the interface.The distribution of noticeably different from the bulk data.This does influence of the core hole created by absorbing holes over the two Cu orbitals cannot be precisely not imply,however,that the Mn ions maintain the photon,the relationship between the x-ray determined,because the XLD experiment probes their bulk charge density and electronic structure absorption edge and the Cu valence is not not only the CuO2 layer directly at the interface at the interface.Indeed,as a result of charge straightforward,but a comparison to XAS spectra but also the deeper layers (albeit with exponen- conservation,one generally expects a shift in Mn of reference materials containing Cu+and Cu2* tially reduced sensitivity).However,the nearly valence matching that of the interfacial Cu ions ions [for a review,see (19)]yields a rough esti- isotropic cross section shown in Fig.2 implies (Fig.2),but because of the strong multiplet 复 mate of 0.2e (where e in the charge on the elec- that the hole content of the Cud32-2 orbital is at broadening of the Mn peak,such a shift is much tron)per copper ion for the charge-transfer least equal to that of the dorbital.We re- harder to recognize than in the case of Cu (20). 9 amplitude.At first sight,this seems to correspond peated the measurement at several temperatures Based on the data of Fig.3,one can set an upper with the line shape of the interfacial absorption (from 300 to 30 K)and confirmed that the peak bound of 0.4 eV on the difference between the peak,which bears a strong resemblance to XAS position and polarization dependence do not de- positions of Mn L absorption edges in bulk-and data in undoped YBCO (/6).Notably.however. pend on temperature.Similar observations were interface-sensitive detection modes.Because a numerous XAS experiments on YBCO and other also made on heterostructures in which the valence change from Mn to Mn results in a bulk hole-doped high-temperature superconduc- doping level of YBCO was raised into the over- shift of the L edge of~1.5 eV,this translates into tors have shown that the position of the Cu doped regime by Ca substitution.The orbital re- an upper bound of~0.3e per Mn atom on the L-absorption peak is independent of doping.This construction and the charge transfer are hence amplitude of the charge transfer across the has been attributed to the Zhang-Rice singlet state general.robust characteristics of the YBCO- interface.which is consistent with the estimated U and,consequently,the doped holes have predom- LCMO interface. amplitude of~0.2e based on the Cu XAS spectra inantly oxygen character (/7).The observed Before discussing possible mechanisms and discussed above.Likewise.because the polariza- shift of the La absorption peak in our interface- potential implications of the orbital reconstruc- tion dependence of the intensity at the Mn L edge sensitive experiment thus cannot be attributed to a tion,we briefly discuss XAS spectra near the Mn is influenced to a large extent by the completely readjustment of the hole density alone and indicates L2 and L3 absorption edges taken in bulk-and unoccupied minority t2 and eg orbitals,it is dif- an extreme modification of the electronic structure interface-sensitive modes (Fig.3).The bulk- ficult to see a rearrangement of the majorityd of the CuOz layer adjacent to the interface sensitive data are again in good agreement with and d3orbitals comparable to that observed on Cu. Fig.2.Normalized x-ray absorption spectra Mechanism of orbital reconstruction.Our 2.0 at the Cu L3 absorption edge,taken in bulk- Polarization along c-axis data therefore imply that the interfacial Cud Bulk FY Polarization in ab-plane sensitive (FY,top panel and interface- orbitals,which are fully occupied in bulk YBCO, sensitive (TEY,bottom panel)detection (·50) 1.5 Cu edge are partially populated by holes at the interface. modes with varying photon polarization as In principle,two distinct physical mechanisms indicated in the legend.a.u.,arbitrary units. 