Evidence for charge redistribution below Tc in YBa2Cu3O7−δ

PflY$iCA

PhysicaC 191 (1992) 57-71 North-Holland

Evidence for charge redistribution below

in YBa2Cu307_6

J. RiShler ~, A. Larisch a n d R. Sch~ifer 1I. Physikalisches Institut, Universitiit zu KOln, Ziilpicherstr. 77, W-5000 KOln 41. Germany

Received24 September 1991

Wehavemeasuredthe temperaturedependenceofthe K absorption near-edgefinestructureof Cu in polycrystailineYBa2Cu3OT_6 between 47 and 300 K. Temperaturedependent differencesin the fine structure are resolvedwithin 0.2%. The most pronounced temperature dependence is observed at = I 1 eV, and is attributed to intensity variations of the out-of-plane transitions Cu(2) 1s~ 4p=n, and Cu( i ) Is~4pxn. At T¢ their average intensity exhibits a distinct minimum. Below T, the intensityof the transition Cu( 1) I s--.4px~ is found to increase by ---+ 1%, indicatingan increaseof electron densityat the Cu( 1) site. Weattribute the experimentalfindingsto a redistribution of charge between chains and planesbelowthe onset of superconductivity.The associated structuraldistortion is interpreted to increasethe symmetryof the Cu(I)02 ( 1)02 (4) rhombsin the dopingblock of the crystal. No evidenceis foundfor a bimodai axial latticeinstabilityacross the superconductingtransition.

1. Introduction X-ray absorption spectroscopy has yielded important contributions to the exploration of the electronic and local atomic structure of the high-To cuprate oxides. Numerous experimental [1-7] and some theoretical studies [8-10] have been dedicated to the fine structure at the Cu K absorption edge of YBa_,Cu3Or_6, 6 = 0 ~ 1. K (Is) core-electrons, excited beyond the absorption threshold, according to the dipolar selection rules, probe unoccupied states with p-type symmetry. Quadrupolar transitions into unoccupied d-states, if allowed, are very weak, only about 1% intensity of the dipolar transitions. The K absorption edges of the copper oxides and related materials exhibit a complex fine structure, which can be understood in terms of one-electron excitations into the molecular orbita!s of the nearest neighbor oxygen cluster. Manybody corrections to this picture might be necessary [ 11 ]. The typical two-fold coordination of monovalent Cu-compounds, and the typical square-planar, octahedral or pyramidal coordinations of divalent Cu-compounds leave their fingerprints in the K absorption edges. They may serve the detection of the E-mail address: [email protected]

nearest neighbor clusters and the corresponding copper valences. Apart from the different types of nearest neighbor clusters, [CuOn], n = 2 , 4, 5, 6, experimental information on the valence comes from the relative binding energy. Generally speaking, the core-level binding energy shifts by several eV to higher energies, as the effective charge, screening "he core-hole, decreases (cf. fig. 1 ). Since configurational changes of the Cu 3d-electrons affect the Coulomb screening of the p-type final states, their relative binding energy comprises information on the number of 3d-electrons. K absorption edges were extensively investigated to elucidate the relationships between the Cu valence (or Cu 3d-configuration ) and superconductivity. The variation of the Cu + 1fraction in YBazCu307_~ has been determined as a function of the oxygen deficiency, ~, from quenched powdered samples at 300 K [12], and in situ from both powders and single crystals, in high temperature oxygen atmospheres at different pa~lial pressures [ 3 ]. The intensity of the sharp ls--,4pn transition, emergi~lg at E - E o = 2.5 eV in monovalent Cu20 (fig. l ), rr~ay serve as a reference for the number of twofold coordinated Cu + atoms in YBaaCu307_a, 6 = 1-,0. From the apparent non-linear dependence of the Cu + ~fraction [ 12 ] on

0921-4534/92/$05.00 © 1992 ElsevierScience Publishers B.V. All rightsrcscrved.

58

J. Rrhler et aL /Evidence for charge redistribution

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Fig. 1. Normalized Cu K edges of Cu-metal (dots), Cu20 (short dashes), CuO (long dashes) and YBa2Cu3OT_6 (drawn line, at room temperature). The energy scale refers to the inflection point below the peak in the rising edge of Cu-metal (E0 = 8976.8 eV ). The bandwidth ofthe spectrometer is about !.5 eV.

5, one might get important experimental information on the configurations of the ordered and disordered oxygen at the chain sites [ 13 ]. The problem of trivalent copper has stimulated other experimental Cu K absorption studies [ 8,7 ]. The assignment of features in the K absorption spectrum to a fraction of trivalent copper (3d8), has been controversiaUy discussed, tt is now well established, that the fraction of nominal trivalent copper in the h-type high-To materials has to be attributed to a divalent Cu 3dgL - i ( L - t = ligand hole ) configuration, not to trivalent Cu 3d s. As proven by Cu L absorption [ 14] and EELS [ 15], the additional holes, introduced by oxygen doping, are transferred into oxygen p-bands, not into localized copper 3d-states. Even if trivalent copper (3d 8) existed in high-T¢ materials, it would hardly be discernible in the K nearedge structure. The relative binding energy of the oneelectron excitation, is~([3dg]4p) *, in Cu 3÷, turns out to be about 26 eV [16]. At high kinetic energy the inelastic scattering of the photoelectron increases dramatically the inverse lifetime of the photoelectron and thus smears possible discrete quasi-atomic excitations. In addition, they are merged with strong multiple-scattering contributions from the clusters, surrounding the mono- and divalent Cu-sites. The discussion on the "trivalent" resonance in the

Cu K absorption edges of the h-type high-T¢ superconductors indicated the limitations of the molecular orbital ansatz and strengthened the need for more exact approaches, e.g. multi-scattering calculations. Nevertheless, the excitations of Cu in its structurally well defined nearest neighbor environments may be safely attributed to one-electron transitions into molecular orbitals. Analyzing the spectra o f the layered high-To cuprate oxides, one can take advantage of the well defined types of nearest neighbor clusters, and of the pronounced structural anisotropy. Linearly polarized X-rays with the electric field vector ellc and ¢_1_c, are able to discern the excitation of states with p~- and po-symmetry from each other. Although the different types o f clusters around the Cu ( 1 ) and Cu (2) sites in YBa2Cu307_a complicate the analysis, most of the resonances on the rising edge can be assigned with the aid of polarized data [4,3] to one-electron transitions into molecular orbital states of the nearest neighbor [CuO,], n=2, 3, 4, 5, clusters. Two resonances were tentatively assigned to shake down charge transfer transitions, simply in analogy to the shake down satellites discussed in the Cu K spectra of Cu 2+ complexes [ 11 ]. Multi-scattering calculations [9,10 ] have been performed in the framework of the theory of Durham e t a ' . [ 17]. Agreement between the polarized absorption data and the calculations was obtained by extending the size o f the clusters to about 35 atoms and a to a spherical radius of 5 A. All transitions, experimentally found from polarized absorption, could be reproduced in a one-electron excitation scheme. In this paper we put emphasis on two items. ( 1 ) The low temperature investigation of the charge transfer between chains and planes and the associated structural distortions at fixed oxygen stoichiometry. (2) The investigation of a possible lattice instability around and below the onset of superconductivity. The relationships between the lengths of the axial bonds, the number of charge carriers, and the critical temperature have been intensively investigated at fixed temperature and variable oxygen concentration [ 18 ]. It is well established that the "short" axial bondlength of dcu~ ~.o~ increases as the oxygen deficiency, 5, decreases. The exact non-linearity of

