Electronic structure of high-Tc superconductors studied using photoelectron spectroscopy

Vacuum~volume39/numbers 7/8/pages 611 to 61 5/1989

0042-207X/8953.00+.00 Pergamon Press plc

Printed in Great Britain

Electronic structure of high-To superconductors studied using photoelectron spectroscopy* Richard L Kurtz, Steven W Robey and Roger L Stockbauer, Surface Science D/vision, Nationallnstitute of Standards and TechnologyL Gaithersburg, M D 20899, USA and

D M u e l l e r , A Shih, L Toth, A K Singh and i USA

Osofsky, NavalResearch Laboratory, Washington, DC 20375,

Fundamental information about the structure of the valence band and the chemical valence states of the various constituents of the La-Sr-Cu, Y-Ba-Cu, Bi-Sr-Ca-Cu and TI-Ca-Ba-Cu oxides have been obtained using photoelectron spectroscopy. These results show that the one-electron theories do not adequately describe the electronic structure of these superconductors. The atomic origins of the features observed in the valence bands have been investigated by studying photoemission resonances and changes in excitation cross-sections with photon energy. Results to date suggest that these materials have varying densities of states at the Fermi level valence bands composed of 0 2p and Cu 3d states, and display no significant changes in the band structure associated with the superconducting behaviour when the temperature is lowered below To. In addition, the complex surface chemistry of these oxides make it essential to study the surface stoichiometry and the interaction of simple molecules. 02 and CO are found to interact only weakly with the surfaces of the materials studied to date, while the H20 and C02 react strongly, forming hydroxides and carbonates.

1. Introduction For a number of years, the critical temperature of superconducting materials was increasing at a rate of ~ 0.3 K year -~ but the discovery of superconductivity in a new class of materials, perovskite oxides, by Bednorz and Miiller ~drove this rate up to what is now ~ 45 K year -~. These materials exhibit a weak isotope effect on the critical temperature 2 and this observation, together with the theoretical prediction that the electron-phonon interaction cannot provide the strong coupling necessary for the high T~'s, prompted many of the efforts aimed at understanding their electronic structure. Although much has been learned about the electronic structure of these materials, many questions still remain. Electron spectroscopies have provided most of the direct information about the electronic configurations in this class of perovskite oxides and the information obtained from measurements of the valence electronic structures is discussed here. Photoelectron spectroscopy is a direct method of measuring the occupied states of these materials and it will be shown that additional information regarding the strengths of electron-electron interactions can also be obtained. Care must be exercised in the interpretation of these data, however. The measurements are surface sensitive due to the short escape depth of electrons with energies between 10 and 100 eV. Consequently, the data are sensitive to variations in the composition of the surfaces. Subtle changes in the measured photoelectron spectra have been attributed to small variations in surface composition; particularly

* Invited. "~Formerly the National Bureau of Standards.

affected are the intensity of the emission at the Fermi level and, perhaps, the intensity of photoelectron satellites3'4. The primary features of the spectra are essentially identical when data are obtained from polycrystalline or single crystal surfaces, however. In the following discussion, we will present evidence for the strong hybridization of the Cu 3d and the O 2p levels. Resonant valence satellites are observed in the spectra and the origin of these satellites will be discussed. They will be shown to imply that the Cu is a 2 + configuration and that the Cu 3d and the O 2p levels are highly correlated. The reactivity of these materials with atmospheric gases will also be discussed and data will be presented that indicate the types of compounds that form on the surfaces of these materials upon contamination. 2. Experimental The data presented here were obtained at the National Institute of Standards and Technology SURF-II synchrotron storage ring using a monochromator and surface analysis system that have been described previously5'6. Briefly, the photoelectron spectra were obtained with a double-pass cylindrical-mirror analyzer operated in a constant pass energy mode. These data are angleintegrated and are shown corrected for incident photon flux. Bulk polycrystalline samples were produced by mixing the appropriate ratios of the oxides and carbonates, calcining, regrinding and compressing. They were then sintered at high temperature in air followed by a slow cool. The samples were scraped or filed at 300 K in uhv (base pressure 1 × 10 ~0 torr); this has been shown to fracture the crystalline grains and expose fresh surfaces 7. For the La- and Y-based materials, there was no observable radiation damage and the only changes observed in

