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Evidence of 3d9-ligand hole states in the superconductor La1.85Sr0.15CuO4 from L3 X-ray absorption spectroscopy
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EVIDENCE OF 3d9-LIGAND HOLE STATES IN THE SUPERCONDUCTOR La1,83Sr0,15CuO4 FROM L3 X-RAY ABSORPTION SPECTROSCOPY A. BIANCONI a J, BUDNICK b A.M. FLANK ‘~,A. FONTAINE C P. LAGARDE C A. MARCELLI d H. TOLENTINO c B. CHAMBERLAND b, c~ MICHEL e B. RAVEAIJ e and G. DEMAZEAU Dipartimento di Fisica, Universitd degli Studi di Roma “La Sapienza“, 00185 Rome, Italy Department of Physics and Institute ofMaterials Science, University of Connecticut, Stoyrs, CT 06268, USA LURE, CNRS—CEA—MEN, Bdtiment 209 D, Université Paris Sud, 91405 Orsay, France Laboratori Nazionali di Frascati, INFN, 00044 Frascati, Italy Laboratoire de Cristallographie el Science des Matériaux, CNRS, Universitb de Caen, 14032 Caen, France Laboratoire de Chirnie de Solide, CNRS, 351 Cours de Ia Liberation, 33405 Talence, France Received 4 November I 987; revised manuscript received 19 January 1988; accepted for publication 19 January 1988 Communicated by A.A. Maradudin
The Cu L3 X-ray absorption spectra of La2_,Sr~CuO4~ (x= 0, 0.15 and 0.88) have been measured with high resolution. For comparison the L3 X-ray absorption spectrum of the formally trivalent Cu compound La2Li05Cu05O4 is reported. The semiconductor La2CuO4, with Cu formal valence v=2, shows a single white line due to the 2p3d~° final state arising from the Cu 3d° initial state. The9Lsuperconducting (where Lisa hole sample in thewith oxygen x=0. derived 15 (T~= band, 37 ligand K) shows hole). theThe additional characteristic 2p3d’°Lfinal feature ofthe states superconducting arising from the initial states material is that 3d there is no energy gap between the final states 2p3d’°and 2p3d’°L.In the semiconducting compound, formed by increasing Sr concentration (x=0.88), a red shift of 0.2 eV ofthe 2p3d~° final state is enough to create a gap between the two states. The conductivity in these ceramic materials is assigned to the itinerant 3d9L states (probed by the 2p 3d’°Lfinal state) which indicate the presence ofoxygen hole carriers. These results support a microscopic mechanism for high T~superconductivity due to pairing of oxygen holes coupled with localized Cu 3d9 localized states i.e. of two itinerant 3d9L states.
1. Introduction The understanding of the local electronic configuration in the new high TC superconductors is clearly a prerequisite for the comprehension of high T~superconductivity. This paper reports results of the copper L3 edge X-ray absorption spectroscopy performed on six systems of the original La2CuO4 superconductor family [1—41,which crystallize in the K2NiF4 type structure [5,6]. The electronic band structure of La2CuO4 and of related Sr-doped superconductors has been calculated [6—8]and valence band photoemission experiments [9—11]have shown a large disagreement with predictions of the one-electron theories due to the important role of electronic correlation effects and localization of Cu 3d orbitals. A local experimental probe, like X-ray absorption spectroscopy, is required to determine the local electronic configura-
tion ofCu ions in the high TC superconducting copper oxides. The Cu K-edge X-ray absorption spectra, probing the unoccupied p-like final states, •of La 2_~Sr~CuO4..~ compounds have been measured [12—15]but conflicting interpretations of the data to extract the Cu valence states have been reported. The L23 X-ray absorption, probing the unoccupied Cu “d” and “s” final states, selected by the dipole selection rule E~1=±1, is particularly appropriate to determine the local electronic configurationof the 3d levels of copper ions. Previous L3 X-ray absorption joint with X-ray photoelectron spectroscopy (XPS) of the Cu 2p levels [17] measurements on the superconducting systems YBa2Cu3O7 [16] have given evidence for itinerant holes in the oxygen derived band in the superconducting phase in agreement with the analysis of valence band photoemission [181. In fact the L3 X-ray absorption measurements on
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the superconducting systems YBa2Cu3O7~~ exhibits acharacteristicfeature in the range of2eV above the white assigned to the 2p3d’°L(where is a state hole in the line oxygen derived band, ligand hole) L final due to the 3d9L initial state. Moreover the 2p3d9 final state, due to the Cu 3d8 (Cu3~)in the ground state, was not found in the L 3 X-ray absorption spectrum of the YBa2Cu3O7 ~in the energy range of 942 eV as expected from X-ray photoelectron spectroscopy (XPS) of the Cu 2p 1ev8 els [17] ruling out the presence of the Cu 3d configuration. In this work the presence of the 3d9L itinerant configuration in the high T~ superconductor La 1 85Sr0 5CuO4 with T~=37Khas been investigated. We have investigated a set of samples which crystallize in the La2CuO4 structure: the formally divalent La2CuO4 compound which can exhibit superconductivity if prepared under oxygen pressure [19,20] and the nonsuperconducting antiferromagnetic phase [20,21] with the Néel temperature TN quite sensitive to the oxygen deficiency y, La2CuO4 ,, TN 290 K for y = 0.03; the non-integer valent Sr doped compounds which exist as superconductor La1 85Sr01 5CuO4 (formal valence v= 2.15) and as semiconductor La1 2Sr088CuO3 65(v=2.l8). These samples are compared to the formally trivalent Cu compound La2Li05Cu05O4 [22]. In the many body description9 La2CuO4 and 3d10L(v=2) config-is expected like to be a mixture of 3d uration, CuO and other divalent copper compounds [23]. The doping by Sr2~should induce new states 3d8 and 3d9L, if the hole is created at the Cu or at the oxygen site respectively and 3d’°L2states if both holes are in the oxygen site. Moreover, a distortion of the structure can induce a variation of the mixing between the 3d’°Land 3d9 configuration. Therefore the ground state of La 19,85Sr0 15CuO4 is ex3d10L configurapected to be mixture of the(Cu0 3d tions for thea local clusters 6 and of the 4) for the local 3d9L, 3d’°L2and 3d8 configurations clusters (Cu0 5. The weight of these configurations depends 4) on their relative energy position and hybridization, In this paper the copper L 3 soft X-ray 9L absorption with Sr spectra formation 3d body condoping. show Thesethe states are dueoftostates a many figuration with itinerant holes in the oxygen valence —
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band coupled with localized 3d electrons on the Cu sites which is close to the Fermi level in 8theconfigusuperconducting phase, No evidence of Cu 3d ration has been found. The conductivity in this highly correlated system is assigned to the presence of the electron—hole configuration 3d°Lat the Fermi level and superconductivity to pairing of the oxygen holes. Recently, nonconventional BCS theories of the high T~oxides have been putsupport forwardto[24—311.Our findings experimental the description of thegive superconductivity as pairing of oxygen holes [30,311.