1.0 could lead to such an orbital reconstruction.First, Interface it is possible that the different crystal-field 20.5 TEY environment of Cu ions at the interface could raise the energy of the d32 orbital above that 0.0 of thedorbital Because the ligand positions at the interface are not precisely known,this 926 930 934 938 scenario cannot be firmly ruled out,but it is Photon Energy (eV) highly unlikely because of the large energy dif- ference between Cu d3-and d-derived Fig.3.Normalized x-ray absorption spectra 2.5 Polarization along c-axis bands in bulk YBCO.A reversal of this hierarchy at the Mn L2 and La absorption edges,taken -Polarization in a-b plane would require a substantially shorter distance in bulk-sensitive (FY,top panel)and interface-sensitive (TEY,bottom panel) 204 Mn edge between the copper and apical oxygen O(2)ions as compared with the in-plane Cu-O bond length, detection modes with varying photon Bulk FY which is unrealistic.Furthermore,the major dif- polarization as indicated in the legend. ference between the bulk and interface crystal- The line shape of the FY spectra is distorted field environments is the substitution of Cu-chain 1.0 by self-absorption effects. ions(with valence close to 2+in bulk YBCO)by 0.5 Interface TEY Mn ions (with valence~3.3+in bulk LCMO). The higher ligand charge should lower the energy 0.0 of the d3 orbital and further increase the 635 640 645 650 energy difference with thed level.A major Photon Energy (eV) rearrangement of the orbital level scheme due to 1116 16 NOVEMBER 2007 VOL 318 SCIENCE www.sciencemag.org

the interfacial absorption peak is shifted to lower energy with respect to the bulk by ~0.4 eV and that the high-energy shoulder is no longer present. The shift of the peak is evidence of a change in valence state of Cu ions near the interface. This indicates that charge is transferred across the interface and that a charged double layer is formed, as generally expected for heterostructures of materials with different work functions. In agreement with specific predictions for the system at hand (11), the charge-transfer direction is such that the hole density in YBCO is reduced at the interface. Because of the strong influence of the core hole created by absorbing the photon, the relationship between the x-ray absorption edge and the Cu valence is not straightforward, but a comparison to XAS spectra of reference materials containing Cu1+ and Cu2+ ions [for a review, see (19)] yields a rough estimate of 0.2e (where e in the charge on the electron) per copper ion for the charge-transfer amplitude. At first sight, this seems to correspond with the line shape of the interfacial absorption peak, which bears a strong resemblance to XAS data in undoped YBCO (16). Notably, however, numerous XAS experiments on YBCO and other bulk hole-doped high-temperature superconductors have shown that the position of the Cu L-absorption peak is independent of doping. This has been attributed to the Zhang-Rice singlet state and, consequently, the doped holes have predominantly oxygen character (17). The observed shift of the L3 absorption peak in our interfacesensitive experiment thus cannot be attributed to a readjustment of the hole density alone and indicates an extreme modification of the electronic structure of the CuO2 layer adjacent to the interface. In order to uncover the origin of the unexpected shift of the absorption peak and to obtain further information about the electronic states at the interface, we have varied the photon polarization in the interface-sensitive detection mode (Fig. 2). In marked contrast to the bulksensitive data, the strengths of the absorption signals for polarization perpendicular and parallel to the layers are almost equal. This is a manifestation of an “orbital reconstruction.” Whereas the holes are constrained to the Cu dx2−y2 orbital in the bulk, at least some of them occupy the d3z2−r2 orbitals at the interface. The distribution of holes over the two Cu orbitals cannot be precisely determined, because the XLD experiment probes not only the CuO2 layer directly at the interface but also the deeper layers (albeit with exponentially reduced sensitivity). However, the nearly isotropic cross section shown in Fig. 2 implies that the hole content of the Cu d3z2−r2 orbital is at least equal to that of the dx2−y2 orbital. We repeated the measurement at several temperatures (from 300 to 30 K) and confirmed that the peak position and polarization dependence do not depend on temperature. Similar observations were also made on heterostructures in which the doping level of YBCO was raised into the overdoped regime by Ca substitution. The orbital reconstruction and the charge transfer are hence general, robust characteristics of the YBCOLCMO interface. Before discussing possible mechanisms and potential implications of the orbital reconstruction, we briefly discuss XAS spectra near the Mn L2 and L3 absorption edges taken in bulk- and interface-sensitive modes (Fig. 3). The bulksensitive data are again in good agreement with corresponding data in the literature. The spectra are much broader than those taken near the Cu L edge, because all of the five Mn d orbitals are partially occupied, giving rise to a complicated multiplet splitting of the absorption peak. The peak intensity is independent of photon polarization within the experimental error. This finding has been taken as evidence of an orbitally disordered state with equal occupation of Mn dx2−y2 and d3z2−r2 orbitals in bulk metallic LCMO. In the interface-sensitive detection mode, neither the peak position nor its polarization dependence are noticeably different from the bulk data. This does not imply, however, that the Mn ions maintain their bulk charge density and electronic structure at the interface. Indeed, as a result of charge conservation, one generally expects a shift in Mn valence matching that of the interfacial Cu ions (Fig. 2), but because of the strong multiplet broadening of the Mn peak, such a shift is much harder to recognize than in the case of Cu (20). Based on the data of Fig. 3, one can set an upper bound of 0.4 eV on the difference between the positions of Mn L absorption edges in bulk- and interface-sensitive detection modes. Because a valence change from Mn3+ to Mn4+ results in a shift of the L edge of ~1.5 eV, this translates into an upper bound of ~0.3e per Mn atom on the amplitude of the charge transfer across the interface, which is consistent with the estimated amplitude of ~0.2e based on the Cu XAS spectra discussed above. Likewise, because the polarization dependence of the intensity at the Mn L edge is influenced to a large extent by the completely unoccupied minority t2g and eg orbitals, it is difficult to see a rearrangement of the majoritydx2−y2 and d3z2−r2 orbitals comparable to that observed on Cu. Mechanism of orbital reconstruction. Our data therefore imply that the interfacial Cu d3z2−r2 orbitals, which are fully occupied in bulk YBCO, are partially populated by holes at the interface. In principle, two distinct physical mechanisms could lead to such an orbital reconstruction. First, it is possible that the different crystal-field environment of Cu ions at the interface could raise the energy of the d3z2−r2 orbital above that of the dx2−y2 orbital. Because the ligand positions at the interface are not precisely known, this scenario cannot be firmly ruled out, but it is highly unlikely because of the large energy difference between Cu d3z2−r2 – and dx2−y2 –derived bands in bulk YBCO. A reversal of this hierarchy would require a substantially shorter distance between the copper and apical oxygen O(2) ions as compared with the in-plane Cu-O bond length, which is unrealistic. Furthermore, the major difference between the bulk and interface crystalfield environments is the substitution of Cu-chain ions (with valence close to 2+ in bulk YBCO) by Mn ions (with valence ~3.3+ in bulk LCMO). The higher ligand charge should lower the energy of the d3z2−r2 orbital and further increase the energy difference with the dx2−y2 level. A major rearrangement of the orbital level scheme due to Fig. 2. Normalized x-ray absorption spectra at the Cu L3 absorption edge, taken in bulksensitive (FY, top panel) and interfacesensitive (TEY, bottom panel) detection modes with varying photon polarization as indicated in the legend. a.u., arbitrary units. Fig. 3. Normalized x-ray absorption spectra at the Mn L2 and L3 absorption edges, taken in bulk-sensitive (FY, top panel) and interface-sensitive (TEY, bottom panel) detection modes with varying photon polarization as indicated in the legend. The line shape of the FY spectra is distorted by self-absorption effects. 1116 16 NOVEMBER 2007 VOL 318 SCIENCE www.sciencemag.org RESEARCH ARTICLES on November 26, 2007 www.sciencemag.org Downloaded from