59

J. Rrhler et al. ~Evidencefor charge redistribution

dc.(, ).o(4) versus

~ depends on the thermal history of the samples. Temperature dependent structural investigations of R E ( R = Y , Eu)Ba2Cu3OT_,~, performed with neutron diffraction [ 19,20 ] and EXAFS [21 ], indicated an anomalous increase of the average "'short" axial Cu( 1 ) - 0 ( 4 ) bondlength by up to =0.01 ~ at decreasing temperature from 300 K to -~ 100 K. This negative thermal expansion of the Cu( 1 ) - 0 ( 4 ) bond might indicate a temperature driven charge transfer at fixed oxygen concentration. The driving mechanism could be in the metastability of defect Cu( 1 ) O , ( 1 ) configurations [22]. If one increases the number of completely emptied and completely filled chain segments e.g. by thermal treatment (~ is kept fixed), one expects to increase the number of the residual Cu +~ atoms by destroying the instable "T-shaped" CuO~ ( 1 ) O2 (4) configurations. Here we assume that Cu( 1 ) is divalent ( 3d 9) in both, the "'T-shaped" and the square planar CuO2( 1 )O2(4) configurations. Thus the intensity of the residual Cu ~+ "'fingerprint" lines may be used to monitor the oxygen ordering at the chain sites. For example, we expect, that the residual Cu +~ "finger~ print" line increases slowly as a function of time in ultra-rapidly quenched samples [23]. Lattice instabilities around Tc in YBa2CuaOT_6 have been reported from many spectroscopic investigations, e.g. Raman [24,25], IR- [26], NMR/ NQR- [27] and EXAFS [28,29,30,31]. Less evidence for lattice anomalies around T¢ comes from non-local techniques as diffraction [ 32,33 ]. Ultrahigh resolution measurements of the macroscopic thermal expansion [ 34,35 ], however, revealed a clear structural anomaly in the a-b plane at T¢. The Cu K absorption edge and the extended absorption fine structure, have been studied across the critical temperature by Conradson et al. [ 36,37 ]. Differences of the absorption edges were extracted from pairs of spectra, recorded at temperatures between 10 and 150 K. The temperature dependent variations of the absorption edge were reported to be in the order of 2% and are attributed to a bimodal lattice instability (or anharmonicity ), centered at the bridging (apex) oxygen atom 0 ( 4 ) . In the direct vicinity o f the critical temperature the vibronically driven overlap between the "'long" Cu ( 2 ) - O ( 4 ) pairs is believed to become smaller (or more hmxnonic). Temperature dependent EXAFS measurements of polycrystalline [36]

and oriented grain samples [37] are stated to be consistent with this type of perturbation in the atomic and electronic structure. Bistable positions of the bridging oxygen, spaced by -- 0.1/~ along the c-axis, should be detectable also by high resolution diffraction techniques, at least appearing as an anomalous Debye-Waller factor. So far, a bimodal position of the bridging oxygen, or a related strong anharmonicity could not be confirmed with X-ray or neutron diffraction measurements [20,38]. We have measured the K absorption near-edge structure in YBa2Cu307_~ between 47 and 297 K. The subtle temperature dependent variations in the rising edge of the absorption were analyzed with the aid of the difference spectra.

2. Experimental details 2.1. Sample characterization

The nnlv rv~l..dlin~ vRa.(",.cI, ~ m n l a was prepared from the metal-citrates. The citrate route yields extraordinarily dense, small, and homogeneous~.y sized crystallites, which is of advantage for the preparation of thin absorption foils with randomly oriented material. The thermal, electrical and magnetic properlies of the sample were extensively studied and described in detail elsewhere [39]. The lattice parameters were determined to be a=3.86 A, b=3.88 A, c = 11.762 ~i, from X-ray powder diffraction at 300 K. From these data we estimate the nominal oxygen deficiency d~<0.1. The electrical resistivity dropped to zero at T¢ = 91.8 K and had a transition width of 0.9 K. The measurement of the Meissner-effect at 25 G gave 30% of the full - 1/4n flux expulsion. The specific heattemperature ratio versus temperature, C(T) / T, exh;h;t~,~ ~ ;,,,~n ~t 09 7 K about 2 K wide. The thermal expansion of a rectangular block ( --8 n,m long), cut from the pressed pellet, was measured ~ith a capacitive dilatometer between 80 and 110 K [ 39 ]. The thermal expansion-temperature ratio versus temperature, o~(T)/T, had a discontinuity at To, indicating a second order phase transition with a jump, A o ~ 8 × 10 -8 K -t. o

60

J. R6hter et al. /Evidencefor chargeredistribution

2.2. Spectroscopy 2.2.1. Data acquisition The absorber was prepared from the same pellet, used in the thermal expansion measurement. The pellet was carefully powdered with an agate mortar, sieved through a 5 pan-mesh, and homogeneously spread on a Kapton-tape. The absorption spectrum exhibited no measurable texture effects, confirming that the grains had been randomly oriented. The thickness was adjusted to give the absorption contrast at the Cu K absorption threshold, A/zd-~ 0.7. The temperature gradient across the foil was minimized by sandwiching the absorber between two berylliumfoils, pressed in a Cu-frame. The temperature was measured with a Si-diode, directly attached to be Befoils, and stabilized within + 0.2 K o f the nominal temperature. Two low temperature runs were carried out. In run 1 the sample was cooled down from 300 K to 47 K within 1 h. Subsequently it was warmed at a rate of 0.09 I~,/min and 12 spectra of the near-edge and extended absorption floe-structure were taken up to 111 K. Run 2 was started after further warming of the sample to 300 K within 5 h. Again the sample was cooled down to 53 K w'ithin 1 h and subsequently warmed up at a rate of 0.6 K/min. The X-ray-absorption spectra were recorded with synchrotron radiation at HASY-LAB, DESY (FRG) under parasitic conditions. DORIS lI operated at 5.4 GeV and with electron currents of typically 35 mA. Average lifetimes of the stored electrons were ~<1 h. We used the vacuum spectrometer EXAFS II (beamline E4) equipped with a focussing Au-coated premirror and a Si 111 double-crystal monochromator. The harmonics in the monochromatic beam were eliminated by detuning the monochromator to 40% of the maximum of the rocking curve. A piezoelectric feed-back system stabilized that adjustment of the monochromator. The position of the sourcepoint was vertically stabilized within + 20 p.m by the DORIS position control system, switched to the monitors in the beamline E. Both feedback systems reduced the fluctuations of the relative intensity by more than one order of magnitude. Low-noise spectra of both the superconductor and the Cu-metal calibration reference, are a necessar, prerequisite for a resolution of ~ 1%. The required

low-noise spectra o f the superconductor and also reasonably low noise spectra of the Cu-metal reference were only achieved with the relatively large energy bandwidth of A E = 7.5 eV. The corresponding loss of details in the fine structure is dramatic (of. fig. 2). In order to achieve the maximum sensitivity for the detection of the expected subtle intensity variations, we decided to tolerate the lack o f energy resolution. 2.2.2. Energy. calibration Since we had expected very small temperature induced energy shifts of the near-edge fine-structure, if any, we took special care of a precise relative energy 1,5

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Fig. 2. Top: Normalized Cu K edge of YBa~Cu30~_6at 300 K, and bottom: first (dashes) and second (drawn line) derivatives. Same energy -esolution as in fig. 1. Thin dashed vertical lines indicate the energy positions of the fine structures as determined from the minima of the second derivative. Position D (bottom) is split into two positions D and d (top). For the labeling and the assignment see text and table I.

