HIGH-ENERGY SPECTROSCOPIES AND HIGH-Tc SUPERCONDUCTORS : AN OVERVIEW

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HIGH-ENERGY SPECTROSCOPIES AND HIGH-Tc SUPERCONDUCTORS : AN OVERVIEW

G. Wendin

To cite this version:

G. Wendin. HIGH-ENERGY SPECTROSCOPIES AND HIGH-Tc SUPERCONDUCTORS : AN OVERVIEW. Journal de Physique Colloques, 1987, 48 (C9), pp.C9-1157-C9-1178.

�10.1051/jphyscol:19879211�. �jpa-00227332�

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JOURNAL DE PHYSIQUE

Colloque C9, suppl6ment au n012, Tome 48, dbcembre 1987

HIGH-ENERGY SPECTROSCOPIES AND HIGH-T, SUPERCONDUCTORS : AN OVERVIEW

G. WENDIN

Laboratory for Atomic, Molecular and Radiation Physics,

Department of Physics, The University of Illinois at Chicago, Chicago, IL 60680, U.S.A.

and Institute of Theoretical Physics, Chalmers University of Technology, 5-412 96 Gateborg, Sweden

RESUME

-

La d h u v e r t e recente de supraconductivib5 dans la gamme 90- 160 K dans les cornposh types Y-Ba-Cu-0 a prwoque une activite experimentale

et

^thbrique

intense. Les problemes fondamentals mncerne le type d'interaction responsable de la supramnductivite observe, la stabilite des phases supracondudeurs, et comment atteindre Tc=300 K. Ces questions ont stimule un grand nombre d'etudes experi- mentales de la structure et de l a Wnamique electronique de ces m m p d en utilisant des spectroscopies hautes-energies differentes, comme XAS, UPS, XPS, AES, XES, BIS, et EELS.

Ce papier discutera les interpretations des r b u l t a t s r h n t s des spectrowpies hautes-energies, et commentera sur les implications pour les t h h r i e s des mechanisms de la supraconductivite.

ABSTRACT

-

The recent dismvery of superconductivity i n the 90- 160 K range i n Y-Ba-Cu-0 type of compounds has provoked an intense experimental and theoretical activity. Fundamental problems concern the type of interaction responsible for the observed superconductivity, the stability of the superconducting phases, and how to reach TC%3OO K. These questions have stimulated a large number of experimental investigations of the electronic structure and dynamics of these compounds using different types of high-energy spectrosmpies like XAS, UPS, XPS, AES, XES, BIS, and EELS.

This paper w i l l discuss interpretations of recent experimental results from high- energy spectr~scopies, and comment on the implications for theories of mechanisms for the supermnductivity.

The discwery of superconductivity i n the 3 0 - 4 0 K range i n La-Ba-Cu-0 and La-Sr-Cu-0 systems by Bednorz and Muller [I], awarded the 1987 Nobel Prize i n Physics, has paved the w v for another scientific breakthrough, namely achieving superconductivity above the liquid nitrogen temperature of 77 K [2-131. Superconductivity at room temperature might be around the corner, which would have immense implications and applications. The discoveries have raised widespread excitement among solid state physicists and chemists, and the field i s rapidly expanding.

At present, critical temperatures i n the 90- 100 K range can be obtained routinely with a number of Y-Ba-Cu-0 type compounds [2-121. Considerably higher T 's, around 155 K [8,101, 2 6 0 K [I21 or even above room temperature [ 131, have been reported. E n f o r t u n e t e ~ ~ , the results are very difficult to reproduce since the very-high-Tc samples tend to return to -90 K after a short while

.

Very recently, however, it has been demonstrated [ 111 that Tc= 1 6 0 K can be reached i n a systematic manner by "freezing in" a high-temperature phase by cycling the sample from below T up to a specific (structural) phase transition temperature (239 K for Y-Ba-Cu-0 and 2 6 0 K For Y-Ba-Cu-0:F; resistivity anomalies have previously been observed around 2 4 0 K i n Y-Ba-Cu-0). This new phase i s very "fragile", and the samples revert back to T,% 9 0 K if heated above the phase transition temperature.

A guideline i n the work of Bednorz and Muller [ I ] was to look for materials with high- frequency modes and Jahn-Teller type of lattice instabilities, to obtain strong electron-phonon coupling and strong attractive electron-electron interaction within a conventional BCS mechanism.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19879211

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C9-1158 JOURNAL DE PHYSIQUE

The surprising thing with the new ceramic superconductors i s that they are very poor metals i n the normal state (even semiconductors if not doped) and not obvious candidates for superconductivity.

