Cu 3d9 - LIGAND HOLE CONFIGURATION IN YBa2 Cu3 O 7 BY X-RAY SPECTROSCOPIES

Cu K edge and L3 edge x-ray absorption near edge structure (XANES) and Cu L3 x-ray photoelectron spectroscopy (XPS) of YBa2Cu307 have been measured to determine the local electronic structure. The K-edge XANES have been interpreted by multiple scattering approach. The data show the formation of the 3 d 9 ~ (where L is a hole in the oxygen derived band, ligand hole ) many body configuration in the initial state of the superconducting oxide. INTRODUCTION Recent experimental results obtained by x-ray spectroscopies have shown that in the new high Tc superconducting the Cu 3d states are localized in the sense that the intra-atomic Coulomb repulsion Udd-6 eV is greater than the band ~ i d t h ~ ~ . Therefore the strong correlation effects induce a distortion of the calculated one-electron density of states1'-l4 and they will influence the electronic properties of the new high Tc superconducting oxides. The preliminary point to be establish is therefore the origin of conductivity in these unusual metallic ceramic materials. In high correlated oxides with high electronic correlation like NiO, CuO ( and Ce02 and Pro2, where the localized states are the 4f states) systems the first available state above the ground state 3dn is the 3 d n + ' ~ configuration (where L is a hole in the oxygen derived valence band, ligand hole).15-l8 Therefore the gap in insulating systems is not of the standard Mott-Hubbard type( from 2 x 3dn to 3dn+l, 3dn-l) but of charge transfer type. The energy separation AE between the two ionic (without including the hybridization V) configuration 3dn and the 3 d n + ' ~ is therefore smaller than Udd ( AE < Udd) The 3dn+lL configuration has a large width due to the convolution of the oxygen valence band with the metal localized narrow level. The presence of a large covalent bond between the metal ion and the oxygen ligands is indicated by the large value of the hybridization V of the same order of magnitude as AE which determines the strong mixing between the two configurations i.e. the interatomic intermediate valent state of these systems. YBa2Cu306.5+x for x=O is a semiconductor with formally bivalent Cu ions. The transition from the semiconducting to metallic phase is observed for x larger than zero and the superconductivity critical temperature increases with x for Wxe0.5. In the many body description YBa2Cu306.5 (v=2) is expected to be a mixture of 3d9 and 3d1% configuration for the local cluster (cuo~)-~ . The additional oxygen x in YBa2Cu306.5+x should induce new states 3d8 and Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19879212 C9-1180 JOURNAL DE PHYSIQUE 3d9k, if the hole is created at the Cu or at the oxygen site respectively and 3d1 t2 if both holes are in the oxygen site for the cluster (cuo~)-~. The weight of these configurations depends on their relative energy position and hybridization. We repolt evidence for : a) no evidence of Cu 3d8 mfigufation from Cu K-edge XANES b) the formation of states due to a many body configuration with itinerant holes in the oxygen valence band coupled with localized 3d electrons on the Cu sites (called 3 d 9 ~ ) in the superconducting phase of YBa2Cu307 ; The conductivity in this highly correlated system is assigned to the presence of the electron-hole configuration 3d9k at the Fermi level and supemnductivity to pairing of the oxygen holes. Recently non conventional BCS theories of the high Tc oxides have been put f ~ r w a r d l ~ ~ ~ . Our findings give experimental support to the description of the superconductivity as pairing of oxygen holes 25.26. The samples have been prepared by starting from powder of ultra pure Y203, BaC03 and CuO using the standard p d u c e . The K-edge XANES experiment was performed at the Frascati synchrotron radiation facility using AWNE 1.5 GeV storage ring using a Si(220) monochromator. The XANES experiment has been carried out using as x-ray source the ACO storage ring LURE synchrotron radiation facility in Orsay. A double crystal Beryl 1010 monochromatar has been used. The x-ray absorption has been measured by detecting the emitted electrons by a channeltron in the total yield mode. The Cu 2p XPS spectra have been recorded using a Vacuum Generator ESCA LAB MKII system and the Mg Ka line as x-ray source. Fig. 1 Cu 2~312 XPS specmm of YBa2Cu30-7 (dotted line ) and the Cu 2 ~ 3 1 2 XANES spectrum of YBa2C~30,~ (solid line). The white line is due to the 2p3d1° final state and a new feature at -933 eV the same enery as the peak B in XPS spectrum &3d x i s observed. The presence of this XANES peak is associated with2p3d1% final state i.e. to the formation of 3d4, configuration in the ground state. The absence of a peak 2p3d9 at 942 eV in the XANES spectra indicates the absence of Cu 3d8 in the ground state. 