J. RShler et al. ~Evidence for charge redistribution

calibration. To let relax the monochromator crystals close to their thermal equilibrium, the data acquisition was only started at --- 15 min after each new injection. Nevertheless, the nominal energy, read from the Bragg angle, was found to vary up to = + 1.5 eV. Three ionization chambers in line detected simultaneously the temperature dependent absorption of the superconductor and ofa Cu-metal foil at room temperature, serving as an energy calibration standard. The first inflection point of the Cu-metal absorption edge tsuperconductor at 47 K) was set to Eo= 8976.8 eV on the absolute energy scale. Note, that relative energy scales of Cu K absorption edges, displayed in the Eterature [4], are often calibrated according to the first maximum at the rising edge. The Cu-metal spectrum, traced simultaneously with the superconductor at 47 K served as a reference in run 1. All other Cu-metal spectra were adjusted to this fixpoint, u~ing a least-squares fit procedure. As shown in fig. z (left), we could reduce the uncertainty of the re!ative energy scale to ~<+ 50 meV.

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2.2. 3. Normalization Absorption differences can easily reveal subtle variations of spectra features, occurring just beyond the noise level of the measurement, which was adjusted to be in the order of 10 - 3 . It is a drawback of the difference analysis, however, that positional changes, variations of intensities or linewidths cannot easily be discerned from each other. Advantageously, difference spectra can be directly extracted from unnormalized raw data, provided the adjustments of the spectrometer (collimation, resolution, monochromator detuning, ionization chambers) have remained unchanged during the whole run, and the beam has penetrated an absorber of constant thickness. Absorption foils, prepared from powders, however, exhibit modulations of d. Applying standard preparation techniques, these modulations cannot be removed completely, at best reduced to + 5%. Practically one takes care of probing the very same region of the absorber. The raw data exhibited a monotonous decrease of the pre-edge background,/lbd, by about 5% between 47 and 300 K, and correspondingly of the absorption contrast, Aad= (#c-#b)d. /~c denotes the average continuum background. Because all other adjust-

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Fig. 3. Top: Normalized Cu K edge of YBa2Cu307_~ at 300 K, recorded with a bandwidth of -~ 7.5 eV. Bottom: first (dashes) and second (drawn line } derivatives. Dashed vertical lines indicate the positions of the fine structures determined in fig. 2. From the second absorption derivative (~, A, a), (B, C), (D, d, E) are grouped together in ,~, e, I), respectively (drawn vertical lines).

ments had been kept fixed, the background variation pointed to a temperature induced variation of the effective absorber thickness. Closer inspection of the cryostat revealed, that the temperature gradient across the stainless steel suspension of the sample holder moved the sample by about 0.8 mm away from the "warm" positie:L Therefore we had to normalize the raw data. Clearly, the resolution of absorption differences close to the noise level of 0.1% requires a careful normalization procedure. For two reasons one cannot simply maximize the overlap before and after the edge. ( l ) The thermal disorder and lattice expansion modify drastically (i.e. on the 0.1% scale) the XANES or EXAFS features, usually chosen as a ref-

62

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Fi~. 4. K spectra of Cu-metal used as a reference for the energy .Ion of the temperature dependent spectra of the super~onductor (run 1). Right hand side: 13 calibrated Cu-metal references (drawn lines) plotted together with the highly resolved Cu-metal spectrumshown in fig. 1 (dotted line). Lefthand side: Expanded view of the reference spectra at the rising edges in the vicinity of Eo. The uncertainty of the relative energy calibration is within + 50 meV. erence for the average continuum absorption. These effects may introduce considerable errors, particularly in the amplitudes of the difference spectra, taken at high temperatures. Figure 5 exhibits A # x ~ d ( x ) = (/tc-Pb) x ~ d ( x ) versus P b X ~ d ( x ) of run 1.5d denotes the thickness variation relative to the thickness o f the reference absorber at 47 K (run l ). x is the actual and temperature dependent position of the beam on the absorption foil. g ~ S x d ( x ) was determined from the increment o f a second order polynomial, #c~5X d ( x ) from the average high energy absorption (60-100 eV), after having removed the extrapolated pre-edge background (fixed linear and squared coefficients). The experimental points are expected to be on a straight line, given by (/it/ /~b) -- 1. The least squares fit to the low temperature points (47 K-101 K) gave (P¢/#b) - 1 =0.48_+0.01, in good agreement with (#c/#b)--1 =0.475, found from the effective mass absorption coefficients of Cu in YBa2Cu307 determined from the tabulated numbers. Within the error of the fit the high temperature data points (104, I I I , 150, 297 K) turn out to be on the straightline as well. The fit confirms that the continuum absorption has been correctly determined, and that the variation of the absorption con-

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Fig. 5. The K absorptioncontrast, AgX 6d, of Cu in YBa2Cu3OT_~ vs. the pro-edge absorption/~bx 5atas measured between 47 and 297 K (run I ) at different thicknesses, d. The straight line connecting the data points with the slope (/k/ttb) -- 1=0.475, has been obtained from the absolute K absorption contrast of Cu in YBazCu3OT_6as calculated from the tabulated numbers of the mass absorption tA/P of the continuum and the pro-edge mass absorption #b/P.

trast results from varying absorber thicknesses, not from instabilities of the spectrometer, e.g. a varying content of harmonics. Thus we can definitely rule out, that experimental artefacts have produced the temperature dependent amplitudes of the absorption differences, displayed in figs. 6 and 7. (2) The normal thermal expansion of the lattice may introduce a "volume effect", which modifies the intensity of the continuum transitions even at constant d ( x ) . In solids under high pressure [40] sizable variations of the continuum step have been observed as a function of volume. As is well-known, the free electron density of states, N ( E ) , varies with volume pc V -2/s. Therefore, according to Fermi's golden rule, the intensity of the transitions at high kinetic energies is expected to decrease as the volume expands. The relative volume, AV(T) / V, of YBa2CusOT_a increases from 4 to 300 K [19] by ~ 7 X I 0 -3, decreasing the average intens ~ty of the continuum transitions by ~<0.4%. Betwe~::n 47 and 150 K this "volume effect" is within the noise of the measurement and was neglected. No correction was made for the data at 297 K.