2. Experiment The L23 XANES experiment has been carried out on the ACO storage ring synchrotron radiation facility LURE at Orsay. A double crystal Beryl monochromator using the 1010 reflection has been used [32]. The resolution has been measured to be about 0.35 eV at 900 eV in agreement with previous measurements [33,34]. The X-ray absorption spectra were recorded by scanning the photon energy with steps of 0. 1 eV. The accuracy in determining the energy shift of the spectroscopic lines is halfthis value. In order to check the energy calibration a systematic measurement of the CuO sample was collected and the white line maximum was days foundoftomeasurements. be exactly reproducible through the five The energy scale was fixed by taking the white line maximum at 931.25 eV according to Koster [35]. The X-ray absorption has been measured by detecting the emitted electrons by a channeltron in the total electron yield mode. Repeated scans of the samples have been carried out to check reproducibility and to improve statistics. No deterioration of samples irradiation.has been observed under soft X-ray In our experiments which required between one and two hours the sample vacuum was between I O~ and lO_6 mm. Typically a sample was exposed to this pressure for a total time of about four hours. The features observed in our spectra were independent of scanning intervals. Just scraped prior totomounting, the surface of each sample was remove some face layers. No subsequent surface treatment was used. In total yield experiments such as this we es-
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timate the sampling depth [34] to be as much as 100 to 150 A.
[35] has shown that the white line moves toward higher energy with a decrease of ionicity. It is interesting to remark that this is in agreement with the fact that an increase in conductivity of a factor 10~ has been observed in going from Gd2CuO4 to
3. Results
La2CuO4 [36,37]. The spectrum of the trivalent compound La2Li05Cu05O4 in fig. 1 shows two lines (at 931.05 eV and 933.3 eV) having similar intensity and the same full width at half-height 1 eV, separated by ~E= 2.25 eV. The intensity ratio of these two lines does not change with repeated scans and gives confidenceThe about sample stability during the experiment. presence of two well-resolved sharp lines seems to be characteristic of insulating formally Cu trivalent compounds and in fact also the L 3 spectrum of (KCu (III)(biuret) 2) [16] exhibits two similar lines but the first one at lower energy and the other one at higher energy (933.6 eV) giving a larger energy separation between the two final states of ~E= 2.9 eV. We have found the presenceof two lines also in the L3 spectrum of NaCuO2 which is a third
In fig. 1 the spectra of formally divalent CuO, La2CuO4 and Gd2CuO4 compounds and of the tnvalent compound La2Li05Cu05O4 are shown. In the formally bivalent compounds La2CuO4 and Gd2CuO4 we observe a single strong absorption line as in CuO (called white line) and in other divalent Cu cornpounds as reported by Koster [35]. Assuming the 9 and 3d’°Lthe ground state to beto athe mixture of1°final 3d white line is due 2p state. This is 3123d a bound state formed by the strong Coulomb interaction, Qhd —.9 eV [17,23], in the final state between the 2p core hole and the 3d electron. In the Cu perovskites the 2p3123d’°final state is wider than in CuO and located at higher and lower energy. We observe that in going from the Gd to the La compound the energy of the main line increases by 0.2 eV. Koster JO
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Energy (eV) Fig. I. The L3 X-ray 1°final absorption state.spectra The spectrum of non-metallicbivalent of the trivalentCu Cu compound compounds:(d) (a)LaCuO, (b) La2CuO4 and (c) Gd2CuO4 show a single line due to the 2p3d 2Li05Cu05O4 shows two sharp, well-resolved lines at 931.05 eV and 931.30 eV final states, both full widths at half height being 1 eV, assigned to 2p3d’°Lfinal states. The integrals of the peaks have been arbitrarily fixed to be one.
