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 1115FULL-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, high￾mobility electron systems with tunable density have led to prominent advances in science and technology over the past decades. Such sys￾tems have recently been replicated in hetero￾structures of complex transition metal oxides (1), leading to the observation of transistor effects (2) and the quantum Hall effect (3). Because transi￾tion metal oxides exhibit a notably rich phase be￾havior in the bulk (4), these developments have raised expectations that quantum states with properties and functionalities qualitatively be￾yond those attainable in semiconductors can be generated at oxide interfaces. The large variety of phases (often with radi￾cally different physical properties) in transition metal oxides is due to the delicate sensitivity of the charge transfer and magnetic interaction be￾tween metal ions to the occupation of d orbitals (5). Which linear combination of the five possi￾ble 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 dif￾ferent at the interface than in the bulk. Despite its pivotal role in determining the phase behavior and physical properties of oxides, almost no ex￾perimental 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 or￾bital occupation at the interface. The cuprate￾manganate interface is well suited as a model system for this purpose, because nearly strain-free, atomically sharp heterostructures can be syn￾thesized (12–14) and because the electronic properties of both materials have been studied extensively in the bulk. Probing heterostructure interfaces. The ex￾periments were performed at the 4-ID-C beam￾line 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 character￾ization methods (15). In order to discriminate the electronic structure at the interface from surface and bulk contributions, we have performed a sys￾tematic 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 in￾fluence 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 in￾terface 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 ex￾periments in the bulk-sensitive fluorescence￾yield (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 ex￾cellent 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 right￾hand 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 informa￾tion 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 high￾temperature 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 Depart￾ment 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 di￾chroism, 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