Cu VALENCE d STATES IN HIGH-Tc SUPERCONDUCTORS FROM SOFT X-RAY FLUORESCENCE SPECTROSCOPY

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Cu VALENCE d STATES IN HIGH-Tc SUPERCONDUCTORS FROM SOFT X-RAY

FLUORESCENCE SPECTROSCOPY

J.-M. Mariot, V. Barnole, C. Hague, V. Geiser, H .J . Güntherodt

To cite this version:

J.-M. Mariot, V. Barnole, C. Hague, V. Geiser, H .J . Güntherodt. Cu VALENCE d STATES IN HIGH-Tc SUPERCONDUCTORS FROM SOFT X-RAY FLUORESCENCE SPECTROSCOPY.

Journal de Physique Colloques, 1987, 48 (C9), pp.C9-1203-C9-1206. �10.1051/jphyscol:19879217�.

�jpa-00227338�

1987

Cu

VALENCE d STATES IN HIGH-T, SUPERCONDUCTORS FROM SOFT X-RAY FLUORESCENCE SPECTROSCOPY

J.-M. MARIOT, V. BARNOLE, C.F. HAGUE,

V.

and H . -

.

Laboratoire de Chimie Physique (cNRS u~-176), Universite Pierre et Marie Curie, 11, Rue Pierre et Marie Curie, F-75231 Paris Cedex 05, France

*~nstitut far Physik, Universitdt Easel, CH-4056 Basel, Switzerland

R1sumB.- La densit6 d'etats locale'cu 3d dans les supraconducteurs La-Ba-Cu-0 et Y-Ba-Cu-0 a Ate Ctudibe & partir des spectres d9&mission de rayons X mous induits par le rayonnement fl1 Ka. Les spectres sont compares aux spectres de photoCmission et aux densit1s.d'Btats theoriques. Les d6saccords entre thkorie et expCrience sont discutes.

Abstract.- The local Cu 3 d density o f states in La-Ba-Cu-0 and Y-Ba-Cu-0 superconductors has been investipated by soft x-ray emission spectra induced by 6 1 K a radiation. The spectra are compared to photoemission spectra and theoretical densities of states. Discrepancies between theory and experiment are discussed.

INTRODUCTION

The recent discovery by Bednorz and Muller 1 1 1 that the La-Ba-Cu-0 compounds are superconductlng with onset temperatures of around 30 K has inltrated considerable sctivlty zn the search for materials wlth even hzgher crltrcal temperatures ( T c ) and the mechanisms leadlng to high-Tc superconductivity. The electronic structure of these perovskite-type materials 1s clearly of prlmary importance zn the understendrng of the ~nteractlons anvolved and a number of theoretical predictlons for the electronic densities of states ( 0 0 s ) 12-61 and angle-integrated photoemrssion spectra t6.71 have already been ~mplemented.

X-ray photoemission spectroscopy (XPS) and ultraviolet photoemlssion spectroscopy (UPS) ~nvestigations 18-13] show that, by and large, the calculated DOS are correct, though they mostly set the valence states 1-2 eV too close t o

EF.

EF

EXPERIMENTAL

The x-ray spectra were excited in the fluorescence mode using A1 Ka radiation. This has the advantage of ensuring that the genuine bulk properties of these sintered materials are probed and avoids possible contamination, or variations in oxygen content, which could be produced by local heating were electron-beam excitation used. We estimate the sample thickness contributing to

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19879217

C9-1204 JOURNAL

DE

the x-ray emission to be zl pm. The spectra were recorded with a 0.5 m radius bent-crystal Johann-type spectrometer, using cleaved gypsum crystals. The residual pressure was Pa. The direction of observation was 30' with respect to the sample surface and normal to the direction of incident photons. The absolute energy positions of the x-ray emissions were determined from two reference lines: Ca K a a n d Te La, recorded in fourth order reflection. We measured the Cu L 3 x-ray transition energy for metallic Cu to be 930.22(5) eV.

in good agreement with tabulated values [141.

The samples were prepared from appropriate mixtures of high purity La203 or Y2O3

.

RESULTS AN0 DISCUSSION

The Cu L 3 emissions for LBCO, YBCO, CuO, and metallic Cu are presented in Figure 1 . They are set on a binding energy scale obtained by subtracting the Cu L 3 t r a n s i t i 0 n energies from the L j core level binding energies taken from the lltterature 19,131. ( I n fact the L 3 binding energy for LBCO has not been published, so we used the value given for a La-Sr-Cu-0 compound o f similar composition). We observe that: ( i ) the superconducting oxides lie at a bindine energy of 3.5 eV whach 1s intermediate between that of metallic Cu (2.4 e V ) and CuO (4.0 eV); ( l i ) the uncorrected full-width at half-maximum amplitude (FWHM) of the emissions are 2.2 eV and 2.6 eV for LBCO and YBCO respectively i.e.

significantly narrower than in Cu (3.4 eV), but broader than in CuO (1.6 eV);