J. RShler et al. ~Evidence for charge redistribution

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Fig. 6. (a) The normalized C u K edges of YBa2Cu307_6 recorded between 47-300 K (run 1 ). 13 spectra are plotted together. Same energy resolution and labeling as in fig. 3. The vertical lines at high energies indicate minima and maxima of the EXAFS. (b) Absorption differences of the temperature dependent spectra shown in (a). The spectra at higher temperatures were substracted from the 47 K spectrum. Top: Difference between the 48 K and 47 K spectra, exemplarily demonstrating the noise level. The thick drawn line was obtained from Gaussian smoothing of the noise difference. The average mean square deviation is determined to be + 0.02. Bottom: ! 3 smoothed absorption differences of run 1 (47, 48, 51, 61, 70, 80, 85, 91, 93, 97, 104, 11 I, 297 K). The pronounced double minimum at (: deepens with increasing temperature from 48 to i04 K. The two pronounced maxima at high energies are due to the thermal damping of the EXAFS.

3. Results

Figure 1 compares with each other highly resolved (AE--1.5 eV) K absorption edges of Cu in monovalent CuO2 (short dashes), divalent CuO (long

Fig. 7. Expanded view ofthe absorption differences shown in fig. 6(b), bottom. Drawn lines are T~< T¢, dotted lines are at T>.>-T¢. The vertical lines at the top marked with ~--,F indicate the positions of the fine structure extracted in fig. 2 and listed in table I. The vertical lines at the bottom marked with A, ¢:, I5 indicate the average positions of the fine structure extracted in fig. 3 and listed in table 1. The positions ofthe minima at c move to higher energies from 48 K to 104 K. The energy shift of the maximum I) to higher energies is indicated by the dashed vertical line at 15.

dashes), elemental Cu (dots), and Y B a 2 C u a O T _ a (drawn line). Note in the spectrum of Cu20 the sharp resonance (Is-~4pg) at E - E o = 2 . 5 eV, which may serve as a fingerprint of the nearest neighbor dumbbell Cu-O configuration of Cu +t (3dl°). The spectrum of divalent (3d 9 ) CuO is shifted by several eV to high energies. It exhibits a shoulder at 5 eV, and a broad maximum at about 17 eV. This spectrum may be taken as a fingerprint of square-planar coordinated Cu +2. The shape and the energy position of the Cu K absorption features in Y B a 2 C u 3 0 7 _ , 5 a r e very. similar to that in CuO, coarsely confirming that Cu is square-planar coordinated in the fully doped superconductor. In fig. 2 we resolve the detailed fine structure of YBa2Cu3OT_6 with the aid of the first (dashes) and second (drawn line) absorption derivatives. The energy positions of the fine structure are determined from the dips in the 2nd derivative and indicated by

Z ROhler et al. / Evidence for charge redistribution

64

Cu( 1 ) ls--,3d. Label (F) at 24 eV denotes the resonance, controversally assigned either as due to a oneelectron excitation trivalent Cu 3d s [ 8 ], or to a CuO bond indicating Cu(Y) ant±site disorder [41 ], or to multiple-scattering [9] in an extended cluster [ Cu ( 1 )O, ], n >> 6. The analysis of our temperature dependent data will focus on the excitations occurring between 0 and 20 eV. (A), (B) and (E) denote excitations ofCu( 1 ) and Cu(2) in oxygen depleted YBa2Cu3OT_,s, ~-~ 1. In a nearly full oxygenated sample, ~--,0.1, these excitations are expected to be very weak. (A) is situated close to the quadrupolar excitation (¢) and is not resolved in the second absorption derivative. Traces of (B) and (E), however, may be identified from the subtle shoulders close to the minima (c) and (D),

the dashed vertical lines. The assignment of the transitions ( ~ ) - ( F ) is discussed in many experimental studies and in particular determined from polarized measurements of samples with different oxygen concentrations. The resonances between 0 and 20 eV are listed in table 1 and are assigned according to several sources. The upper tableline lists the orientation of the electric field vector, e, relative to the crystalline c-axis in the experiments of Tolentino et al. [ 3 ] and Heald et al. [ 4 ]. The next tableline lists the excited molecular orbitals together with the labels used in the literature [ 3,4,36,9 ]. The transitions excited with c.l.c are labelled with the capital letters (A), (B), (D), (E), (F); the transitions excited with ellc have small letters (a), (b), (c), (d) [3]. (~) at - 1 . 4 eV denotes the weak quadrupolar transition

Table 1 Energy positions in eV ofthe fine-structures in the Cu K edge of YBa2CusOT_a from different sources. All numbers refer to the inflection point below the peak in the rising edge of the Cu-metal (cf. text ). The energy positions are determined from the first or second absorption derivatives. Numbers given in parentheses indicate relatively weak intensities. Figures 2 and 3 refer to this work. Refs. [ 3 ] and [ 4 ] refer to polarized absorption studies of a single crystal by Tolentino et al., and of oriented grains by Heald et al., respectively. Ref. [ 9 ] compares results obtained by Garg et al. from multi-scattering calculations (The.) to experimental data (Exp.) from a polycrystalline sample. Multi-scattering calculations by Della Longa (ref. [ 10] ) of the Cu( ! ) c_l_c absorption in YBa2Cu3OT_6, d/m0 and I have yielded the positions and relative intensities of the maxima and minima observed in the difference spectra of ref. [3], Ref. [36] refers to the work ofConradson et al. Ref. [3] [41 [36] [9] Fig. 2 Fig. 3 [3] ± [3] IJc)

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a) Ref. [ 3 ] assigns the position of the maximum for both polarizations to D. b~ Ci and CIM~ in ref. [9] assign the same resonance Cu2+px,~. Ct refers to the maximum, C~Ma~to the node in the first derivative. The more accurate position is obtained at C~M~. ¢~ Polarization vector 30 ° offthe c-axis. d) Read from fig. 2 in ref. [4] and shifted by t 2 eV. "~ Read from fig. 1 in ref. [36].

Z R6hler et al. / Evidencefor charge redistribution

(d), respectively. The excitation (A) at about l eV is attributed to the characteristic ls-,4prc transition in dumb-bell coordinated Cu +1. It is seen to be shifted to low energy from the position of the "fingerprint" transition in Cu20, most probably due to the additional screening of conduction electrons in YBa2Cu30~_~. Excitation (B) has been always detected simultaneously with (A). From comparative multi-scattering calculations [ 10 ] of the near-edge structures in fully oxygenated ( t ~ 0) and oxygen depleted (t~= 1 ) YBa2Cu3OT_6, both excitations, (A) and (B), have been found to be associated with structural changes in an extended cluster (including the Ba atoms) surrounding Cu( l ) ( e l c ) . Label (E) denotes the Cu ( 2 ) 1s-,4lX~x,y transition (absorption maximum) in oxygen depleted and semiconducting YBa2Cu30@ The absorption maximum is observed at 17.1 eV. Both labels, (D) and (d) (split by about 0.7 eV), denote transitions into unoccupied 4p-states with ~symmetry. The splitting into (D) and (d) is not resolved in our unpolarized data. The strongest excitation (D) arises predominantly from transitions into the 4px,yt~ states of the square-planar coordinated Cu (2) sites. (D) might comprise also transitions into Cu(l )4pra-states along the Cu(1 )O2(1 )-chains. Polarized X-ray absorption measurements of untwinned samples are expected to discern different intensities of the Cu(2) ls~4px~ and the two superimposed transitions Cu(1), Cu(2) ls--,4pyc~. Such experiments, however, were not yet performed. Label (d) denotes the transition Cu( I ) ls--,4p.~. Since the superconducting Cu (2) 02 (2) 02 (3)-planes are oriented perpendicularly to the doping Cu(1)O2(1)O2(4)-plane, (D) is found to be strongest on excitation with e_l_c, but (d) on excitation with ~llc [4]. The Cu( 1 ) plane in the doping block of the crystal comprises the "short" (d= 1.86 A) bond Cu( 1 ) - 0 ( 4 ) , which links the bridging oxygen with the Cu( 1 )02( 1 )-chain. According to the k × d = c o n s t , rule [42,43] (k denotes the wavenumher of the photoelectron, d the bond length), the excitation energy of the transition Cu( 1 ) ls--,4p~6 is seen at higher energy than the transition (D) into the molecular orbital states 4px,yc of the "middle" bonds ( d = 1.94 A). The transitions on the rising edge are labelled (a), (b) and (c). All three of them are deafly resolved