Horewer, T,'s as high

as

90- 160 K or higher suggest that the electron-phonon mechanism might not be responsible for the interaction leading to superconductivity.

Some important general properties of the new CuO-based superconductors m8y be summarized

8s follows:

1. The materials are layered structures involving Cu-0 planes and Cu-0 chains

1

14-16] which carry the supercurrents. The importance of electron correlation effects i n these types of systems i s well known [ 17-191.

2. The resistivity decreases linearly I201 with temperature down to Tc and then drops to zero within a few degrees K, or less, i n the best materials.

3. The magnetization reveals a Meissner effect [2] and a temperature dependent magnetic field penetration depth [211.

4. Hall effect measurements [22] show that the charge carriers are holes. Tc seems to be proportional to the carrier concentration which i s proportional to the dopant concentration 122.2.3l.

5. Infrared absorption measurements 1241 shows gaps i n the order of A s 1 0 - 2 0 meV.

6. Tunneling measurements 1251 (normal-to-supermnductor) show gaps i n the order of A 10-20 meV (there are indications of gaps i n the 20-30 or possibly even 6 0 meV range i n thin films of La-Sr-Cu-0 [261).

7. Tunneling of electron pairs i n a Josephson junction between superconducting NbZr and Y-Ba-Cu-0 indicates pairing compatible with BCS type of pairing (s-wave, spin-singlet) [271.

8. There i s a very weak o isotope effect i n the 9 0 K superconductor Y-Ba-Cu-0 (T lowered by 0.3-0.5

K

when Oi6-mv%)

[ N ,

sugpesting that oxygen vibrations and electron-p%~n~n coupling play a noticeable but minor role i n the mechanism of superconductivity.

9. Anisotropic critical currents and critical magnetic fields have been observed i n single crystals of Y-Ba-Cu-0 1291.

10. The specific heat shows a sharp diswntinuity at T [301.

The fundamental quastion of course concerns the nafure af the microscopic mechanism for the observed superconductivity. A large number of papers have already been written on the subject, and Refs. 131-381 provida exmllent "introductory" reading, mvering a brqad spectrum of opinions and results.

Assuming that an electron (or hole) pairing mechanism i s relevant, there are two general limits, namely (1) the BCS l i m i t with condensation of Cmper pairs (kt,-kl) and (ii) the bipolaron l i m i t describing Bose condensation of real-space pairs (e.g. holes i n adjacent Cu-0 bonds). This i s independant of which mechanism causes the attractive interaction (phonons, plasmons, excttons, spin fluctuations, super-exchange, etc.).

The BCS l i m i t involves a very large number of pairs within the coherence length (roughly the size of a pair) and may be described by mean field theory. I n the bipolaron l i m i t the pairs are tight- l y bwnd and the pairing occurs i n real space. There are very few pairs within the coherence length, and the BCS mean field approximation does not apply.

Wing from the BCS to the bipolaron limit, the pairs may keep their character of spin-singlet pairing whlle the way the pairs build up the macrosmpic wave function of the superconducting state w i l l be very different. The question therefore not only concerns whether we are dealing with weak or strong caupling BCS theory, but whether BCS theory i s applicable at all. Neverthless, the experimental observations of tunneling of pairs between an ordinary superconductor ( NbTe) and superconducting Y-Be-Cu-0 1271 and the T-dependence of the magnetic penetration depth 1211 suggest that the character of the pairs i s similar i n the sense that there i s non-vanishing werlap between BCS pairs and pairs i n the high-T superconductors.

Within the BCS model, the values of h / k T,

-

^3-1 0 can be interpreted as evidence for strong coupling BCS superconductivity. On the other land, analysis of data from measurements of specific heat, critical magentic fields, etc. suggests a situation intermediate between the BCS and bipolaron limits [381.

Questions regarding mechanisms for superconductivity seem to be quite open at present. HOW- ever, there i s already a large amount gf results from high-energy spectrosmpies. Although incomplete, these establish a number of important facts which must be seriously considered by any model aiming at describing high-Tc superconductivity i n La-Ba-Cu-0 or Y-Ba-Cu-0 type of materials.

The role of high-energy spectroscopy i s t o study spectra of electronic excitations, charge distributions and geometric structure both i n the normal system and i n the superconductor. This may be done by photoabsorption or reflection or by inelastic scattering of photons or electrons. The b/namics of these excitations may also be studied by photoelectron spactroswpy, i n terms of the

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intensity distribution of valence- and core-hole levels and their associated satellite excitations.