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R L Kurtz et al: Electronic structure of high- Tosuperconductors

the spectra over time were due to the adsorption of a small amount of H20, present as a background gas in the chamber. 3. Results In Figure l the photoelectron spectra obtained from five different superconducting oxides are presented for a photon energy of 76 eV. The shallow valence region, from about 15 eV to Ev, shows similar structures for all of these oxides. They have a low density of states at the Fermi level and a valence band that is about 5 eV wide and centred at ~ 4 eV. All of these materials show a distinct peak at ~ 9 . 5 eV except for the Bi material; there the peak is still present but with a substantially lower intensity. These materials also show an additional peak at ~ 12.5 eV. In the TI material, this feature overlaps with the TI 5d levels which appear at binding energies of 13.1 and 15.0 eV. At higher binding energies, the shallow core levels of the other components of these materials are evident. F o r all of these materials, no sharp Fermi edge is observed and the density of states observed near Ev appears to be consistent with a semiconducting behaviour in the normal state. The remarkable similarity of the photoelectron spectra from these different materials underscores the similar natures of their electronic structures. The broad band centred at ~ 4 eV can be decomposed into two features; in YBa2Cu307 ,~ these two features are located at 2.3 and 4.5 eV. These two components are attributed to Cu 3d and O 2p features, however, in the early literature there was a disagreement between experimentalists as to the assignment of these peaks ~'9. Using synchrotron radiation, we can resolve this question by studying the cross-section of the two valence band features as a function of incident photon energy. F r o m atomic cross-section data, the O 2p level is known to decrease in cross-

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section relative to the Cu 3d (ref 10). Comparing the ratio Cu 3 d : O 2p for 35 and 80 eV photon energies, we expect the Cu component to increase by a factor of ~ 8 . 4 relative to the O component ~°. In Figure 2, we show the photoelectron spectra obtained from YBa2CusO7 , for photon energies of 35 and 80 eV. The two components of the valence band are indicated with the arrows and it is clear that the factor of > 8 times change in intensity is not observed. The relative intensities of these features vary by only 18%. This implies that an ionic decomposition of this band into its constituent components is not a good description of this material. In comparison, ZnO, which is a prototypical ionic semiconductor, appears to fit the expected atomic cross-section behaviour well] ~. The fact that such a small variation in the crosssection of the superconductor valence band is observed implies that it is highly hybridized and that there are strong covalent interactions between the Cu 3d and the O 2p levels. We can learn more about these materials if we follow the intensities of the various valence features over an extended photon energy range t2. Over this larger photon energy range, we are able to observe photoemission resonances 13 which allow the identification of the atomic origin of the observed features. The resonant process results in enhanced emission of these valence features when the photon energy is tuned near the onset of a core-electron excitation. The components of the valence bands that are associated with that core level or atomic species can therefore be identified. The enhancement arises from a coupling of the excitation and decay channels associated with this coreelectron excitation and results in a Fano intensity profile as a function of incident photon energy ~3. In Figure 3, we present such photoemission data for YBa2 Cu307 ~ with photon energies ranging from 60 to 106 eV (ref 12). In this range, we are able to span the excitation of two shallow core levels; the Cu 3p at ~ 73 eV and the Ba 4d near 104 eV. For photon energies that just exceed these binding energies, we get an enhancement of the features due to Cu and Ba, respectively. For hv = 74 eV, we find that two features are strongly enhanced at 12.5 and 9.5 eV and we assign them to Cu excitations. For hv = 104 eV, we observe that two features at 15.0 and 28.8 eV are enhanced and we assign these to the Ba 5p and 5s levels, respectively. By using such resonances, it is possible to decom-

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Figure I. Photoelectron spectra obtained from five different superconductors: La2 xSrxCuO4, YBa2Cu307_.,, HoBa2Cu307 x, Bi Ca-SrCu~O and T1 Ca Ba-C~O. These different materials give similar spectra in the range 15 eV to Ev although the T1 5d levels are evident near 15 eV in that material. 612

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R L Kurtz et al: Electronic structure of high- T~superconductors

d 8 state corresponds to the two (multiplet-split) satellites that are observed at 12.5 and 9.5 eV. The satellites resonate with the excitation of the Cu 3p level and this is due to the interference of excitation (1) with,

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Figure 3. Photoelectron spectra obtained from Y B a 2 C u 3 0 7 x for photon energies ranging from 60 to 106 eV in 2 eV increments. The 106 eV spectrum is at the bottom of the figure and the spectra are offset by constant vertical increments for clarity.