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example of a formal trivalent copper compound [38]. The electronic structure of formally trivalent sys9L, 3d’°L2 tems3d8 is described by a for mixture the 3d(Cu0 5 and configurations a localofcluster 4) 9L. and 3d’°L2 The energy separation 3d required to configuration is given between by the the energy transfer an electron from the ligand to the Cu 3d level ts.E. The energy separation between 3d9L and 3d8 will .
be given by Udd AE. Assumingthat the strong white lines are due to 2p—+3d transitions the two lines can 9 final be associated with 2p3123d’°L and 2p1123d states. The energy separation is expected to be ~E= Q~ + Udd AE because of the larger final state electron—hole Coulomb interaction Qhd 9 eV acting on the 2p 10L. It is clear that the expected value for 3E is 3123d much larger than the energy separation of about 2 eV observed for the two lines in the trivalent compounds. Moreover in all these trivalent cornpounds both lines do not show the expected multiplet structure for the 2p3d9 final state which was observed in the X-ray absorption spectra of Ni2~ compounds (with 3d8 high spin ground state configuration [34]). Their energy positions and the lack of multiplets is in agreement with the assignment of both final states to 2p 10L configuration. There3123d fore in order to explain the presence of two lines it —
—
is possible that two types of holes, one more localized more originUtoand the another two peaks 2p delocalized 10L’ andL*2pcould give 3/23d 3/23dIOL*, respectively. The fact that the intensity of the two lines is of the same order of magnitude indicates that the two states are strongly mixed, In the antiferrornagnetic La2CuO4 (TN = 240 K) the transition to the 2p3d’°state is shifted significantly with respect to the CuO line to higher energy (0.15 eV) and the contribution due to the 2p3d’°L final state is not visible. We have investigated both superconducting and antiferrornagnetic samples of La2CuO4~and no appreciable difference has been observed between the different samples. This result is not surprising since only a very small variation of stoichiornetry (~y-.O.O3)is enough to induce the change from antiferromagnetic to superconducting behavior. In the superconducting compound, La1 85Sr01 5CuO4 (formal valenceand v=wider 2. 15),as the L3 absorption becomes asymmetric shown in fig. 2a. The main line remains at the same energy 288
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as in La2CuO4 and a broad high energy asymmetric contribution appears. This contribution is assigned to the 2p3d’°Lcontribution. In the semiconducting compound La 2 CSrCCuO4 (x=0.88, v=O.35), with formal Cu valence v=2.l8, the main line (at 931.2 eV) shifts by 0.2 eV to lower energy. Comparing our results with L 3 absorption spectra recordedby measuring the electron energy loss in thin films [39]threshold our spectra do not show anyeV spectral ture with in the region of 2.5 abovefeathe white line which is characteristic for the final state 2p3d’ 04~1 due to Cu + ions, formed by sample reduction [40]. This peak was observed in the energy loss spectra indicating the reduction of the sample under electron bombardment. The presence of Cu 3d8 in the initial state should give the final state 2p3d9. The Cu 2P2,3 XPS data of La 9 final states in the 1 85Sr015CuO4 2p3d The fact that no energy region of exhibit 942 eVthe [11,39]. spectral peak is observed in this energy region of the X-ray absorption spectrum, where the similar final state is expected, rule out the presence of Cu 3d8 configuration. In fig. 3 we show the difference spectrum between superconducting La 1 85Sr0 15CuO4 and La2CuO4 assuming that this difference gives the 2p3123d°Lcon9L states induced by Sr tribution arising from the 3d doping. The difference spectrum shows a broad, about 1.7 eV wide, asymmetric feature with a peak at about 932.2 eV. The peak of the 2p3/23d’°final state due to the initial state configuration 3d°is assumed to be that of La2CuO4 at 931.4 eV and is 1.45 eV wide. The energy separation between the 2p3123d” and the 2p3123d’°Lfinal states is a measure of the ligand ionization energy. The energy separation between the maxima ofthe two peaks is 0.8 eV but because of the band width of the two configurations (mainly of the 2p3123d’°Lconfiguration) there is no gap separating the two final states. This result indicates that there are ligand holes close to the Fermi level in the superconductor. Thence the metallic behaviour is indicated by the overlapping of the two configurations. The width of the absorption feature due to the the 1 °Lconfiguration is 1.7 eV, considering 2p3,23d instrumental broadening and the core hole life time, the intrinsic width is reduced to 1.3 eV. Assuming -
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Energy (eV) Fig. 2. The L
3 X-ray absorption spectra of the superconducting compound La,85Sr015CuO4 (formal Cu valence v=2.15) (curve (e)) and ofthe semiconducting La1 2Sr0 88CuO365 (v= 2.18) (curve (f)) are compared to the purely divalent La2CuO4 (curve (b)).
that is state, closethis to the width the 9L this in thevalue initial stateband crossing the of Fermi 3d level will be itinerant with a life time of about 10—’ ~ s. The experimental finding a many body configuration 3d°L indicates the of strong correlation between the localized Cu 3d9 and itinerant ligand holes and large hybridization between the oxygen 2p orbitals and the Cu 3d orbitals. The presence ofthe superconducting phase should be associated with the ligand hole pairing. If we consider the two-dimensional Cu0 2 network, starting with a probability of p, = 0.15/4 to have one hole into a Cu—O ligand (4 oxygen bonds per copper) the probability to get a second hole around the same copper sites given by a random distribution is p1(N— l)p,N/2 with N=4 for the square planar considered herein. Then the probability for pair 6p~) is having iSp, = a0.225 ofligand in the same siteone ( single ligand hole times theholes probability to get (4p,). The value of Udd of about 6 eV [171 indicates the presence of well localized magnetic moments on the
Cu site, which is inorder agreement with evidence of antiferromagnetic in non-superconducting La 2CuO4~.In superconducting Cu oxides, where the antiferromagnetic interaction established between 9 local moments an is ordering of the ligand the Cu 3d holes should be induced by the presence of the 3d9L. configuration and therefore the pairing of the ligand holes become possible. Therefore this experiment demonstrates that pairing of oxygen holes can be driven by ordering of local moments on the Cu 3d9 sublattice. Recently some models have been proposed for the microscopic mechanism for high T~ superconductivity determined by pairing of oxygen holes in an antiferromagnetic two-dimensional layer of CuO2 [31,32].