65

in our data. Polarization dependent studies [4,3 ] find them to be strongest on excitation with ~Hc and have them assigned to final states with l~-symmetry. The strongest feature (c) gives rise to the shoulder at 12 eV in fig. 2 (top). It can be safely attributed to the one-electron transition Cu(2) ls~4pg.., i.e, out-ofplane. The shoulders (a) and (b) seen around 5 eV are controversially attributed either to Cu(2)Cu(1) ls--,4p*~ shakedown charge transfer satellites (the asteriks ~enotes a many body excitation) [3,4,2], or to one-electron final states delocalized over a larger duster than considered in the molecular orbital approximation [ 9,44 ]. Figure 3 shows the same spectrum as in fig. 2, however, recorded with low resolution (AE_~ 7.5 eV). Clearly, the fine structure visible in the highly resolved spectrum (AE-~ 1.5 eV) is almost wiped out. The absorption maximum is seen damped and shifted from 17.1 eV to 18 eV. From the absorption derivatives, however, we still resolve on the rising edge fine structure, labelled (A) and (C). As indicated by the 7.5 eV wide horizontal bars, we have grouped together the excitations seen in fig. 2 (dashed vertical lines) into the broadened lines: (A), (C), (IT)). (~), (A), (a) concur with (A); (B), (c) with (~); (D), (d), (E) with (15). Note that (~) and (D) interfere with each other. As discussed in section 2, we had to take into account this dramatic loss of energy resolution in order to gain high intensity resolution between the temperature dependent absorption differences. The effect of temperature on the near-edge fine structure E - E o = - 4 0 - 1 0 0 eV is displayed in figs. 6(a) and (b). Figure 6(a) exhibits the 13 spectra recorded in run l between 47 and 297 K. They have been normalized as described in section 2. The vertical lines between 0 and 20 eV indicate the positions of the averaged excitations (A), (~) and (I)) displayed fig. 3. The vertical lines at high energies indicate wiggels of the extended absorption fine structure (EXAFS). Figure 6 (t3) shows the absorption differences [/~d(48 K ) - , u d ( 4 7 K)] to [/~d(297 K) -/1d(47 K) ]. The 47 K-spectrum was subtracted from the higher temperature spectra. For the sake of clarity the data points were averaged with a standard smoothing procedure, as examplarily demonstrated in fig. 6(b) (top) for [/~d(48 K) - I u / ( 4 7 K) ]. The average noise level is read to be within + 0.002. The

66

t. ROhler et al. / Evidence for charge redistribution

absorption differences at high temperatures (cf. fig. 6(b) (bottom)) exhibit a clear minimum o f up to - 0 . 0 1 at about 11 eV, i.e.. dose to the position (C). A weak and nearly temperatare independent shoulder is seen at about 2 eV, i.e. close the position (A). The minimum at (~) deepens as the temperature increases from 47 K to ~ 100 K. At ambient temperature the intensity at (~) turns out to be weaker than at about 100 K. The absorption differences seen close to the absorption maximum (15) are positive at all temperatures and increase by about +0.005 at high temperatures. The EXAFS at 297 K are strongly damped, mostly due to the thermally excited disorder at high temperatures. Figure 7 magnifies the temperature dependent absorption differences between - 10 and 30 eV. Drawn lines indicate data at T~< To, dashed lines data at T> T¢. From the thick line connecting the minima at (e) it is deafly visible that its energy position shifts to high energy at T> T¢. The effect of temperature on the intensities o f the shoulder at (A) is weak, within the noise level of + 0.002. Close to (IS)) both relative intensities and positions move with temperature. Seemingly, the temperature dependence of the nearedge structure at (F) interferes with the temperature effects at (13). Figures 8 and 9 display the temperature dependences of the relative intensities and positions of the minima at (e) and the maxima at (IS)), respectively. The numbers are read graphically from fig. 7. We find the absorption at (~) to decrease between ambient temperature and _ 100 K, and to increase between = T¢ and 47 K. The position of the minimum at (~) is temperature independent between I00 K and 297 K, but decreases by - 1 eV at T< T¢. The temperature effects visible at the position (I)) are less definite. Here the intensity decreases weakly by 0.004 + 0.002 at T< T¢ and remains nearly constant between 297 K and T~. The positional variation of ( I3 ) has been resolved more clearly. At T< T¢ we observe an energy shift by about - 2 eV.

4. Discussion

We believe that the most important result has to be seen in the temperature dependence of the min-

i

i

¢'3 x

d--5 v

ol e--

E -!0 T

,

-15

I'Cl

,

100

I

200

,

300

Temperature [K) %, 16

!'

OJ

~

~

12

.~

O

~

8

i

0

I

100

n

|

200

300

Temperature [K] Fig. 8. Top: the intensity of the absorption differences at position e as a function of temperature. Closed circles are from run 1, open circles from run 2. Same error bars as indicated in fig. 6(b). Bottom: temperature dependence of the energy position of the dip 4. Error bars are estimated to be + 0.5 eV. The drawn lines are guides to the eye.

imum labelled (~) in figs. 7 and 8. The absorption features close to (~) exhibit a distinctive anomaly at T¢, indicating a correlation with the onset of superconductivity. Clearly visible, the spectral weight o f (e) increases below T¢ by ~0.01, well outside the error bars. We have shown in section 1, that the systematic errors, possibly introduced by the normalization procedure, are within the statistical noise o f +_0.02. The same has been verified for the uncertainty of + 50 meV arising from the calibration o f the relative energy scale. We attribute the temperature dependent absorption seen at (e) to the transitions C u ( l ) ls-,4pxn, and Cu(2) Is-,4pzn, labelled (B) and (e), respectively. Both, molecular orbital and multi-scattering calculations, have shown that the fine structure on the Cu K absorption edge is intimately connected with the size and geometry of the nearest neighbor clusters around Cu. Therefore we start to discuss the observed temperature induced variations in terms of

J. R6h ter ~t al. / Evidence for charge redistribution

~I

10

0 x A

5

~c -

0

t-

100

200

300

T e m p e r a t u r e [K]

24 ~- 20

.o 4,--'

~ ~6 i

'

I

,

I

100 200 Temperature [K]