930 (a5 0 gqS ENERGY (ev) RESULTS a) Evidence of 3d9-ligand-hole configuration Fig. 1 shows the Cu Lg absoqikion specrmm of a bulk YBa2Cu30,7 sample and its Cu 2pZl3 XPS spectrum. The Cu 2p312 XPS spectra of YBa2Cu30-7 shows two lines A and B, as in bivalent Cu compounds, assigned mainly to 2p3dy and 2p3d1% final states respectively. The white line maximum at the % absorption edge is found at 931.1 eV. The full width at half high is r= 1.25 eV. The white line is assigned to Cu a3d1° final state for the Cu 3d9 initial state. A broad shoulder indicated by the arrow in Fig.1 appears at about 933 eV on the high energy side of the white lme. The L2 edge shows a similar feature at about 1.8 eV above the white line. This feature is characteristic of supemnducrhg materials and it is at about the same energy, -933 eV, as the peak B in XPS spectrum due to a d 1 % final state. Therefore this peak is assigned to the transition from 3d9!, initial state to a final state &3d1°~ indicating the formation of 3 d 9 ~ state in the ground state. The energy separation between the two final state configurations 293d1° and&3d1% gives the ligand ionization energy. For the metallic superconductors no gap is observed between the two final states. The lack of a gap indicates that there are ligand holes close to the Fermi energy. Fig.2Cu K-edge XANES spectrum of YBa2Cu3q upper curve and calculated XANES spectrum using the multiple scattering approach from the two clusters having as central atom the four fold coordinated Cul (curve a) and the five-fold coodimted Cu2 in the Cu% layers normal to the c-axis (curve b) and their weighted sum (curve c). b) Lack of 3d8 (cu3+) configurationThe high oxidation state of Cu ions in YBa2Cu307 is usually expected to give cu3+, 3d8 states in the ground state. This configuration should give a 2p3d9 final state in the Cu L3 XANES expected at 942 eV. In fact the energy of this final state should be close to that of the peak A , &3d9, in the XPS spatnun. The absence of a peak at 942 eV in the XANES spectrum indicates the absence of Cu 3d8 states (cu33 in the ground state. Cu K-edge XANES of (Lal-xSrx)2Cu04 27-30 and of YBa2Cu307 31J5 have been measured by several groups. Controversial interpretations of the data have been reported in order to extract the Cu valence state. The main point is if the presence of cu3+ (Cu 3d8) can be inferred from this spectrum. We have performed a C9-1182 JOURNAL DE PHYSIQUE one-electron multiple scattering calculation in order to determine the role of the multiple scattering signal determined by the complex crystalline structure which can provide an explanation for the various spectral features observed in the spectra XANES calculations have been carried out using the multiple scattering program36 and the coordinates given by x-ray diffraction Previous K-XANES calculations for the La2Cu04 29 were limited to a small cluster of two neighbor shells around the central atom. We have extended the calculation to the Y-Ba -Cu oxide taking into account the orthorombic distorsion and the presence of two different Cu sites. A cluster of 40 atoms including four shells has been used in the calculation. In Fig. 2 we report the experimental Cu K-edge XANES spectrum (upper curve) and the calculation for the Cul ,and Cu2 sites and the weigthed sum to be compared with the experimental spectrum. In fig. 3 the non self-consistent XANES calculations for the site Cu2 are reported showing the effect of the size of the cluster. Fig.3XANES calculations of the Cu2 site in YBa2Cu3% by increasing the size of the cluster with central Cu ion, from a single shell cluster (lower curve) to a cluster of four shells (upper curve). Curve 1 : CuOg. Curve 2 : CuOgBaqYq .Curve 3 : CuOgBaqYq Cug .Curve 4 : CuOgBaqYq C u 6 0 ~ ) . It is clear that increasing the size of the cluster the agreement between the calculations and the experiment increases. Similar results have been obtained by P.J. Durham et al. using self-consistent calculations?' The agreement remains qualitative however the calculation gives account for al l experimental features, therefore no peak can be associated with the presence of cu3+ ions. These results are confmed by the observed spectral changes obtained by varying oxygen concentration. This experiment gives direct evidence that increasing oxygen content x>O in YBa2Cu306.5+x does not give cu3+ ions with Cu 3d8 configuration but gives the 3 d 9 ~ electronic configuration. These many body states have an energy width of about 2 eV being determined by the convolution of the oxygen 2p band and the Cu 3d states. These states fill the gap between the 3d9 and the 3d1% configuration of the bivalent compound YBa2Cu306,5 , In conclusion in order to interpret the x-ray spectroscopies of the superconducting copper oxides it is necessary to describe the ground state by many body configurations as shown in fig. 4. A ----------.3d8 -------------' 10 2 -, 3 d I, ----------'3d1 'I, / 3d9 -----------_ _ _ 3d9 \ Fig. 4 Schematic picture of the many body configurations for a ( ~ " 0 ~ ) ~ cluster in a bivalent Cu wm und (left side) and for Y B V u 3 O7 (right side) where additional states 3d%, 3d1q2 and 3d8 (dashed lines) for a cluster ( ~ 0 0 ~ ) ~ are formed. Our results show that the the itinerant states are not 3d hole in Cu sites, as assumed in the Anderson mode120,21, but are holes in the oxygen band. These holes are coupled with the localized Cu 3d electrons giving a single 3d91, many body configuration. The high Hubbard correlation energy Udd indicates the presence of local moments in the Cu sites that can give local antifemmagnetic order in the Cu02 planes which could induce the pairing between two holes25,26 AcknowledgmentsWe would like to thank A. Kotani , G.A. Sawatzky and X.Fontaine for stimulatingdiscussions and A. Scacco, C. Sanipoli and F. Stazi for technical help. 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