4. Conclusion Doping La 3+ 2CuO4 the with Sr (where formally ions are expected) holes are formed in theCuoxygen-derived band forming the many body configuration 3d9L in the ground state. In the 289
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Fig. 3. The L3 X-ray absorption spectra of the superconducting compound La, 85Sr,1 5CuO4 has been deconvoluted in two components: the 2p3d’°plus the 2p3d’°L configuration. The 2p3d’°L component (curve (g)) has been obtained by the difference between La1 85Sr0 5CuO4 and La2CuO4, assuming the La2CuO4-type profile for the 2p3d’°configuration also in the9L superconducting states in the ground Sr-doped state of the superconducting The spectrum of the formal insulating compound material. The spectrum material. of the 2p3d’°Lcomponent (curve (g))trivalent is assigned to the presence of La the itinerant 3d 2Li05Cu05O4 (curve (d)) is plotted here for comparison.
9 and superconductor the absence ofa gap between 3d 3d9L in the ground state leads to the metallic-like conductivity. The characteristic aspect of these states is that the Cu 3d states are localized but strongly hybridized with the oxygen 2p orbitals. The present results show that the presence of itinerant 3d9L configuration is characteristic of superconducting materials studied so far, both the class of La 2~Sr~CuO4 (x<0.2) and the previously studied YBa2Cu3O7~(0
to the Laboratoire pour I’Utilization du Rayonnement Electromagnetique (LURE) technical staff for their valuable aid. This work was supported by the European Community under CEE Contract STP0040-l-(CD). The work of J.I. Budnick carried out at the University of Connecticut is supported by the US Department of Energy under contract No. DE0S05-80ER10742.
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[31Ri. Acknowledgement
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We would like to thank A. Kotani and G.A. Sawatzky for stimulating discussions. We are grateful
Appl. Phys. 26(1987) LI; C. Politis, J. Geerk, M. Dietrich and B. Obst, Z. Phys. B 66 (1987) 141.
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M. Hervieu and B. Raveau, J. Solid State Chem. 39 (1981) 120. [5] R.M. Fleming, B. Batlogg, R.J. Cava and E.A. Rietman, Phys.Rev.B35(1987)7191. [6] L.F. Mattheiss, Phys. Rev. Lett. 58 (1987) 1028. [7] i.Yu, Al. Freeman and 1.-H. Xu, Phys. Rev. Lett. 58 (1987) 1035. [81 W.M. Temmerman, G.M. Stocks, P.J. Durham and PA. Sterne,J. Phys. F 17(1987) L135. [9] T. Takahashi, F. Maeda, S. Hosoya and M. Sato, Japan. J. Appl. Phys. 26(1987) L349. [10] B. Reihi, T. Riester, J.G. Bednorz and K.A. Muller, Phys. Rev. B, to be published. [II] A. Fujimori, E. Takayama-Muromachi, Y. Uchida and B. Okai, Phys. Rev. B 35(1987)8814. [12] J.M. Tranquada, S.M. Heald, A. Moodenbaugh and M. Suenaga, Phys. Rev. B 35 (1987) 7187. [13] E.E. Alp, G.K. Shennoy, D.G. Hinks, D.W. Capone It, L. Soderhoim, H.B. Shattler, J. Guo, D.E. Ellis, PA. Montano and M. Ramanathan, Phys. Rev. B 35(1987) 7199. [14] H. Oyanagi, H. Ihara, T. Matsushita, M. Tokumoto, M. Hirabayashi, N. Tereda, K. Senzaki, Y. Kimura and T. Yao, Japan. I. Appi. Phys. 26 (1987) L488. [15] F.W. Lytle, RB. Greegor and A.J. Panson, Phys. Rev. B,to be published. [16] A. Bianconi, A. Congiu Castellano, M. De Santis, P. Rudolf, P. Lagarde and AM. Flank, Solid State Commun. 63 (1987) 1009. [17] A. Bianconi, A. Congiu Castellano, M. Dc Santis, P. Delogu, A. Gargano and R. Giorgi, Solid State Commun. 63 (1987) 1135. [181 A. Fujimori, E. Takayama-Muromachi and Y. Uchida, Solid State Commun. 63 (1987) 857. [19] J. Beille, R. Cabanel, C. Chaillout, B. Chevallier, 0. Dcrnazeau, F. Deslandes, J. Etourneau, P. Lejay, C. Michel, J. Provost, B. Raveau, A. Sulpice, J.L. Tholence and R. Tournier, C.R.Acad. Sci. 11304(1987)1097. [20] P.M. Grant, S.S.P. Parkin, V.Y. Lee, EM. Engler, ML. Ramirez, J.E. Vazquez, 0. Lim, RD. Jacowitz and G.L. Greene, Phys. Rev. Lett. 58 (1987) 2482. [21]Y.J. Uemura, W.J. Kossler, X.H. Yu, J.R. Kempton, HE. Schone, D. Opie, C.E. Stronach, O.C. Johnston, M.S. Alvarezand D.P. Goshorn, Phys. Rev. Lett. 59 (1987) 1045.