,

300

Fig. 9. Top: t h e i n t e n s i t y a b s o r p t i o n differences at p o s i t i o n 1) as

a function of temperature.Closed squares are from run 1, open squares from run 2. Same error bars as indicated in fig. 6(b). Bmtom: temperature dependence of the energy position of the maximum I). Error bars are estimated to be _+2 eV. The drawn lines are guidesto the eye. structural distortions o f the nearest neighbor clusters. Possible relationships between distortions of the local structures and the electronic structure will be addressed further below. The temperature dependence of the various Cu-O bond lengths is well established from diffraction data with a resolution of up to + 0.005 A. No structural anomalies were detected at To. The "large" axial C u ( 2 ) - O ( 4 ) bond contracts by - 0 . 0 2 2 A, between room temperature and 100 K and is nearly independent on temperature ( ~<0.002 A) between 100 K and 4 K [ 19,20]. The "short" axial bond Cu( 1 ) - 0 ( 4 ) turns out to be nearly temperature independent between 300 K and 4 K. A weak increase by = + 0.005 ..,u,.~v 1. ~,~,.,v ..... experl,-~ is ,er, o,~,~, by Kwei ~,~ ~" al. traQ ",n~ ments [21] report -~ +0.01 A. or temperature independent behavior [ 5]. We believe, that the normal thermal expansion of the C u ( 1 ) - O ( 4 ) bond reported by Maruyama et al. [29 ], most probably is due to correlations with the "long" C u ( 2 ) - O ( 4 ) bond in their EXAFS multi-shell fitting procedure. The temperature dependence of the basal bond lengths is generally reported to be very weak and close

67

to the experimental resolutions [ ! 9,5 ]. The average thermal expansion of our polycrystalline sample [39], however, was found to be anomalous at To. We have evidence that the observed second order phase transition occurs preferentially within the a-b plane, since the polyerystalline sample was mounted in the capacitive dilatometer with the measurement direction perpendicular to the pressing direction o f the original powder. Recent thermal expansion measurements of untwinned single crystals [35] detected distinctive second order thermal expansion jumps only within the a-b plane, not along the c-axis, t~, measured along the c-axis, changed its slope at T¢ by 2.2× 10 - s K -2. Very interestingly, the values of Acta.b were found to exhibit inverse signs and are one order o f magnitude larger than Aot, extracted from the measurement of our unoriented or from another measurement of oriented grain samples [ 34 ]. The transition Cu(2) ls-~4p=~ may be taken as a probe of the geometry and the length of the "long" axial bond Cu ( 2 )-O ( 4 ). Due to the relatively large absolute length of -~ 2.3 A, its sensitivity is low. The large thermal contraction by - 0 . 0 2 2 A, however, is expected to increase the overlap between the Cu(2) 4p=~- and O(4)pz-orbitals and thus to delocalize the Cu ( 1 ) l s4p final-state. Transitions into localized states show up intense quasi-atomic resonances; transitions into delocalized states exhibit broadened absorption features [45 ]. Therefore we expect the intensity of the transition Cu (2) I s--, 4p=~ to decrease with the thermal contraction between 297 K and 100 K. From fig. 8 we read - 0 . 0 5 for T:> T < 300 K. The position of the minimum (~:) remains unchanged within the scatter of the data. From comparison of the high temperature (T> 100 K) with the low temperature behavior one could naively conclude, that the bridging oxygen O (4) starts to move apart from the superconducting planes at T < T¢. If we had simply scaled the intensity variation at T< T~ with the variation of the Cu ( 2 ) - O ( 4 ) bond length and with the absorption at T> T¢, dc~2)-o~4) would have been found to expand drastically by -~ - 0 . 0 5 A below T:. Such a strong effect, if it is really existent, should have been unambiguously extracted from diffraction. To our knowledge, this has not been the case. The remarkable variation of the absormion feature at e finds a straightforward explanation through

68

J. R6hter et al. / Eridence for charge redistribution

the temperature de r~zJndence of the transition Cu( 1 ) Is--,4p.jL labelled (B) and buried at about 9.5 eV m the broad minimum (~). Due to its short al~soh~te length o f ~ 1.85 A, even subtle variations o f the axial "'short" bond Cu ( 1 ) 0 ( 4 ) are expected to affect the transition Cu( 1 ) l s ~ 4 p x ~ Note that the Cu( 1 ) I s-,4p~,,x outof-plane transition is directed along the crystalline aaxis. Thus the bulk orbital overlap with ~he bridging oxygen occurs via the O ( 4 ) p ~ - s t a t e s . The v,v,ak negative thermal expansion of the C u ( 1 ) - O ( 4 ) bond, reported in the literature, is expected to increase the atomic-like character o f the transition Cu( 1 ) ls-*4p~, i.e. to increase its intensity as the temperature is lowered. Therefore we state, that the normal thermal contraction of the unit cell between 297 K and 100 K has opposite effects on the intensities of the Cu( 1 ) and Cu(2) out-of-plane transitions, associated with the "'long" and "'short" axial bonds. Low temperature delocalizes the final state (c), Cu(2) ls4p=n, but localizes the final state (B), C u ( l ) ls4p~. We find from fig. 8 (bottom), that the net effect on the position of (e), given by the weighted average of (B) and (c), turns our to be zero at T¢> T~<300 K. The effects of intensity variations, however, do not compensate completely. At T< T~ both position and intensity indicate a s',rong transfer of spectral weight to the final state (B), Cu(l ) Is4p~n. Between T~ and 47 K the position shifts by - 1 eV and the intensity increases by +0.01. In terms o f an effective charge, acting on the photoelectron in the Cu( 1 ) 4p~n-state, the increasing intensity below T¢ must be attributed to an increase of charge at the Cu( 1 ) site. Introducing into the discussion the subtle contraction of the Cu( 1 ) - O ( 1 ) bond lengths along the b-axis and simultaneous expansion of the a-axis [35 ], we may state, that the Cu( ! )O2( 1 )O2(4) plane lowers its rhombic distortion at T¢, (cf. fig. 10). We do not known to which extent the "short" axial bonds expand as the "middle" chain bonds shorten. However. from the difference in the absolute bond lengths, we expect that the "'short' axml bonds are affected more strongly than the "middle" chain bonds. The "'long" axial bond C u ( 2 ) - O ( 4 ) might contract as the "'short" axial bond C u ( l ) - O ( 4 ) expands, or may remain unchanged. In the latter case the total axial bond C u ( 1 ) - C u ( 2 ) would be ex-

/ -Cu(?_.)

/I

IJ /

0(1)--..