High-energy spectroscopies involve e.g. photoelectron spectrampy (PES) with X-ray (XPS), ultraviolet (UPS) or tunable synchrotron radiation, (soft) X-ray abmrption s ectrosoopy (XAS), X-ray emission spectr-py (YES), bremsstrahlung i w h r o m a t spectrmpy PBIs), and electron energy loss spectroscopy (EELS). These are well suited to probe filled and empty electronic levels, as well as the dfnamics of screening and correlation of holes and electrons i n valence bands and core levels. PES also gives information about excited states of hole levels (shake-up and shake-down satellites; two-hole-one-electron levels) and therefore also about the Coulomb interaction Uhh between screened holes. Resonant PES (RESPES) can be used to further characterize the valence band and the satellites, by stuwing variation of electron emission from different regions of the valence band, from satellites and from core levels, at core level resonances.

Systems related to the Cu-0 based high-Tc superconductors (transition metal oxides and other compounds; Ni metal) have been intensely studied by various high-energy spectroscopies and theoretical approaches [ 17-19,3+42] since around 1970. In the high-energy spectra of the high-Tc superconductors one can therefore immediately find conclusive evidence for the importance of b C u charge transfer screening and for strong correlation effects among the 3d-electrons.

There are a number of questions which can be addressed, and have been addressed, with success by high-energy spectroscopies [ 18,43-751:

a) , 4 v e r w &sitlyor*stafes

(1?03I

^andpwtia/ I n the region around EF ( Fermi level) it seems to be lower, or much lower, than predicted by bandstructure calculations.

b ) Y a / m M ( Y 5 , ) structure be& &ahwe f~ Compar isons with bandstructure calculations clearly indicate strong effects of correlation.

c) Excitm?ceHmtd' Resonances i n XAS at the Cu 2p and 3p thresholds (2p ,3p+3d) p r w e the existence of empty Cu 3d levels. RESPES at the Cu 3p thresholds( 3 ~ 3 d ) proves the existence of empty Cu 3d levels and indicates hybridized 3d character i n the valence band all the way up to the E [631. Resonances i n XAS at the 0 I s threshold ( 1 ~ 1 2 p ) and i n RESPES at the 0 2s threshold [59E ( 2-213) prove the existence of empty 0 2p levels. RESPES at the Ba and La 4d thresholds (4d-+4f) indicates hybridized 4 f and 5d character i n the valence band

[Ell.

Resonances at the La 2p- thresholds (2p+5d) depend on doping which indicates La 5d-0 2p hybridization

[@I.

d) L?7rre/..t1bn energ~h, /mlrktriu;r Setellife energies indicate large Cu 3d-3d and Cu 2p-3d Coulomb interaction energies, U3 3 d ~ 6 eV and U2p3dLJ9 eV.

a) O%r@ dr3trraut~iws trmsfer.. A mixed-valence picture emerges for the ground state. Estimates b don 3 8 - 3 d

C

ground state configuration mixing indicates an effective Cu 3d-occupation consider bly larger than 9 and indicates important Cu 3d-0 2p hybridization 1631.

f) C u f f / C ~ ' / d f r a t i i The high-Tc superconductors show "fingerprints" similar to CuO, indicating a predominance of cu2+, mixed with cul+. No pure

cu3+

^{( 3 8 )}valence is observed.

However, there are clear indications of ( c u ~ + o - ) ~ + ( 3 8 ~ ) cluster ions from Cu 2p XAS and XPS.

g) &ys&/strmture, &wt r w ma&-, vacwciks E M S abwe the Cu K edge ( I s threshold) agrees with X-ray and neutron scattering reprding structure and coordination. XANES at the Cu 1 s edae cannot find evidence of cu3+. Qives clear evidence of c u t + for laroe axMen deficiencies,

-

^{. -}

like i n Cu306.

h)

2--

^d81-h) with tmper~ture, from rmm temperature C we// below TG so

we//

as

#undL

~ h e e e are also questions which the abwementioned high-energy spectroscopies cannot address directly, for example transport properties. However, the results from high energy spectroscopies w i l l , without any doubt, be very important for determining essential parameters i n microscopic models for high-T,..superconductivity. In particular, information about possible change of the elec- tronic structure wlth decreesing temperature may be of vital importance.

2. LATTICE STRUCTURE.

X-r8y f14.761 and neutron [15,771 crystallography and electron diffraction and microscopy [78-801 have revealed the general structure of the oxygen deficient perovskite YBa2Cu307-6 as a function of temperature and 0-vacancy concentration (0s S 1 1

1.