RESEARCH ARTICLES readjustments of ligand positions is therefore im-are formed.The charge-transfer transition occurs at the interface.However,given the complexity plausible.The second scenario is based on the when the antibonding molecular orbital crosses of the unit cells and the broken translational sym- observation that the Cud3 orbital points the Cud level,as shown in the insets.The metry at the interface,this presents a formidable directly toward the interface and can hybridize change in hole density on Cu induced by the challenge to current computational capabilities. effectively with the Mnd orbital via the charge-transfer transition is another signature of Concluding remarks.Our experiments show apical oxygen ion [O(2)in Fig.1B],generating a the formation of a covalent bond:Figure 4 shows that the electronic structure of the CuO2 layer is covalent chemical bond bridging the interface.In that only about half of the hole charge ends up on modified by covalent bonds across the interface. this scenario.covalency results in the formation Cu;the remaining hole charge is distributed over These results suggest that the orbital rearrange- of extended "molecular orbitals"consisting of the Mn-and O(2)-derived components of the ment and strong hybridization are at least partial- atomic Cu and Mn d3 orbitals with an molecular orbital. ly responsible for the unusual magnetic behavior admixture of the p-orbitals on the apical oxygen The cluster calculation demonstrates that the previously observed at cuprate-manganate inter- (insets in Fig.4). formation of a strong covalent bond between Cu faces (22,23)and contribute to the suppression Covalent bonding at the interface.To and Mn ions at the interface is indeed realistic. of superconductivity near the interface (/3).Fur- check whether the covalent-bonding scenario is Partial occupancy of the corresponding antibond- ther,the valence electrons of a large variety of viable,we performed exact-diagonalization cal- ing molecular orbital qualitatively explains the transition metal oxides,whose properties in het- culations of the MnCuOo cluster shown in Fig. modification of the XLD spectra at the interface. erojunctions have been extensively investigated 1B,including a single hole and the full set of Cu The filling factor of this orbital is expected to [including manganates (24),titanates (25),vana- d orbitals and interaction parameters described in match that of the Mn d3 orbital,which is dates (26),ruthenates (27),and ferrites (28),re- the literature (/5,21).Because the Mn de about one-third in bulk LCMO.According to the side in nearly degenerate d orbitals and are hence 意 orbital does not hybridize with Cu.the Mn ion is calculation.a substantial fraction of the charge subject to hybridization at interfaces. represented by a single d3 orbital with a density in the hybrid orbital resides on Cu.which classical Hund's rule coupling to the core t2g References and Notes 只 is in good agreement with the experimentally 1.A.Ohtomo,H.Y.Hwang,Nature 427,423 (2004). spins.In order to simulate the difference in observed shift of the Cu valence.Because holes 2.S.Thiel et al.,Science 313.1942 (2006). YBCO and LCMO work functions,we tuned the in this orbital are not subject to Zhang-Rice 3.A.Tsukazaki et al.,Science 315,1388 (2007). on-site energy of the hole on Mn.Figure 4 shows singlet formation,the shift of the Cu XAS peak at 4.E.Dagotto,Science 309,257 (2005). the hole density in the Cu d shell (measured from the interface is also explained.Also,the Cu-Mn 5.Y.Tokura,N.Nagaosa,Science 288,462 (2000) 6.S.Okamoto,A.]Millis,Noture 428,630 (2004) the full-shell configuration 3d)as a function of orbital hybridization naturally results in a strong 7.S.Okamoto,A.]Millis,Phys.Rev.B 72,235108 (2005) 号 this parameter.For large values,the hole resides antiferromagnetic exchange coupling (/5),as 8.S.Thulasi,S.Satpathy,Phys.Rev.B 73,125307 (2006) completely on the Cu ion,and the ionic valencies recently observed at LCMO-YBCO interfaces 9.W.C.Lee,A.H.McDonald,Phys.Rev.B 74,075106 (2006). 