'/

0(4)

/ / Fig. 10. The nearest neighbor rhombic configuration of oxygen

around Cu( 1 ) in YBa:Cu3OT_~ The arrows indicate the structural distortion discussed to be compatiblewith the observed transfer of spectral weight to the Cu(l ) ls-,4px~ transition below the onset of superconductivity. panded and the Cu(2) atom pushed into the wavy C u ( 2 J O 2 ( 2 ) O 2 ( 3 ) planes. If oxygen disorder-order processes had increased the number of dumb-bell coordinated Cu ( i ), then the "fingerprint" transition (A) at 1 eV had been seen to be increased simultaneously with the excitation (B). It is tempting, to identify a temperature dependent variation of residual Cu +t contributions close to the position of the shoulder (~,). However, due to the lack of resolution in energy and intensity, we do not discuss the temperature dependence o f this absorption feature. The variations of intensity and position seen at ( D ) in fig. 7 are plotted versus temperature in fig. 9. Both intensity and position exhibit an anomaly at T¢. Contrary to (e), the intensity of (1)) decreases below T~. The po~ition, h,~. . . . . . . h;r.~ in the same diredion as (~) does. Due to the lack of energy resolution (~) interferes with (I)), on the one hand, and (I)) interferes v,fith (F), on the other hand. Thus the temperature effects seen at (I)) must be partially attributed to the overlap with the neighboring excitations, and no further conclusion can be drawn here. So far we have discussed a model of structural

J. RiJhlet et al. / Evidence for charge redistributie-,

changes, compatible with the observed temperature dependence of the absorption fine structure and connected with the onset o f superconductivity. The related changes o f the electronic structure may be regarded to be consistent with the mechanisms of charge redistribution below To, recently proposed by Khomskii and Kusmartsev [46 ]. These authors show that the onset o f superconductivity enhances the charge transfer between planes and chains. The number of holes in the superconducting planes, nh, is expected to follow the relation: nhOt:(A/EF) 2. A denotes the superconducting gap and EF the Fermi energy. The relative amount of charge redistributed below Tc is estimated to be ~<1%. The associated structural distortions are therefore expected to be extremely weak. We coarsely estimate them to be ~<1% of the bond length variations occurring on oxygen doping, i.e. in the order of 1 X 10 -3 A. The weakness of the effects may explain the absence of structural anomalies at T¢ in the published diffraction data and other structural studies. At the present state of art the near-edge absorption spectroscopy does not yield quantitative information on bond lengths variations as EXAFS might do. The extreme sensitivity of multi-scattering processes to geometrical distortions and orbital symmetries, however, shows up effects of structural distortions more clearly at low /<- than at high k-numbers. Nevertheless, a thorough analysis of the EXAFS is expected to give compatible results [47 ]. Our data indicate an axial lattice instability below T~. An axially centered lattice stability around T¢ has been derived by Conradson and Raistrick [ 36 ] from the Cu K X-ray absorption near-edge and in addition from an analysis o f the beats in the extended fine structure [37,30]. From the present investigation we can confirm their general finding that low temperature affects the intensities of the out-of-plane transitions Cu(l ) ls~4pxn and Cu(2)ls--*4p=rt. These and ~ : " "-" transitions are labelled by ~onl~td~uit ...... ~,aistn~r, [ 36 ] "'C" and "D", respectively. In our data the contribution of "'C" emerges more clearly at T< T~. We do not agree, however, with their relative amplitudes and, most importantly, with the temperature behavior reported by them around T~. The intensities of "C'" and "D", are claimed to be strongly enhanced at T= T~, but markedly damped within the temperature range of the superconducting fluctuations just

69

above and below T¢. An interpretation was given in terms of an anharmonic (bistable) position o f the bridging oxygen O (4). Anharmonicities involving the bridging oxygen 0 ( 4 ) are considered to be of crucial importance in some models of the microscopic mechanism of high-temperature superconductivity [48-50]. We do not find evidence for this type o f lattice instability around To. At present we do not have a conclusive explanation for the apparent discrepancy between our resuits and the results of Conradson and Raistriek [ 36 ]. A possible source of errors might be in the procedure chosen in ref. [36] for the correction of variations in the background and for different absorber thickness. Further experiments might be necessary to elucidate the origin of the discrepancy. The analysis of our EXAFS data will be presented in a forthcoming paper [47].

5. Summary In conclusion, we have found that the fine structure at E - Eo-- 11 eV on the rising edge of the Cu K absorption exhibits variations of intensity and energy position of -- 1% between 47 and 300 K. The temperature dependent excitations have been attributed to the out-of-plane transitions (B), Cu( l ) ls-,4pxn, and (c), Cu(2) ls~4pzr~. The decreasing intensity between 300 K and Tc has been ascribed to the normal thermal contraction of the "'long" axial bond linking the superconducting plane and the bridging oxygen. The monotonic increase of intensity below To, and the associated shift of the absorption at (4) to low energy have been attributed to a transfer of spectral weight to the transition (c), Cu( 1 ) ls-,4pxn. It is interpreted to be due to an increase of electron density at the Cu(l ) site. The associated structural distortions concern the bonds within the Cu( 1 )O~( 1 ) 0 2 ( 4 ) configuration in the doping block of the crystal. They are estimated to be in the order of only 1X 10 -3. The results give evidence for a redistribution of charge between chains and planes in YBa2Cu3OT_~ below the onset of superconductivity.

70

J. Rfhler et al. / Evidencefo, charge redistribution

Acknowledgements Thanks are due to the staff of HASYLAB and DORIS for prodding izeam time and their hospitalit),., H. General for preparing the sample and H. Broieher for the thermal expansion data. Fruitful discussions are acknowledged with A. Bianconi, A. Fontaine, and in particular with D.I. Khomskii.

References [ 1] A. Bianconi, A. Congiu Castellano, M. De Santis, C. Politis, A. Marcelli, S. Mobilio and A. Savoia, Z. Phys. B 67 (1987) 307. [ 2 ] F. Baudelet, G. Collin, E. Dartyge, A. Fontaine, J.P. Kappler, G. KrilL J. Jegoudez, M. Maurer, Ph. Monod, A. Revcolevschi, H. Tolentino, G. Tourillon and M. Verdagner, Z. Phys. B69 (1987) 141. [3] H. Tolentino, E. Dartyge, A. Fontaine, G. Tourillon, T. Gourieux, G. Krill, M. Mater and M.-F. Raver, in: High-To Superconductors: Electronic Structure, Proc. Int. Symp. on the Electronic Structure of High-T¢ Superconductors, Rome, 5-7 Oct 1988, eds. A. Bianconi and A. Marcelli (Pergamon, Oxford, 1989) p. 245. [4] S.M. Heald, J.M. Tranquada, A.R. Moodenbaugh and Youwen Xu, Phys. Rev. B 38 (1988) 761. [5 ] J.B. Boyce, F. Bridges and T. Claeson, Phys. Rev. B 36 (1987) 5251. [6]H. Oyanagi, H. lhara, T. Matsushita, M. Tokumoto, M. Hirabayashi, N. Terarda, K. SenzakL Y. Kimura and T. Yao, Jpn. J. Appl. Phys. 26 (1987) L638. [ 7 ] B. Lengeler, M. Wilhelm, B. Jobst, W. Schwaen, B. Seebacher and U. Hiilebrecht, Solid State Commun. 65 (1988) 1545. [8] E.E. Alp, G.K. Shenoy, D.G. Hinks, D.W. Capone II, L. Soderholm, H.-B. Schufler, J. Guo, D.E. EUis, P.A. Montano and M. Ramanathan, Phs~. Rev. B 35 (1987) 7199. [9] ILB. Garg A. Bianconi, S. Della Longa, A. Clozza, M. De Santis and A. Marcelli, Phys. Rev. B 38 ( ! 988) 244. [10IS. Della Longa, N. De Simone, C. Li, M. Pompa and A. Bianconi, in: High-To Superconductors: Electronic Structure, Proc. i':t. Syrup. on the Electronic Structure of High-T¢ Superconductors, Rome, 5-7 Oct 1988, eels. A. Bianceni and A. MarcelF. (Pergamon, Oxford, 1989) p. 259. [ I 1] N. Kosugi, T. Yokoyama, K. Asakura and H. Kuroda, Chem. Phys. 91 (1984) 249. Strong shakedo~-n charge transfer satellites of l s~4pr¢ out-of-plane transilions are believed to be a signature of the K absorption in Cu(3d 9) with squareplanar coordinated nearest neighbors. Following the many body excitation picture, on creation of the core hole charge is transferred from the ligand to the metal. Concomitantly the screening of the core hole increases and shifts the excitation ener~' below that of the corresponding one electron transition.