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[22] G. Demazeau, C. Parent, M. Pouchard and P. Hagenmuller, Mater. Res. Bull. 7 (1972) 913. [23] G. van der Laan, C. Westra, C. Haas and G.A. Sawatsky, Phys.Rev.B23(1981)4369. [24] D.J. Scalapino, E. Loh Jr. and J.E. Hirsch, Phys. Rev. B 35 (1987) 6694; H.B. Schutter, J.D. Jorgensen, D.G. Hinks, D.W. Capone and Di. Scalapino, to be published. [25] P.W.Anderson, Science 235 (1987) 1196. [26] P.W. Anderson, G. Baskaran, Z. Zou and T. Hsu, Phys. Rev. Lett. 58 (1987) 2790; 0. Baskaran, Z. Zoo and P.W. Anderson, Solid State Cornmon. 63 (1987) 973. [27] M. Cyrot, Solid State Commun. 62 (1987) 821; 63 (1987) 1015. [28] C. Noguera and P. Garoche, Solid State Commun. (1987), tobe published. [29] P.A. Lee andN. Read, Phys. Rev. Lett. 58 (1987) 2691. [30] V.J. Emery, Phys. Rev. Lett. 58 (1987) 2794. [311 J.E. Hirsh, Phys. Rev. Lett. 59 (1987) 228. [32] M. Lemonnier, 0. Collet, C. Depautex, J.-M. Esteva and D. Raoux, NucI. Instrum. Methods 152 (1978) 109. [33] J.M. Esteva and R.C. Karnatak, in: Giant resonances, eds. J.P. Connerade, J.M. Esteva and R.C. Karnatak (Plenum, New York, 1987); G. van der Laan, J. Zaanen, GA. Sawatzky, R.C. Karnatak and J.-M. Esteva, Phys. Rev. B 33 (1986) 4253. [34] PA. Citrin, J. Phys. (Paris) 47 (1986) C8-437. [351AS. Koster, Molecular Physics 26 (1973) 625. [36] C. Michel and B. Raveau, Rev. Chim. Mm. 21(1984) 407. [37] T. Kenjo, and S. Yajima, Bull. Chem. Soc. Japan. 46 (1973) 2619. [38] A. Bianconi, J. Budnick, E. Dartyge, AM. Flank, A. Fontaine, P. Lagarde, A. Marcelli, A. Clozza, B. Chamberland, i. Jegoudez and A. Revcolevshi, to be published. [39] N. Nucker, J. Fink, B. Renker, D. Ewert, C. Politis, P.J.W. Weijs and J.C. Fuggle, Z. Phys. B 67 (1987) 9. [40] A. Bianconi, A. Clozza, A. Congiu Castellano, S. Della Longa, M. De Santis, A. Di Cicco, K. Garg, P. Delogu, A. Gargano, R. Giorgi, P. Lagarde, A.M. flank and A. Marcclii, in: Proc. Adriatico Research Conf. on High temperatore superconductors, Trieste, Italy, 6—8 July 1987, eds. Y. Lu, M. Tosi and E. Tosatti (World Scientific, Singapore).
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EVIDENCE OF 3d9-LIGAND HOLE STATES IN THE SUPERCONDUCTOR La1,83Sr0,15CuO4 FROM L3 X-RAY ABSORPTION SPECTROSCOPY A. BIANCONI a J, BUDNICK b A.M. FLANK ‘~,A. FONTAINE C P. LAGARDE C A. MARCELLI d H. TOLENTINO c B. CHAMBERLAND b, c~ MICHEL e B. RAVEAIJ e and G. DEMAZEAU Dipartimento di Fisica, Universitd degli Studi di Roma “La Sapienza“, 00185 Rome, Italy Department of Physics and Institute ofMaterials Science, University of Connecticut, Stoyrs, CT 06268, USA LURE, CNRS—CEA—MEN, Bdtiment 209 D, Université Paris Sud, 91405 Orsay, France Laboratori Nazionali di Frascati, INFN, 00044 Frascati, Italy Laboratoire de Cristallographie el Science des Matériaux, CNRS, Universitb de Caen, 14032 Caen, France Laboratoire de Chirnie de Solide, CNRS, 351 Cours de Ia Liberation, 33405 Talence, France Received 4 November I 987; revised manuscript received 19 January 1988; accepted for publication 19 January 1988 Communicated by A.A. Maradudin
The Cu L3 X-ray absorption spectra of La2_,Sr~CuO4~ (x= 0, 0.15 and 0.88) have been measured with high resolution. For comparison the L3 X-ray absorption spectrum of the formally trivalent Cu compound La2Li05Cu05O4 is reported. The semiconductor La2CuO4, with Cu formal valence v=2, shows a single white line due to the 2p3d~° final state arising from the Cu 3d° initial state. The9Lsuperconducting (where Lisa hole sample in thewith oxygen x=0. derived 15 (T~= band, 37 ligand K) shows hole). theThe additional characteristic 2p3d’°Lfinal feature ofthe states superconducting arising from the initial states material is that 3d there is no energy gap between the final states 2p3d’°and 2p3d’°L.In the semiconducting compound, formed by increasing Sr concentration (x=0.88), a red shift of 0.2 eV ofthe 2p3d~° final state is enough to create a gap between the two states. The conductivity in these ceramic materials is assigned to the itinerant 3d9L states (probed by the 2p 3d’°Lfinal state) which indicate the presence ofoxygen hole carriers. These results support a microscopic mechanism for high T~superconductivity due to pairing of oxygen holes coupled with localized Cu 3d9 localized states i.e. of two itinerant 3d9L states.