乞 are close to their bulk values (i.e.,Cu is in the (22.23. 10.A.Ruegg,S.Pilgram,M.Sigrist,Phys.Rev.B 75,195117 (2007). formal valence state 2+,and the Mn valence is 3+). Needless to say,the cluster calculations have 11.S.Yunoki et al.,Phys.Rev.B 76,064532 (2007). The formal Cu-valence state.realized at high some limitations.In particular,the energy levels 12.H.-U.Habermeier et al,Physica C 364-365,298 (2001). Mn hole on-site energy,corresponds to ~0.76 in the cluster are sharp,and the Cu-Mn molecular 13.Z.Sefrioui et al.,Appl.Phys.Lett.81,4568 (2002). holes in the Cu d shell.which reside in the Cu orbitals are nearly orthogonal to Cu d,be- 14.T.Holden et al.,Phys.Rev.B 69,064505 (2004). 15.See supporting material on Science Online. d orbital.The remaining hole density is in cause mixing is only due to weak spin-orbit 16.N.Nuicker et ol.,Phys.Rev.B 51,8529 (1995) the in-plane oxygen p orbitals [O(3)in Fig.1B],coupling effects.The charge-transfer transition 17.F.C.Zhang,T.M.Rice,Phys.Rev.B 37,3759 (1988). W which hybridize strongly with Cud.The plot shown in Fig.4 is therefore abrupt.In an ex- 18.C.T.Chen et ol.,Phys.Rev.Lett.66,104 (1991). also shows that the charge transfer across the tended system,the energy levels are broadened 19.F.M.F.de Groot,J.Electron Spectrosc.Relat.Phenom. 67.529(19940. interface leads to a major rearrangement of the into bands,and the two bands near the Fermi 20.M.Abbate et al.,Phys.Rev.B 46,4511 (1992). electron distribution in the Cu eg orbital mani- level can be partially mixed.The charge carriers 21.M.A.van Veenendaal,H.Eskes,G.A.Sawatzky,Phys. fold.Indeed,when the transfer is complete,the on interfacial Cu ions are thus generally expected ReYB47.11462(1993). Cudorbital is completely full,and holes to have mixed d and d3 character,as 22.1.Stahn et al.,Phys.Rev.B 71,140509(R)(2005). partially occupy thedorbital.This indicates experimentally observed.More elaborate ab 23.]Chakhalian et al,Nat.Phys.2,244 (2006). 24.H.Yamada et al.,Science 305,646 (2004). that the Mn and Cu d32 orbitals are indeed initio calculations are required to obtain a 25.A.Ohtomo,D.A.Muller,]L.Grazul,H.Y.Hwang, strongly hybridized and that molecular orbitals quantitative description of the band dispersions Nature419,378(2002). 26.L.F.Kourkoutis,Y.Hotta,T.Susaki,H.Y.Hwang. Fig.4.Occupancy of Cu d or- 2+ D.A.Muller,Phys.Rev.Lett.97,256803 (2006). bitals at the LCMO-YBCO inter- Cu 27.X.Ke,M.5.Rzchowski,L.]Belenky,C.B.Eom,Appl. Phys.let84.5458(2004). face as a function of Mn hole 0.8 28.K.Ueda,H.Tabata,T.Kawai,Science 280,1064 (1998). on-site energy,as predicted by 29.M.Varela et al.,Solid State Electron.47,2245 (2003). the exact-diagonalization calcu- 3 Cu x2-y2 B 30.The authors would like to acknowledge G.Sawatzky, 3z2-r2 lations described in the text.The D.Khomskii,S.Okamoto,F.M.F.de Groot,and A.Millis 0.6 CU 372-2 occupancy is given by the total 5 for useful discussions.M.v.V.was supported by the U.S. Cu x2-y2 number of holes,measured $ AB Department of Energy (DOE),No.DE-FG02-03ER46097. Mn from the full-shell (3d)elec- Cu 3z2-r2 三 3z22 J.C.was supported by the U.S.Department of Defense- Army Research Office under Contract No.0402-17291. tron configuration.The corre- 0.4 Work at Argonne National Laboratory was supported by sponding formal Cu valence 6 B the DOE,Office of Science,Office of Basic Energy states are indicated for dlarity. Sciences,under contract no.DE-AC02-06CH11357. The insets show the orbital level 0.2 Supporting Online Material scheme at the interface,includ- www.sciencemag.org/cgi/content/full/1149338/DC1 2 ing extended bonding (B)and Materials and Methods -0-3z-r Figs.51 to $3 antibonding (AB)"molecular 0.0Q References orbitals"formed by hybridized 1.20 Cu and Mn d3zr orbitals.The 1.22 1.24 1.26 1.28 1.30 16 August 2007;accepted 2 October 2007 1+ Published online 11 October 2007; hole is indicated as the green Cu On-site energy on Mn(eV) 10.1126/cience.1149338 cirde. Include this information when citing this paper www.sciencemag.org SCIENCE VOL 318 16 NOVEMBER 2007 1117