pose the photoelectron spectra into their constituent components ~2. In this valence region, we find that the O 2s and the Y 4p levels are located at 20.1 and 24.0 eV but aside from being Cu related, the features at 12.4 and 9.5 eV cannot be ascribed to any specific core level. Rather, they have been identified as resonant satellites of Cu (ref 12). Less than half of the intensity of the feature at 9.5 eV can be accounted for as a Cu satellite, however; the remaining intensity in that region is due to a resonant satellite of O (ref 14, 15). Resonant satellites have been observed previously in photoemission from Cu and its oxides, Cu20 and CuO (ref 16). It has been observed that Cu metal (Cu °) has two satellites at 14.6 and 11.9 eV, Cu20 (Cu +) has a single satellite at 15.3 eV and CuO (Cu 2+) has two satellites at 12.9 and 10.5 eV (ref 16). Also in that work, it was observed that the photon energy of resonance was characteristic of the oxidation state ; the photon energy that the satellite intensity was half of its maximum was observed to be 75.4, 74.5 and 73.1 for the Cu, Cu20 and the CuO, respectively~6. In the case of the superconductor YBazCu3OT_x, the corresponding photon energy is 73 eV (ref 12). Therefore, in comparison with the photoemission results from Cu and its oxides, both the satellite energies and the photon energy at which they resonate are characteristic of a CuO-like (Cu 2+) oxide. More information can be obtained by modelling these data using a configuration-interaction cluster calculation. Several groups have reported such studies ~7'18 and the origin of these satellites has been described in detaiP 9. Briefly, the ground state is found to be an admixture of d ~ and d ~°L where the notation d" refers to the occupation of the Cu 3d level and the L refers to an 0 2 ligand with a 2p hole (i.e. ls22s22pS). The admixture of d~°L into the ground state is due to the small charge-transfer energy, A ~ 0.25 eV, that is obtained in the calculation. The satellites arise from a Coulomb correlation of C u d holes in the dSconfiguration that results from the photoemission process,

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where the second step in equation (2) is a super Coster-Kronig Auger decay of the excited, intermediate configuration. It is the interference of these two channels that gives rise to the resonant profile of these features as a function of photon energy 12. From the energy difference between the satellites and the valence band, as well as their relative intensities, the configuration-interaction calculations conclude that the Cu d~t correlation energy, U~d, is ~ 6 eV. It has also been observed that the feature at 9.5 eV resonates with the excitation of the O 2s level 14'L5.This resonant excitation can be explained, as in the case of the Cu, as an interference of the direct excitation (equation (1)) with the resonant decay of the excited core,

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where the 2s refers to an O 2s hole. From Auger and XPS data, it is known that the correlation energy of two O 2p holes on a single site is greater than 8 eV (ref 20). The observation that this feature resonates at with the O 2s, and that the satellite is closer to the valence band than the Cu satellites (and corresponds to a smaller U) implies that it must be associated with two O 2p holes on different, neighbouring ligands. This final state is written as d I°LL and we estimate that the associated correlation energy, Uee,, is ~4.5 eV. Both of these results, obtained from the resonant photoemission measurements, imply that correlation effects dominate in the YBa2Cu307 x- Also, similar resonant photoemission data are obtained from the La- and Tl-based materials2L'22,indicating that they are characterized by strong correlations as well. This means that one-electron theories of the electronic structure are not able to adequately describe these materials. The lack of a distinct Fermi edge and the presence of strong correlations have important implications on many theories for the superconducting behaviour. Recently, questions have been raised about the density of states at E~., By cleaving single-crystals at 20 K, one group has been able to observe an appreciable Fermi edge4. Upon warming to T ~> 55 K they find that the surface undergoes an irreversible change, giving spectra that are essentially identical to those obtained from room-temperature cleaved or scraped samples. These measurements merit close attention and careful study to determine if a subtle change in stoichiometry or crystal structure at the surface could give rise to such behaviour. In addition to the application of photoemission to understanding the fundamental properties of these materials, several studies have been conducted to determine the nature of the surface species that are formed in the initial stages of decomposition due to atmospheric exposure 22'23. In Figure 4(a), we present the results of a study of the interaction of H20 with LaL.sSr0.2CuO4. In the upper panel, we plot the h v = 40 eV photoemission spectrum obtained from the freshly scraped surface with the dashed curve and the spectra obtained after exposure to increasing amounts of H20 with the solid curves. It is clear from these data that the surface saturates by a dose of 1 L, corresponding to an adsorbate coverage of 1 monolayer. We can obtain more information about the species that form 613

R L Kurtz et al: Electronic structure of high-T c superconductors .