The lattice structure of the Tc=9O K superconductor YBa2Cu3 (S=O) i s shown i n Fig. 1. The layers l i e i n the a-b plane and the stacking i s i n the c-direction.

4

he present knowledp'[l4-163 indicates that the yttrium layers contain no oxygen and serve as "spacers", separating slabs of Ba-Cu-0 (the yttrium layer becomes emptied of

The Ba-Cu-0 slabs consist of five 1 y r s in the sequence

O

denotes 0-vacancies): The "surface layers"

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JOURNAL DE PHYSlQUE

Figure 1. Lattice structure of YBa Cu3O7.

a) After Ref. [ 141. b) After Ref.

[dl.

planar Cu02. The copper Cu(2) atoms form a nearly square lattice with oxygen O(2) and O(3) atoms m u p y i n g all the bridge positions (Fig. 1 a). The result i s a nearly square 20 network of Cu-0 chains with Cu ( 2 ) at the crossing points. I n the central layer, chainsof 0-vacancies break this 2D network; as a result, the Cu( l ) atoms form 1 D Cu( 2)-0( I ) chains along the b-axis separated by chains of 0( 5) oxygen vacancies. These vacancy chains result i n a shorter lattice constant along the the a-axis than along the b-axis

-

the orthorombic structure.

The Cu planes may be looked upon as a 20 rectangular network of chains of CuOq "tiles" (Fig.

1 b) connect

3

only via the corner oxygens. The Cu-0 chains i n the central layer may be regarded es chains of Cu04 tiles i n the b-c plane connected at the 0( 1) corners, and the with upper and lower 0( 4) oxygen mrners fixed i n the BaO planes (Fig. 1 b). The Cu( 1 ) chains therefore look a b i t like parallell "fences" of Cu04 tiles at right angles to the two Be0 planes.

The Cu( 1)-O(4) bonds are short and strong, which makes the O(4)-Cu( 1)-O(4) unit very strong: it remains intact wen i f all the 0( 1 ) chain atoms are removed. One can then imagine the CuO4 tiles i n the Cu( 1 ) chains to have a tendency to rotate and bend around the 0( 4)-Cu( 1 1-0( 4) axis. As seen from Fig. 1 a, the O( 1 ) atoms make very large vibrational excursions along the a-axis (no stabilizing O(5) atoms), which might be viewed as a limited rotation and bending of the CuO tiles. As long la the Cu( 1 ) chain i s intact, the Cu04 tiles w i l l be connected at the O( 1 ) corners. an1 the chain could perform large-amplitude transverse wave motion.

It i s of great importance to study oxygen deficient YBa Cu phases w e r a wide range of oxygen vacancy concentration, say 0s 6 11. The doping level

5

^th%$erial and the structure of the Cu( 1 ) chains depend on the number of 0-vacancies, which are therefore of critical importance for the electronic properties of the material.

For S>O, oxygen depletion of YBa Cu3O7-~ forms O( 1 ) vacancies i n the central Cu( 1)-0( 1) chains I771. It i s well established th& anneal~ng at elevated temperatures leads to YBQCU

4-

phases i n the range of, say, 0 . 5 ~ 8 i 1 , i n which the O( 1 ) atoms are disorderd between the 0 d ) an3 0( 5) sites i n the a-b plane. Morewer, for 8 = 1 the entire Cu( 1 ) plane has been emptied of oxygen and the structure hes become tetragonal and non-supermnducting.

However, for p>O essentially nothing i s known about possible orderinq of 0-vacancies i n the Cu( 1

1

plane. Starting from YBa2Cu34 one muld e.g. imagine O( 1 ) vacancies to wipe out one Cu( 1 ) chain after another, maybe forming regions with chains (8 =O) separated from depleted regions ( 6

= 1 ). Alternatively, the vacancies muld be ordered i n a uniform manner or i n density wave patterns, creating supercells. One could also imagine the Cu( 1) chains to be broken into a number of dis- connected Cu04 tiles.

Very recent results f 811 indicate that an elaborate heat treatment applied to the orthorombic YBa2Cu3+ phase can ~ndeed retain the orthorombic symmetry and create ordered O( 1 )-vacancies and

su

erconductin YBa2Cu3O7_~ in the range 0 i 8< 0.7. The variation of Tc with 0-vacancy mnce-7 i s shown i n FI~. 2:

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Tc (K) Figure 2. T as a function of oxygen vacancy

concentration

%

i n YBa2Cu3

-

curve results from oxygen

orthorombic YB CuJ07 at "low" temperature, creating o r d e r 2 O( 1 )-vmncies i n the a-b plane [81]. The dashed curve indicates the result of more "violent" heat treatments leaving disordered 0-vacancies i n the a-b plane. Note that i n the region 0.25< 6 (0.45 Ref. [81] finds a strongly enhanced magnetic susceptibility i n

the normal system. Also note that at 8-0 there 20

-

isapointatTc=130Kfrom Ref. [ I l l .