readjustments of ligand positions is therefore implausible. The second scenario is based on the observation that the Cu d3z2−r2 orbital points directly toward the interface and can hybridize effectively with the Mn d3z2−r2 orbital via the apical oxygen ion [O(2) in Fig. 1B], generating a covalent chemical bond bridging the interface. In this scenario, covalency results in the formation of extended “molecular orbitals” consisting of atomic Cu and Mn d3z2−r2 orbitals with an admixture of the pz orbitals on the apical oxygen (insets in Fig. 4). Covalent bonding at the interface. To check whether the covalent-bonding scenario is viable, we performed exact-diagonalization calculations of the MnCuO10 cluster shown in Fig. 1B, including a single hole and the full set of Cu d orbitals and interaction parameters described in the literature (15, 21). Because the Mn dx2−y2 orbital does not hybridize with Cu, the Mn ion is represented by a single d3z2−r2 orbital with a classical Hund’s rule coupling to the core t2g spins. In order to simulate the difference in YBCO and LCMO work functions, we tuned the on-site energy of the hole on Mn. Figure 4 shows the hole density in the Cu d shell (measured from the full-shell configuration 3d10) as a function of this parameter. For large values, the hole resides completely on the Cu ion, and the ionic valencies are close to their bulk values (i.e., Cu is in the formal valence state 2+, and the Mn valence is 3+). The formal Cu2+ valence state, realized at high Mn hole on-site energy, corresponds to ~ 0.76 holes in the Cu d shell, which reside in the Cu dx2−y2 orbital. The remaining hole density is in the in-plane oxygen p orbitals [O(3) in Fig. 1B], which hybridize strongly with Cudx2−y2 . The plot also shows that the charge transfer across the interface leads to a major rearrangement of the electron distribution in the Cu eg orbital manifold. Indeed, when the transfer is complete, the Cu dx2−y2 orbital is completely full, and holes partially occupy the d3z2−r2 orbital. This indicates that the Mn and Cu d3z2−r2 orbitals are indeed strongly hybridized and that molecular orbitals are formed. The charge-transfer transition occurs when the antibonding molecular orbital crosses the Cu dx2−y2 level, as shown in the insets. The change in hole density on Cu induced by the charge-transfer transition is another signature of the formation of a covalent bond: Figure 4 shows that only about half of the hole charge ends up on Cu; the remaining hole charge is distributed over the Mn- and O(2)-derived components of the molecular orbital. The cluster calculation demonstrates that the formation of a strong covalent bond between Cu and Mn ions at the interface is indeed realistic. Partial occupancy of the corresponding antibonding molecular orbital qualitatively explains the modification of the XLD spectra at the interface. The filling factor of this orbital is expected to match that of the Mn d3z2−r2 orbital, which is about one-third in bulk LCMO. According to the calculation, a substantial fraction of the charge density in the hybrid orbital resides on Cu, which is in good agreement with the experimentally observed shift of the Cu valence. Because holes in this orbital are not subject to Zhang-Rice singlet formation, the shift of the Cu XAS peak at the interface is also explained. Also, the Cu-Mn orbital hybridization naturally results in a strong antiferromagnetic exchange coupling (15), as recently observed at LCMO-YBCO interfaces (22, 23). Needless to say, the cluster calculations have some limitations. In particular, the energy levels in the cluster are sharp, and the Cu-Mn molecular orbitals are nearly orthogonal to Cu dx2−y2 , because mixing is only due to weak spin-orbit coupling effects. The charge-transfer transition shown in Fig. 4 is therefore abrupt. In an extended system, the energy levels are broadened into bands, and the two bands near the Fermi level can be partially mixed. The charge carriers on interfacial Cu ions are thus generally expected to have mixed dx2−y2 and d3z2−r2 character, as experimentally observed. More elaborate ab initio calculations are required to obtain a quantitative description of the band dispersions at the interface. However, given the