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Figure 5. (a) Photoelectron spectra obtained from Lal.sSr0,2CuO4 after

exposure to increasing amounts of H20. The exposures are 0.01, 0.1, 1, 10 and 100 L (1 L = l0 6 torr s-I). (b) The difference spectra, solid curves ; the dashed curve is the difference spectrum obtained by assuming the overlayer attenuates the substrate emission and the curve plotted with the solid squares is that for NaOH (ref 24).

exposure to increasing amounts of CO2. The exposures are 0,01, 0.1, 1, 10 and 100 L. (b) The difference spectra, solid curves ; the dashed curve is the difference spectrum obtained by assuming the overlayer attenuates the substrate emission and the curve plotted with the solid squares is that for CaCO3 (ref 25).

on the surface by producing the difference spectra, plotted with solid lines in Figure 4(b). These curves are produced by subtracting the dashed curve of Figure 4(a) from the spectra obtained after dosing with increased amounts of H20. We find that this procedure produces two peaks, located at 5 and 9 eV and a negative dip at 2.5 eV. This negative dip may be due to some extent to charge transfer from the substrate to the adsorbate or to band-bending but it may also be due to attenuation of the substrate photoelectron intensity by the overlayer that is forming. If we assume that the substrate is being attenuated and scale this spectrum down before subtracting, we can obtain the curve plotted with the dashed lines. When this curve is compared with the spectrum obtained from N a O H (ref 24), we find that there is excellent agreement with the observed splitting of the 3a and In levels of O H - . This implies that the adsorption of H20 is dissociative at 300 K, producing surface hydroxides. We can perform a similar experiment with CO2, and these results are shown in Figure 5. In Figure 5(a) we show the hv = 40 eV n(E) data obtained for exposure of Lal.sSr0.2CuO4 (dashed curve) to 0.01, 0.1, 1, 10 and 100 L of CO2 (solid curves). In this case, saturation is obtained at exposures greater than 1 L and we estimate that the initial sticking coefficient is ~ 1/3. Again we can produce the difference spectra and they are shown with the solid curves in Figure 5(b). As in the case of the H 2 0 adsorption, CO2 adsorption produces an overlayer that attenuates the sub-

strate emission and a difference curve can be produced that accounts for this attenuation ; it is plotted as the dashed curve in Figure 5(b). If we compare this curve with the CO3 features observed in CaCO3, given by the solid squares in Figure 5(b) we find a very good agreement with the relative intensities and splittings. In this case, we can conclude that exposure to CO2 produces surface carbonates. Similar experiments have been performed for 02 and for CO, however these molecules are much less reactive. O2-exposure leaves the surfaces of YBazCu307 x and La~.sSr0.2CuO4 essentially unchanged while CO produces surface carbonates but with an initial sticking coefficient that is 0.01 that of CO2 (ref 23). Recent experiments with the Tl-based compounds report a similar sensitivity to H20 and CO2 exposure and this associated relative insensitivity to O2 and CO. The implication of these studies is that protection of these materials from H20 contamination is not sufficient ; other molecules such as CO2 may be nearly as reactive and damaging.

614

4. Summary We have shown that photoelectron spectroscopy is an extremely important tool for understanding the fundamental properties of superconducting perovskite oxides. These measurements indicate that these materials have a highly hybridized Cu O valence band

R L Kurtz et al: Electronic structure of high-To superconductors

a n d r e s o n a n t satellites are observed t h a t imply t h a t these materials are highly correlated. B o t h C u 3d a n d O 2p electrons are correlated with Udd ~ 6 eV a n d Upp ~ 4.5 eV. N o substantial changes are observed in the electronic structures as the materials are cooled f r o m r o o m t e m p e r a t u r e to T < Tc however questions a b o u t stoichiometry or structural transitions have b e e n raised u p o n w a r m i n g 20 K cleaved crystals. These materials react strongly with H / O a n d CO2 forming hydroxides a n d c a r b o n a t e s b u t they are m u c h less reactive t o w a r d s 02 a n d CO.

Acknowledgements R L Kurtz, S W R o b e y a n d R L S t o c k b a u e r would like to acknowledge the s u p p o r t of the U n i t e d States Office of N a v a l Research. One o f us (D Mueller), would like to acknowledge the s u p p o r t o f the U S N a t i o n a l Research Council t h r o u g h a p o s t d o c t o r a l research fellowship. W e would also like to t h a n k the staff" o f the N I S T S U R F - I I s y n c h r o t r o n light source for their help in this work.

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