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 1117readjustments of ligand positions is therefore im￾plausible. 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 cal￾culations 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 mani￾fold. 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 antibond￾ing 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 , be￾cause mixing is only due to weak spin-orbit coupling effects. The charge-transfer transition shown in Fig. 4 is therefore abrupt. In an ex￾tended 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 sym￾metry 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 rearrange￾ment and strong hybridization are at least partial￾ly responsible for the unusual magnetic behavior previously observed at cuprate-manganate inter￾faces (22, 23) and contribute to the suppression of superconductivity near the interface (13). Fur￾ther, the valence electrons of a large variety of transition metal oxides, whose properties in het￾erojunctions have been extensively investigated [including manganates (24), titanates (25), vana￾dates (26), ruthenates (27), and ferrites (28)], re￾side 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 or￾bitals at the LCMO-YBCO inter￾face as a function of Mn hole on-site energy, as predicted by the exact-diagonalization calcu￾lations described in the text. The occupancy is given by the total number of holes, measured from the full-shell (3d 10) elec￾tron configuration. The corre￾sponding formal Cu valence states are indicated for clarity. The insets show the orbital level scheme at the interface, includ￾ing 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