1. Introduction The understanding of the local electronic configuration in the new high TC superconductors is clearly a prerequisite for the comprehension of high T~superconductivity. This paper reports results of the copper L3 edge X-ray absorption spectroscopy performed on six systems of the original La2CuO4 superconductor family [1—41,which crystallize in the K2NiF4 type structure [5,6]. The electronic band structure of La2CuO4 and of related Sr-doped superconductors has been calculated [6—8]and valence band photoemission experiments [9—11]have shown a large disagreement with predictions of the one-electron theories due to the important role of electronic correlation effects and localization of Cu 3d orbitals. A local experimental probe, like X-ray absorption spectroscopy, is required to determine the local electronic configura-
tion ofCu ions in the high TC superconducting copper oxides. The Cu K-edge X-ray absorption spectra, probing the unoccupied p-like final states, •of La 2_~Sr~CuO4..~ compounds have been measured [12—15]but conflicting interpretations of the data to extract the Cu valence states have been reported. The L23 X-ray absorption, probing the unoccupied Cu “d” and “s” final states, selected by the dipole selection rule E~1=±1, is particularly appropriate to determine the local electronic configurationof the 3d levels of copper ions. Previous L3 X-ray absorption joint with X-ray photoelectron spectroscopy (XPS) of the Cu 2p levels [17] measurements on the superconducting systems YBa2Cu3O7 [16] have given evidence for itinerant holes in the oxygen derived band in the superconducting phase in agreement with the analysis of valence band photoemission [181. In fact the L3 X-ray absorption measurements on
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the superconducting systems YBa2Cu3O7~~ exhibits acharacteristicfeature in the range of2eV above the white assigned to the 2p3d’°L(where is a state hole in the line oxygen derived band, ligand hole) L final due to the 3d9L initial state. Moreover the 2p3d9 final state, due to the Cu 3d8 (Cu3~)in the ground state, was not found in the L 3 X-ray absorption spectrum of the YBa2Cu3O7 ~in the energy range of 942 eV as expected from X-ray photoelectron spectroscopy (XPS) of the Cu 2p 1ev8 els [17] ruling out the presence of the Cu 3d configuration. In this work the presence of the 3d9L itinerant configuration in the high T~ superconductor La 1 85Sr0 5CuO4 with T~=37Khas been investigated. We have investigated a set of samples which crystallize in the La2CuO4 structure: the formally divalent La2CuO4 compound which can exhibit superconductivity if prepared under oxygen pressure [19,20] and the nonsuperconducting antiferromagnetic phase [20,21] with the Néel temperature TN quite sensitive to the oxygen deficiency y, La2CuO4 ,, TN 290 K for y = 0.03; the non-integer valent Sr doped compounds which exist as superconductor La1 85Sr01 5CuO4 (formal valence v= 2.15) and as semiconductor La1 2Sr088CuO3 65(v=2.l8). These samples are compared to the formally trivalent Cu compound La2Li05Cu05O4 [22]. In the many body description9 La2CuO4 and 3d10L(v=2) config-is expected like to be a mixture of 3d uration, CuO and other divalent copper compounds [23]. The doping by Sr2~should induce new states 3d8 and 3d9L, if the hole is created at the Cu or at the oxygen site respectively and 3d’°L2states if both holes are in the oxygen site. Moreover, a distortion of the structure can induce a variation of the mixing between the 3d’°Land 3d9 configuration. Therefore the ground state of La 19,85Sr0 15CuO4 is ex3d10L configurapected to be mixture of the(Cu0 3d tions for thea local clusters 6 and of the 4) for the local 3d9L, 3d’°L2and 3d8 configurations clusters (Cu0 5. The weight of these configurations depends 4) on their relative energy position and hybridization, In this paper the copper L 3 soft X-ray 9L absorption with Sr spectra formation 3d body condoping. show Thesethe states are dueoftostates a many figuration with itinerant holes in the oxygen valence —
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band coupled with localized 3d electrons on the Cu sites which is close to the Fermi level in 8theconfigusuperconducting phase, No evidence of Cu 3d ration has been found. The conductivity in this highly correlated system is assigned to the presence of the electron—hole configuration 3d°Lat the Fermi level and superconductivity to pairing of the oxygen holes. Recently, nonconventional BCS theories of the high T~oxides have been putsupport forwardto[24—311.Our findings experimental the description of thegive superconductivity as pairing of oxygen holes [30,311.
2. Experiment The L23 XANES experiment has been carried out on the ACO storage ring synchrotron radiation facility LURE at Orsay. A double crystal Beryl monochromator using the 1010 reflection has been used [32]. The resolution has been measured to be about 0.35 eV at 900 eV in agreement with previous measurements [33,34]. The X-ray absorption spectra were recorded by scanning the photon energy with steps of 0. 1 eV. The accuracy in determining the energy shift of the spectroscopic lines is halfthis value. In order to check the energy calibration a systematic measurement of the CuO sample was collected and the white line maximum was days foundoftomeasurements. be exactly reproducible through the five The energy scale was fixed by taking the white line maximum at 931.25 eV according to Koster [35]. The X-ray absorption has been measured by detecting the emitted electrons by a channeltron in the total electron yield mode. Repeated scans of the samples have been carried out to check reproducibility and to improve statistics. No deterioration of samples irradiation.has been observed under soft X-ray In our experiments which required between one and two hours the sample vacuum was between I O~ and lO_6 mm. Typically a sample was exposed to this pressure for a total time of about four hours. The features observed in our spectra were independent of scanning intervals. Just scraped prior totomounting, the surface of each sample was remove some face layers. No subsequent surface treatment was used. In total yield experiments such as this we es-
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timate the sampling depth [34] to be as much as 100 to 150 A.