[ 12] J.M. Tranquada, S.M. Heald, A.R. Moodenbaugh and Y. Xu, Phys. Rev. B 38 ( 1988 ) 8893. [13]G. Uimin and J. Rossat-MignoL (1991), submitted to Physica C. [ 14] A. Bianconi, M. De Sands, A. Di Cicco, A.M. Hank, A. Fontaine, P. I ag~'cle, H. Katayama-Yoshida and A. Kotani, Phys. Rev. B 38 (1988) 7196. [ 15 ] hl. Nficker, H. Romberg, X.X. Xi, J. Fink, B. Gegenheimer and Z.X. Zhao, Phys. Rev. B 38 (1988) 11946. [16]U. Murek. Diplomarbcit, Universit/it zu K61n, 1988, unpublished. The core-level binding energy shift is calculated in the Z+ 1 impurity approximation with a Bom-Haber cycle proposed by B. Johansson and B.Martensson, Phys. Rev. B 21 (1980) 4427. [ 17] P.J. Durham, J.B. Pendry and C.H. Hodges, Comp. Phys. Commun. 25 (1982) 193. [ 18 ] R.J. Cava, Science 247 (1990) 656. [ 19 ] M. Francois, A. Junod, IC Yvon, A.W. Hewat, J.J. Capponi, P. Strobel, M. Marezio and P. Fischer, Solid State Commun. 66 (1988) 1117. [ 20 ] G.H. Kwei, A.C. Larson, W.L Hults and J.L. Smith, Physica C 169 (1990) 217. [21]J. R6hler and U. Murek, in: High-To Superconductors: Electronic Structure, Proc. Int. Syrup. on the Electronic Structure of High-T¢ Superconductors, Rome, 5-7 Oct 1988, eds. A. Bianconi and A. MarceUi (Pergamon, Oxford, 1989) p. 271. [ 22 ] A.G. Kachaturyan and J.W. Morris Jr., Phys. R,=v. Lett. 59 (1987) 2776. [23] B.W. Veal, H. You, P. Paulikas, H. Shi, Y. Fang and J.W. Downey, Phys. Rev. B 42 (1990) 4770. [ 24 ] T. Ruf, C. Thomsen, R. Liu and M. Cardona, Phys. Rev. B 38 (1988) ! 1985. [25] U, Weimer, R. Li, R. Feile, C. Tom~-Rosa and H. Adrian, Physica C 175 (1991) 89. [26 ] L. Genzei, A. Wittlin, M. Bauer, M. Cardona, E. SchOnherr and A. Simon, Phys. Rev. B 40 (1989) 2170. [27] H. Riesemeier, J. Pattloch, K. Liiders and V. Miiller, Solid State Commun. 68 (1988) 251. [28] U. Murek, K. Keulerz and J. R~hler, Physica C 153-154 (1988) 270. [29] H. Maryama, T. lshii, N. Bamba, H. Maeda, A. Koizumi, Y. Yoshikawa and H. Yamazaki, Physica C 160 (1989) 524. [30] J. Mustre de Leon, S.D. Conradson and I. Batistic, Phys. Rev. Lett. 65 (1990) 1675. [31 ] L. Tr6ger, D. Arvanitis, J. R6hler, B. Schliepe and K. Baberschke, Solid State Commun. 79 ( 1991 ) 479. [32] P.M. Horn, D.T. Keane, G.A. Held, J.L. Jordan-Sweet, D.L. Kaiser and F. Holtzberg, Phys. Rev. Lett. 59 ( 1987 ) 2772. [33] D. Dernbach and IC-H. Ehses, Ferroelectrics 105 (1990) 63. [ 34 ] C. Meingast, B. Blank, H. Btirkle, B. Obst, T. Wolf, H. W i l l , V. Setvamanickam and IC Salama, Phys. Rev. 41 (1990) ! 1299. [ 35 ] C. Meingast, T. Wolf, H. Wiihl, A. Erb and G. Miiller-Vogt, Ph':'s. Rev. Lett.( 1991 ), submitted.

J. R6hler et al. / Evidence for charge redistribution

[361S.D. Conradson and I.D. Raistr/ck, Science 341 (1989) 1340. [37] S.D. Conradson, I.D. Raistrick and A.R. Bishop, Science 248 (1990) 1394. [38]G.H. Kwei, A.C. Lawson, W.L. Hults and J.L. Smith, Physica C 175 (1991) 615. t39]H.J. Broicer. Diplomarbeit, Universit~t zu K/51n, 1990, unpublished. [40]R. Schiifer. Diplomarbeit, Universit~t zu KiSln, 19q0, unpublished. [41 ] F.W. Lytle, E.M. Larson, R.B. Greegor, E.C. Marques and A.J. Panson, Physica B 158 ( ! 989) 471. [42 ] R. de L. Kronig, Z. Phys. 75 (1932) 191. [43] H. Petersen, Z. Physik 76 (1932) 768. [44] C. Li, M. Pompa, A.C. Castellano, S.D. Longa and A. Bianconi, Physica C 175 ( 1991 ) 369. [45 ] J. RiShler, X-ray Absorption and Emission Spectra Vol. 10, chapter 71, p. 453, in: Handbook on the Physics and Chemistry of the Rare Earths, eds. ICA. Gschneidner Jr., L. Eyring and S. Hfifner (North-Holland, Amsterdam, 1987).

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[46] D.I. Khomskii and F.V. Kusmartsev, Phys. Rev. Left. ( 1991 ), submitted. [47 ] J. R6hler, A. Larisch and tL Sch~er. (1991), in preparation We have found that the beat occurring in the fdtered Cu--O EXAFS at k - 1 2 ]~-' is not attributable to a dynamical instability of the bridging oxygen. The beat is attributed to the temperature dependent static interference between the "'long" axial Cu(2)-O(4) bond and the "short" axial Cu( 1 ) - 0 ( 4 ) bonds. [48] A.R. Bishop, R.L Martin, ICA. Mfiller and Z. Tesanovic, Z. Phys. B 76 (1989) 17. [49 ] A. Bussmann-Holder, A.R. Bishop and I. Batistic, Phys. Rev. B43 (1991) 13728. [50] M. Frick, I. Morgenstern and W. yon der Linden, Z. Phys. B 82 (1991) 339.