( iii ) the d 005 at E F is low; ( i v ) the self-absorption in CuO, which shows up a s a dip in the intensity at 2 2 eU binding energy, is much stronger than in the high-Tc oxides.

flssuming that the substitution of La by Ba or Sr is about equivalent, we can c o m p a r e . our results for LBCO with the XPS data for La-Sr-Cu-0 18.91. In XPS the Cu d states should be dominant since the Cu 3d photoionization cross-section is about one order of magnitude larger than for 0 2p. The XPS valence band maximum has been observed at 2.3.5 e V in good agreement with our results. It is also compatible with the He I 1 UPS data presented by Reihl et al. t101, which are interpreted as an indication that the Cu 3d states lie at approximately 3.3 eV.

The XPS band widths are, however, much broader ( 2 4 e V ) than the SXES emission

10 EF

Binding energy (eV) Fig. 1 : Cu L bands.

3

Binding energy (eV)

Fig. 2 : Cu L band in YBa Cu 0 compared t o tzeoretical d

h s 3

from Ref. 3.

3.4 eV. This disagrees however with the interpretation given by other authors for the He I UPS data, namely that the Cu 3d states lie at 4.5 eV t 1 2 1 . The broader Cu 3d band compared to LBCO which we observe, is not reflected in changes in the XPS valence band widths, which remain about the same.

The dip in the Cu L 3 .emission spectrum of CuO, due to self-absorption.

corresponds to the position of a strong white line in the absorption spectrum which occurs we11 below the normal L 3 excitation threshold as a consequence of electron-hole correlations C9.151. No such dip is observed in the high-Tc compounds. ( W e have checked this by varying the take-off angle). The difference in behaviour indirectly indicates a reduced density of unoccupied d states in the superconductors compared to CuO which is also the deduction drawn from the electron energy-loss data of Nocker et al. 191 and fits in with the low d DOS helow E F which we observe.

As already mentioned, agreement between theoretically calculated photoemisssion spectra C6,73 and experiment, can only be achieved b y shifting the latter - 2 eV closer to EF. It is equally the case when our data are compared to calculated Cu L j spectra C71. Moreover, our results underline another discrepancy with most of the DOS calculations: namely, that the strong tailing of the Cu L 3 emission band towards high binding energies expected from the predicted mixing o f the Cu 3d and O Z p states, is not observed. From this we deduce that, in fact, a rigid shift of the theoretical DOS does not appreciably improve agreement with experiment. It has been suggested that these Cu-0 based compounds may be subject to strong correlation effects with mixed ground state configurations C l l l . The final state in the Cu L j emission might consist qf either two holes localized on the Cu site 1.e. a d configuration, or a d

L

configuration, where

C

Closer inspection o f the local partial d DOS S U Q Q ~ S ~ , however, that the problem may yet be treated within the one-electron approximation. Several of the DOS calculations deal primarily with the case of La2CuO3 and set the maximum in the Cu 3d states at 2 eU, wlth strongly hybridized Cu 3d

-

-

There are important structural differences between YBCO and the La$u03- derived compounds. In particular. in YBCO the Cu atoms lie in two different Cu-0 networks: one parallel to the (a,b)-plane which is two-dimensional, the other.

parallel to the (b,c)-plane forming a one-dimensional chain along the b-direction C161. Linear-muffin-tin-orbltal atomic-sphere-approximation calculations 161 lead to approximately similar results for LaZCuO3 and YBCO. The Cu d states are mainly concentrated in a narrow peak 2 . 1 . 8 eV below EF. The peak is broader in YBCO ( z 2 . 0 e V ) than in La2CuO3 ( z 1.5 eU), but tailing of hybridized states extends down to about 7 eV. (The broadening comes from the d states of the Cu atoms which lie parallel to the (a,b)-plane and which have longer Cu-0 bond lengths than those of the other network). Better agreement with experiment is found for the LAPW calculation of Mattheiss and Hammann C31 (see Figure 2). In this work the width of the Cu d band is also found to be broader

( z Z . 2 e V ) than in a similar calculation for La2CuOg E21, but now the Cu d contributions from either of the Cu-0 networks are situated

-

C9-1206

We note however that the Cu-0 bond lengths for the one dimonslonal network are underestimated in this calculation. Hybridization towards high binding e n e r ~ i e s remains of quite weak intensity. The main contribution from mixed Cu 3d and.O 2p derived states occurs at low binding enerqies just below

EF.

In conclusion, we have shown from Cu t 3 x-ray emission spectra which probe specifically the d OOS at Cu sites that these states are more localized in the high-Tc compounds La1.8fBa0.15CuO~~ and YBa2Cu30%7 than in metallic Cu and are little affected by mixing with the 0 2p derived states. The localization of the d states is significantly less strong, however, than in CuO. There is evidence that the shape and position of the d states with respect to _EF may be explained within the one-electron approximation of electronic structure calculations.

References

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