[35] has shown that the white line moves toward higher energy with a decrease of ionicity. It is interesting to remark that this is in agreement with the fact that an increase in conductivity of a factor 10~ has been observed in going from Gd2CuO4 to
3. Results
La2CuO4 [36,37]. The spectrum of the trivalent compound La2Li05Cu05O4 in fig. 1 shows two lines (at 931.05 eV and 933.3 eV) having similar intensity and the same full width at half-height 1 eV, separated by ~E= 2.25 eV. The intensity ratio of these two lines does not change with repeated scans and gives confidenceThe about sample stability during the experiment. presence of two well-resolved sharp lines seems to be characteristic of insulating formally Cu trivalent compounds and in fact also the L 3 spectrum of (KCu (III)(biuret) 2) [16] exhibits two similar lines but the first one at lower energy and the other one at higher energy (933.6 eV) giving a larger energy separation between the two final states of ~E= 2.9 eV. We have found the presenceof two lines also in the L3 spectrum of NaCuO2 which is a third
In fig. 1 the spectra of formally divalent CuO, La2CuO4 and Gd2CuO4 compounds and of the tnvalent compound La2Li05Cu05O4 are shown. In the formally bivalent compounds La2CuO4 and Gd2CuO4 we observe a single strong absorption line as in CuO (called white line) and in other divalent Cu cornpounds as reported by Koster [35]. Assuming the 9 and 3d’°Lthe ground state to beto athe mixture of1°final 3d white line is due 2p state. This is 3123d a bound state formed by the strong Coulomb interaction, Qhd —.9 eV [17,23], in the final state between the 2p core hole and the 3d electron. In the Cu perovskites the 2p3123d’°final state is wider than in CuO and located at higher and lower energy. We observe that in going from the Gd to the La compound the energy of the main line increases by 0.2 eV. Koster JO
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Energy (eV) Fig. I. The L3 X-ray 1°final absorption state.spectra The spectrum of non-metallicbivalent of the trivalentCu Cu compound compounds:(d) (a)LaCuO, (b) La2CuO4 and (c) Gd2CuO4 show a single line due to the 2p3d 2Li05Cu05O4 shows two sharp, well-resolved lines at 931.05 eV and 931.30 eV final states, both full widths at half height being 1 eV, assigned to 2p3d’°Lfinal states. The integrals of the peaks have been arbitrarily fixed to be one.
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example of a formal trivalent copper compound [38]. The electronic structure of formally trivalent sys9L, 3d’°L2 tems3d8 is described by a for mixture the 3d(Cu0 5 and configurations a localofcluster 4) 9L. and 3d’°L2 The energy separation 3d required to configuration is given between by the the energy transfer an electron from the ligand to the Cu 3d level ts.E. The energy separation between 3d9L and 3d8 will .
be given by Udd AE. Assumingthat the strong white lines are due to 2p—+3d transitions the two lines can 9 final be associated with 2p3123d’°L and 2p1123d states. The energy separation is expected to be ~E= Q~ + Udd AE because of the larger final state electron—hole Coulomb interaction Qhd 9 eV acting on the 2p 10L. It is clear that the expected value for 3E is 3123d much larger than the energy separation of about 2 eV observed for the two lines in the trivalent compounds. Moreover in all these trivalent cornpounds both lines do not show the expected multiplet structure for the 2p3d9 final state which was observed in the X-ray absorption spectra of Ni2~ compounds (with 3d8 high spin ground state configuration [34]). Their energy positions and the lack of multiplets is in agreement with the assignment of both final states to 2p 10L configuration. There3123d fore in order to explain the presence of two lines it —
—
is possible that two types of holes, one more localized more originUtoand the another two peaks 2p delocalized 10L’ andL*2pcould give 3/23d 3/23dIOL*, respectively. The fact that the intensity of the two lines is of the same order of magnitude indicates that the two states are strongly mixed, In the antiferrornagnetic La2CuO4 (TN = 240 K) the transition to the 2p3d’°state is shifted significantly with respect to the CuO line to higher energy (0.15 eV) and the contribution due to the 2p3d’°L final state is not visible. We have investigated both superconducting and antiferrornagnetic samples of La2CuO4~and no appreciable difference has been observed between the different samples. This result is not surprising since only a very small variation of stoichiornetry (~y-.O.O3)is enough to induce the change from antiferromagnetic to superconducting behavior. In the superconducting compound, La1 85Sr01 5CuO4 (formal valenceand v=wider 2. 15),as the L3 absorption becomes asymmetric shown in fig. 2a. The main line remains at the same energy 288