\

. * I # ? L A 1

0 0.5 1.0

The point 8=O.S i s of e particular interest, bearuse i t has usually been obtained by cooling from the high-temperature phases, with the result that the 0( 1 ) atoms bewme disorderd between the 0( 1) and O(5) sites i n the a-b plane, giving a tetregonal non-superconducting semiconductor.

The result of Cava et al. 1811 demonstrates that, for a low concentration of holes, Cu( 1) chain ordering plays a very important role (see also Ref.

I82l).

For example, disorder might induce localization i n the Cu02 planes, directly via scattering or indirectly via small but important adjustments of the electronic structure and of the Fermi level.

The particular role of the Cu-0 chains i s not understood. So far, the highest T, for systems with only C u 4 plenes, like Lq-,Sr,CuO

,

lies around T -40

K.

The Immodiste, p m a ~ b l y naive, conclusidn i s t et the Cu-0 chatns should $ay an importan? role i n the T p 9 O K Y-Ba-CU-0 super- conductors. However, whether the Cu-0 chains act as one-dimensional superconductors, or whether they influence the electronic structure and doping of the Cu02 planes, or whether they participate i n a coupled Cu%-Cu0-Cu02 superconducting unit, are s t i l l open questions. Experiments modifying the Cu-0 chains by replacing Cu and 0 by other elements are i n progress, and there are already a number of interesting results on the variation of Tc with the concentration of various impurities and dopants 182-881 (see Sec. 9 ).

3. ELECTRONIC STRUCTURE 3: I. /nfr&cf~bon

A common starting point i s to assume oxygen, barium and yttrium to adopt formal valencies (I2-, B$+, and y3+ leading to rare gas ionic cores. I f we further assume copper to be divalent, CI.?', charge neutrality requires YBa Cu307- to have 1-28 holes per unit cell. Extreme alternatives are that this hole concentration 2 ~ m e s L i i z e d to one or several types of ions, or delaalized over independent-electron band states. In reality, i t w i l l be distributed i n intermediate ways, with localization to mixed-valent Cu04 clusters, and delocalization over more or less strongly correlated 0- 2p-der ived valence bands.

From a chemical point of view one often assumes the holes to be localized on specific copper ions, leading to a static mixture 2( 1 +8)cu2++ ( 1 -28)cu3+ of divalent and trivalent copper atoms on different sites with average vdency 2 + ( 1 -28)/3. Alternatively, there could be a mixed valence state (cu2+- ~ u ~ ~ c o n f i ~ u r a t i o n mixing on every site).

Still another possibility could be that the Cu ions remain i n thecu2+ valency (or some similar fixed valency) while some of the oxygen atoms edopt 0'- valency (localized 2p-holes) or while itinerant holes are introduced i n the 0 2p-derived valence band. There i s also the possibility that chain and plane oxygen atoms adopt intermediate-valence 02-+ 0- configurations.

A large effort i n high-energy spectroscopy has been focussed on issues of C U ~ + / C U ~ + / C U ~ +

valence. This question i s of central importance for models of superconductivity based on Cu 3d or 0 2p conductivity. The picture which emerges from high-energy spectroscopy i s that a large fraction of the holes seem to go into itinerant and/or localired O(2p)-like levels, h bridized with Cu 3d, i n the Cup! planes and Cu-0 chains. The Cu ions seem to be i n a Cu2'(3db1/ ~ u l ~ ( 3 d " ) mixed valence state which depends on the 0-vacancy concentration S l.891.

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32 Be&tr~~%fur a Mo/c~"u/8r urbife~prcfure of the CuUp/m

Bandstructure calculations based on single-configuration-atomic and plane-wave basis func- tions (LAPW [90-951 or LMTO [%,971) and on the lml-density approximation (LDA) for the exchange-correlation potential [98,991 are capable of giving mutually consistent one-electron pictures of the superconductin compounds, i.e. of the undoped crystal La2Cu04 (Refs. 190-92,963) and of YBaCu& (Refs. 19'3-977). Such independent-electron pictures are very important as a basis for understanding the general electronic structure, and perha s for describing phonon spectra [ l W l , thermwnamic properties [I011 and transport processes

tml.