complexity of the unit cells and the broken translational symmetry at the interface, this presents a formidable challenge to current computational capabilities. Concluding remarks. Our experiments show that the electronic structure of the CuO2 layer is modified by covalent bonds across the interface. These results suggest that the orbital rearrangement and strong hybridization are at least partially responsible for the unusual magnetic behavior previously observed at cuprate-manganate interfaces (22, 23) and contribute to the suppression of superconductivity near the interface (13). Further, the valence electrons of a large variety of transition metal oxides, whose properties in heterojunctions have been extensively investigated [including manganates (24), titanates (25), vanadates (26), ruthenates (27), and ferrites (28)], reside in nearly degenerate d orbitals and are hence subject to hybridization at interfaces. References and Notes 1. A. Ohtomo, H. Y. Hwang, Nature 427, 423 (2004). 2. S. Thiel et al., Science 313, 1942 (2006). 3. A. Tsukazaki et al., Science 315, 1388 (2007). 4. E. Dagotto, Science 309, 257 (2005). 5. Y. Tokura, N. Nagaosa, Science 288, 462 (2000). 6. S. Okamoto, A. J. Millis, Nature 428, 630 (2004). 7. S. Okamoto, A. J. Millis, Phys. Rev. B 72, 235108 (2005). 8. S. Thulasi, S. Satpathy, Phys. Rev. B 73, 125307 (2006). 9. W. C. Lee, A. H. McDonald, Phys. Rev. B 74, 075106 (2006). 10. A. Ruegg, S. Pilgram, M. Sigrist, Phys. Rev. B 75, 195117 (2007). 11. S. Yunoki et al., Phys. Rev. B 76, 064532 (2007). 12. H.-U. Habermeier et al., Physica C 364-365, 298 (2001). 13. Z. Sefrioui et al., Appl. Phys. Lett. 81, 4568 (2002). 14. T. Holden et al., Phys. Rev. B 69, 064505 (2004). 15. See supporting material on Science Online. 16. N. Nücker et al., Phys. Rev. B 51, 8529 (1995). 17. F. C. Zhang, T. M. Rice, Phys. Rev. B 37, 3759 (1988). 18. C. T. Chen et al., Phys. Rev. Lett. 66, 104 (1991). 19. F. M. F. de Groot, J. Electron Spectrosc. Relat. Phenom. 67, 529 (1994). 20. M. Abbate et al., Phys. Rev. B 46, 4511 (1992). 21. M. A. van Veenendaal, H. Eskes, G. A. Sawatzky, Phys. Rev. B 47, 11462 (1993). 22. J. Stahn et al., Phys. Rev. B 71, 140509(R) (2005). 23. J. Chakhalian et al., Nat. Phys. 2, 244 (2006). 24. H. Yamada et al., Science 305, 646 (2004). 25. A. Ohtomo, D. A. Muller, J. L. Grazul, H. Y. Hwang, Nature 419, 378 (2002). 26. L. F. Kourkoutis, Y. Hotta, T. Susaki, H. Y. Hwang, D. A. Muller, Phys. Rev. Lett. 97, 256803 (2006). 27. X. Ke, M. S. Rzchowski, L. J. Belenky, C. B. Eom, Appl. Phys. Lett. 84, 5458 (2004). 28. K. Ueda, H. Tabata, T. Kawai, Science 280, 1064 (1998). 29. M. Varela et al., Solid State Electron. 47, 2245 (2003). 30. The authors would like to acknowledge G. Sawatzky, D. Khomskii, S. Okamoto, F. M. F. de Groot, and A. Millis for useful discussions. M.v.V. was supported by the U.S. Department of Energy (DOE), No. DE-FG02-03ER46097. J.C. was supported by the U.S. Department of Defense– Army Research Office under Contract No. 0402-17291. Work at Argonne National Laboratory was supported by the DOE, Office of Science, Office of Basic Energy Sciences, under contract no. DE-AC02-06CH11357. Supporting Online Material www.sciencemag.org/cgi/content/full/1149338/DC1 Materials and Methods Figs. S1 to S3 References 16 August 2007; accepted 2 October 2007 Published online 11 October 2007; 10.1126/science.1149338 Include this information when citing this paper. Fig. 4. Occupancy of Cu d orbitals at the LCMO-YBCO interface as a function of Mn hole on-site energy, as predicted by the exact-diagonalization calculations described in the text. The occupancy is given by the total number of holes, measured from the full-shell (3d 10) electron configuration. The corresponding formal Cu valence states are indicated for clarity. The insets show the orbital level scheme at the interface, including extended bonding (B) and antibonding (AB) “molecular orbitals” formed by hybridized Cu and Mn d3z2−r2 orbitals. The hole is indicated as the green circle. www.sciencemag.org SCIENCE VOL 318 16 NOVEMBER 2007 1117 RESEARCH ARTICLES on November 26, 2007 www.sciencemag.org Downloaded from

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