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as in La2CuO4 and a broad high energy asymmetric contribution appears. This contribution is assigned to the 2p3d’°Lcontribution. In the semiconducting compound La 2 CSrCCuO4 (x=0.88, v=O.35), with formal Cu valence v=2.l8, the main line (at 931.2 eV) shifts by 0.2 eV to lower energy. Comparing our results with L 3 absorption spectra recordedby measuring the electron energy loss in thin films [39]threshold our spectra do not show anyeV spectral ture with in the region of 2.5 abovefeathe white line which is characteristic for the final state 2p3d’ 04~1 due to Cu + ions, formed by sample reduction [40]. This peak was observed in the energy loss spectra indicating the reduction of the sample under electron bombardment. The presence of Cu 3d8 in the initial state should give the final state 2p3d9. The Cu 2P2,3 XPS data of La 9 final states in the 1 85Sr015CuO4 2p3d The fact that no energy region of exhibit 942 eVthe [11,39]. spectral peak is observed in this energy region of the X-ray absorption spectrum, where the similar final state is expected, rule out the presence of Cu 3d8 configuration. In fig. 3 we show the difference spectrum between superconducting La 1 85Sr0 15CuO4 and La2CuO4 assuming that this difference gives the 2p3123d°Lcon9L states induced by Sr tribution arising from the 3d doping. The difference spectrum shows a broad, about 1.7 eV wide, asymmetric feature with a peak at about 932.2 eV. The peak of the 2p3/23d’°final state due to the initial state configuration 3d°is assumed to be that of La2CuO4 at 931.4 eV and is 1.45 eV wide. The energy separation between the 2p3123d” and the 2p3123d’°Lfinal states is a measure of the ligand ionization energy. The energy separation between the maxima ofthe two peaks is 0.8 eV but because of the band width of the two configurations (mainly of the 2p3123d’°Lconfiguration) there is no gap separating the two final states. This result indicates that there are ligand holes close to the Fermi level in the superconductor. Thence the metallic behaviour is indicated by the overlapping of the two configurations. The width of the absorption feature due to the the 1 °Lconfiguration is 1.7 eV, considering 2p3,23d instrumental broadening and the core hole life time, the intrinsic width is reduced to 1.3 eV. Assuming -
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Energy (eV) Fig. 2. The L
3 X-ray absorption spectra of the superconducting compound La,85Sr015CuO4 (formal Cu valence v=2.15) (curve (e)) and ofthe semiconducting La1 2Sr0 88CuO365 (v= 2.18) (curve (f)) are compared to the purely divalent La2CuO4 (curve (b)).
that is state, closethis to the width the 9L this in thevalue initial stateband crossing the of Fermi 3d level will be itinerant with a life time of about 10—’ ~ s. The experimental finding a many body configuration 3d°L indicates the of strong correlation between the localized Cu 3d9 and itinerant ligand holes and large hybridization between the oxygen 2p orbitals and the Cu 3d orbitals. The presence ofthe superconducting phase should be associated with the ligand hole pairing. If we consider the two-dimensional Cu0 2 network, starting with a probability of p, = 0.15/4 to have one hole into a Cu—O ligand (4 oxygen bonds per copper) the probability to get a second hole around the same copper sites given by a random distribution is p1(N— l)p,N/2 with N=4 for the square planar considered herein. Then the probability for pair 6p~) is having iSp, = a0.225 ofligand in the same siteone ( single ligand hole times theholes probability to get (4p,). The value of Udd of about 6 eV [171 indicates the presence of well localized magnetic moments on the
Cu site, which is inorder agreement with evidence of antiferromagnetic in non-superconducting La 2CuO4~.In superconducting Cu oxides, where the antiferromagnetic interaction established between 9 local moments an is ordering of the ligand the Cu 3d holes should be induced by the presence of the 3d9L. configuration and therefore the pairing of the ligand holes become possible. Therefore this experiment demonstrates that pairing of oxygen holes can be driven by ordering of local moments on the Cu 3d9 sublattice. Recently some models have been proposed for the microscopic mechanism for high T~ superconductivity determined by pairing of oxygen holes in an antiferromagnetic two-dimensional layer of CuO2 [31,32].
4. Conclusion Doping La 3+ 2CuO4 the with Sr (where formally ions are expected) holes are formed in theCuoxygen-derived band forming the many body configuration 3d9L in the ground state. In the 289
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Fig. 3. The L3 X-ray absorption spectra of the superconducting compound La, 85Sr,1 5CuO4 has been deconvoluted in two components: the 2p3d’°plus the 2p3d’°L configuration. The 2p3d’°L component (curve (g)) has been obtained by the difference between La1 85Sr0 5CuO4 and La2CuO4, assuming the La2CuO4-type profile for the 2p3d’°configuration also in the9L superconducting states in the ground Sr-doped state of the superconducting The spectrum of the formal insulating compound material. The spectrum material. of the 2p3d’°Lcomponent (curve (g))trivalent is assigned to the presence of La the itinerant 3d 2Li05Cu05O4 (curve (d)) is plotted here for comparison.
9 and superconductor the absence ofa gap between 3d 3d9L in the ground state leads to the metallic-like conductivity. The characteristic aspect of these states is that the Cu 3d states are localized but strongly hybridized with the oxygen 2p orbitals. The present results show that the presence of itinerant 3d9L configuration is characteristic of superconducting materials studied so far, both the class of La 2~Sr~CuO4 (x<0.2) and the previously studied YBa2Cu3O7~(0
to the Laboratoire pour I’Utilization du Rayonnement Electromagnetique (LURE) technical staff for their valuable aid. This work was supported by the European Community under CEE Contract STP0040-l-(CD). The work of J.I. Budnick carried out at the University of Connecticut is supported by the US Department of Energy under contract No. DE0S05-80ER10742.
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We would like to thank A. Kotani and G.A. Sawatzky for stimulating discussions. We are grateful
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