The celculations show that the bandstructure of the Y-Ba-Cu-0 type superconductors i s domi- nated by electronic states associated with the Cu( 1 ) planes and Cu(2) chains. The gross structure of these states can be understood within a molecular orbital picture (see e.g. Refs. [94,95lJ.

Figure 1 b shows a portion of the unit cell, emphasizing the nearly square-planer structure of the Cu04 units with Cu at the centre and 0 at the corners. Cu 3d orbitals with lobes i n the CuO plane pointing towards the 0-corners may be combined with 0 2p orbitals pointing towards the ceniral Cu atom to form bonding (p&) and antibonding (p&*) sigma-type molecular orbitals with strong hybridization and large bonding-antibonding splitting. Furthermore, Cu 3d orbitals with lobes i n the Cu02 plane pointing between the 0-corners may be combined with the other in-plane 0 2p orbitals to form bonding ( p a ) and antibonding ( p a * ) pi-type molecular orbitals with weaker hybridization and smaller bonding-antibonding splitting than the sigma orbitals.

The 1 D character of the chains and 2D character of the planes are evident from the bandstructure calculations: The bands of the chains only disperse along the chains (b-diredion) while the bands of the planes disperse along any direction i n the a-b plane (essentially no dispersion along the c-direction).

In YB Cu 07, the Cu(2) planes are associated with strongly dispersing, essentially empty, a n t i b o n d i n ~ u ~ u f - 0 2 p (pdu*) bands built from a 2D network of Cu04 tiles. I n the Cu( 1 ) chains there i s also an essentially empty, antibonding Cu3d-02p (pda*) sigma-band, with strong dispersion along the chains and weak dispersion at right angles, built from a 1 D network of Cu04 tiles. I n addition, however, the linear arrangement of Cu04 tiles admits a nearlv full, antibonding Cu3d-02p ( p a * ) pi-band with weak dispersion also

alona

the chain.

Independent-electron bandstructure calculations are of limited value for direct interpretation of electronic excitation spectra, since the CuO-compounds are highly correlated electronic systems with important effects of electron-electron interaction. Nevertheless, the calculations point towards a picture of coupled CuOq tiles, or clusters. The &antage of thinking i n terms of molecular Cu04 clusters is that i f correlation effects are important, these can be handled e.g. via configuration interaction within the molecular cluster, leading to improved cell functions to be used i n the periodic LCAO scheme. The molecular cluster wave functions may then be used i n a LCAO (Linear Combination of Atomic Orbitals) scheme to describe the dispersion of the molecular cluster levels due to hybridization between the different cells. This i s the kind of problem that i s addressed by e.g.

the extended Anderson model.

The most obvious difference between the bands of the Cu( 1 ) chains and the Cu(2) planes seems to be the presence of the nearly filled, anti bonding Cu3d-02p ( p a * ) pi- band with weak dispersion along the chains. It i s not known whether this band s t i l l w i l l cross the Fermi level i n a correlated description. If it does, the associated heavy itinerant 0 2p-holes w i l l be higly correlated objects with strong tendencies towards localization under the influence of a rising Fermi level, disorder, and impurities.

33: Furfib- diswiun of F[oure 2

Having discussed the electronic structurepf YB Cu 4 - 8 i n some detail, we shall return to the very important experimental results for Tc=Tc(8$ln pig. 2.

Figure 2 inspires to the following speculations: ( i ) For O?; 8 (0.25 both the Cu(2) planes and the Cu( 1 ) chains w i l l be superconducting with a resulting T = 9 0 K. The increasing concentration of 0-vacancies may be aaomodeted i n a way that breaks onfy a limited number of chains. Tc de- creases linearly with the decreasing hole concentration on the chains, roughly proportional to

1-26, due to the rising Fermi level. (ii) Around 8=0.25 there i s a transition that may make the chains become non-superconducting, but s t i l l lead to ordered 0-vacancies, Tc dropping down to 6 0 K characteristicof the Cu(2) planes alone. I n the interval 0.2% 6<0.5, T,decreases linearly with the decreasing hole concentration of the planes. For 6-0.5, the simple-minded expression 1-26 for the hole concentration goes to zero, which happens to coincide with a transition to semiconducting be- haviour. In reality, the hole concentration i s decreasing also on the Cu sites, becfiuse the effective CU valence isdecreesing ( i n c r e a s i n g ~ u ~ * weight), which delays the filling of the 0 2p-derived bends.

