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.orgthe 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 inter￾face. This indicates that charge is transferred across the interface and that a charged double layer is formed, as generally expected for hetero￾structures of materials with different work func￾tions. In agreement with specific predictions for the system at hand (11), the charge-transfer di￾rection 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 esti￾mate of 0.2e (where e in the charge on the elec￾tron) 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 superconduc￾tors 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 predom￾inantly oxygen character (17). The observed shift of the L3 absorption peak in our interface￾sensitive 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 un￾expected 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 bulk￾sensitive data, the strengths of the absorption signals for polarization perpendicular and parallel to the layers are almost equal. This is a mani￾festation 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 exponen￾tially 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 re￾peated the measurement at several temperatures (from 300 to 30 K) and confirmed that the peak position and polarization dependence do not de￾pend on temperature. Similar observations were also made on heterostructures in which the doping level of YBCO was raised into the over￾doped regime by Ca substitution. The orbital re￾construction and the charge transfer are hence general, robust characteristics of the YBCO￾LCMO interface. Before discussing possible mechanisms and potential implications of the orbital reconstruc￾tion, we briefly discuss XAS spectra near the Mn L2 and L3 absorption edges taken in bulk- and interface-sensitive modes (Fig. 3). The bulk￾sensitive 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 polariza￾tion within the experimental error. This finding has been taken as evidence of an orbitally dis￾ordered 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 polariza￾tion 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 dif￾ficult 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 dif￾ference 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 dif￾ference between the bulk and interface crystal￾field 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 bulk￾sensitive (FY, top panel) and interface￾sensitive (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