This would explain why YBa2Cu3O7_g i s s t i l l superconducting at 8-0.7 under ordered conditions.

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4. DISCUSSION OF EXPERIMENTAL PHOTOEMISSION RESULTS AT ROOM TEMPERATURE 4 1. Y a I m b#dfYB)photmrn13s1& $omtra

Figure 3 shows experimental valence band PES results for La-Sr-Cu-0 and Y-Ba-Cu-0 by Shen et 81. [631 recorded at the C u 3 ~ 3 d resonance at around h w 7 5 eV. I n Fig. 3 i s also shown theoretical PES spectra based on the independent-electron bandstructure calculations by Nattheiss and Hamann [90,93

3.

From Fig. 3, which i s representative for the present experimental knowledge of these spectra, and from comparison with the results of bandstructure calculations, we may draw the following conclusions:

a) The experimentally observed DOS at the Fermi level EF i s lower, or much lower, than predicted by bandstructure calculations.

b) I n Y-Ba-Cu-0, the main VB band shows two main structures, a structure around - 3 eV which may be associated with mainly 0 2p character, and a structure around -4.5 eV with mainly Cu 3d character. This identification i s based on band structure calculations m o n the photon energy dependence of photoionitation cross sections.

c) The experimentally observed main VB band i s shifted to higher binding energies i n wm- parison with independent-electron bandstructure calculations. This indicates that localization and correlation effects on the Cu 3d and 0 2p hole levels are important.

d) The VB satellite at about

-

12.5 eV i s not accounted for by bandstructure calculations and signals important correlation effects. The satellite may be associated with a Cu 3 8 ( 3 e ) configuration and a Coulomb (Hubbard) correlation energy U3d3d s 6 eV.

e) The VB peak at around -9.5 eV (most clearly seen i n Y-Ba-Cu-0) probably also indicates correlation effects. I would like b propose that the satellite (or part of it) may be associated with a 0 2p4 (2e2) configuration and a Coulomb (Hubbard) correlation energy U2p2p = 4 eV.

I

^PHOTON^ENERGY^{75 eV}

I

-

LapCuO4 (BAND THEORY )

( BAND THEORY )

y 1 2 3 0 7

J ,,

-20 -15 -10 -5 5

ENERGY RELATIVE TO FERMl LEVEL (eV)

Figure. 3. Valence band (VB XPS, from Shen et el. [631.

-970 -960 -950 -940 -930 BINDING ENERGY (eV)

Figure 4. Cu 2p XPS [633. ( 1 ) L Cu04;

( 2 ) CUO; ( 3 ) cu metal; (4) L Ca4;

2

( 5)Lat .a*0.2C~04; ( 6 Y B 9 u3o7-a.

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f) There ere no significant variations between the different room temperature VB photoelectron spectra i n Fig. 3 (and other similar spectra 143-6311 that can be associated with superconductivity. The PES of the superconductor La, Sr CuO (T =37

K)

lmks quite similar to the PES of the serniconductor L Cu04, and also to

~~=BEQD?

thetc-80 K superconductor YB%Cu

.

g) The Y-Ba-Cu-0 sp

2

r a I n Fig. 3 show some clear differences i n the VB region around

-

^eV

and the satellite region around -9.5 eV between different samples (see also Ref. [581).

~ " 3

h) Essentially no variations are seen i n the YB PES spectra goin through Tc [51,59]. I n contrast, important variations are seen between T-300 K and T<Tc [53,59!.

+2 &re-/eve/,uhot~~misim spectra

Figure 4 shows experimental Cu 2p XPS results by Shen et el. [631, comparing La-Sr-Cu-0 and Y-Ba-Cu-0 spectra with those of CuO and Cu metal. Again, Fig. 4 i s representative for the present ex erimental knowledge of these spectra [43-6$1 and we may draw the following conclusions:

a! The L Cu04. La, $r Cu04, and YBaCu

%-a

s@ra lmk very similar to s s h other and to the CUO s p s r u m . The steRi?s structure about

a

eY above the main lines i n these spectra may be associated with a Cu 3 8 (38) mnfiguration and a Coulomb (Hubbard correlation energy U

=

9 eV. The presence of this satellite gives conclusive evidence for the C$+( 36)) mr~fi~uration?%~fhe ground state, allowing efficient ligand ( L

k

Cu 3d charge-transfer screening.

b) By comparison, Cu metal essentally lacks this satellite, as seen i n the 2p XPS i n Fig. 4, because the lack of 3d-holes ( 3d1* ground state) prevents 3d charge-transfer screening.

I n addition, there are no dramatic variations between the different room temperature VB photoelectron spectra i n Fig. 4 (and other similar spectra [43-641) that can be associated with onset of superconductivity. The PES of the superconductor La, Sro CuO (T ~ 3 7 K) looks quite similar to the PES of the semiconductor L%Cu04, and also to t h e ' s ~ s i$the?c=40 K superconductor Y B B ~ C U ~ ~ .

Essentially no variations are seen i n the VB PES spectra going through Tc [53,591. However, important temperature effects are seen between room temperature and temperatures around Tc and below (see

Sec.

8 ) .

5. M~LEcuLAR CLUSTER MODEL OF PHOTOION17ATION. ANALYSIS OF EXPERIMENT.

In order to interpret photoemission spectra, i t i s necessary to understand the process of relaxation of electronic charge around a core hole at an atomic site. It then sems useful to consider a molecular cluster embedded i n an effective medium with a conduction band, and to distinguish between ?htra-cIust&r and extra-chssfer relaxation. intra-cluster relaxation takes into account intra-cluster charge transfer, but cannot neutralize the central cell mntainina the core hole. This neutralization i s akomplisheb by extra-cluster relaxation, involving char& transfer from the conduction band to screenina orbitals of the cluster (Cu 3d. 0 20. Ba 5d. etc.).

Intuitively it isclear

Gat

the ~creenin~charge'in t h e k n t i a l cluster cell may be distributed i n many ways, giving rise to a number of peaks i n the core-hole photoelectron spectrum. I n the simplest case the screening charge may be "delccalized" w e r the ligands or it may be localized to the central atom with the core hole. However, since hybridization effects can be important the screening charge might be spread over the cluster i n a number of ways. One can even imagine a state where an affinity level of the central cluster becomes filled, resulting in a negatively charged cluster screened by a surrounding positive correlation hole from repelled conduction electrons.

To be more specific, let us consider YB Cu

4

(0( 8<0.5), having 1-28 VB-holes per unit cell distributed over the two CU planes a n 8 h e ~ u d c h a i n . Furthermore let US consider cells (not crystallographic unit cells!) con

%

ining central Cu or 0 atoms i n one of the C u 4 a-b planes, as shown i n Fig. 5 (note that these also represent the Cu-0 chains i n the b-c plane).

Figure 5

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Judging from the Cu 2p XPS i n Fig. 4, the predominant valency of Cu i s cu2+, corresponding to the 3dg configuration. Improved states can be built up using configuration interaction involving mainly intra-cluster 02p-Cu3d charge transfer processes to 3d1°t (the bar denotes a hole level).

Neglecting for the moment the presence of empty levels i n the oxygen-ligand conduction band i n the ground state, but admitting intra-cluster charge transfer, we obtain the following typical mixture of configurations:

ground state 13d9 + 3d1°L>

-

^L.^13d

- + -

L> (a) VB-hole state ( 3 8 + 3d9L

-

+ 3d10L2>

-

= 13dZ

-

+ 3dL

_--

+ L2>

-

^(b)

core-hole state 1 2 ~ 3 d ~ + 2p3d10~> = (2p3d +2pL> ( 1 )

- - - - - --

^(c)

2 ~ 3 d electron 12p3d10> = 12p>

hole pair

- -

( d l

The configuration mixing invalves considering a l l possible tigand-metal charge transfer processes, leading to hybridization within the many-electron system. For example, i n a) the ground state i s described as a superpostion of a hole on the Cu site (3d9 o r 3d) and 8 hole on the ligand (3d10L or L). The coefficients have to be determined by solving the Schrwlinger equation, calcu- lating energies and matrix elements anddiagonalizing the Hamiltonian I17,41,42,631.

52 YaIence66VIdPt-S

The VB-hole state i s dwcribed as a superposition of two-hole states distributed i n all possible ways: two hoies on Cu ( 3 8 or 3d2), two holes on the ligand ( 3 d l O ~ ~ or L2), and one hole on each ( 3 8 ~ or 3dL). The energies of these configurations w i l l depend on the hole-hole Coulomb interaction U i.e. U U , and UdL A ~ u O- 0cluster calculation by Fuj imori et 81. 1181 places ~ the mixed 3 & ~ -3#~*bharacter i n the 0 - 7 eV region and places Cu ( 3 8 ) satellites i n the 10- 12 eV region. The VB-photoelectron spectrum i n Fig. 2 might then be characterized as shown i n Fig. 6:

Figure 6. Valence band (VB) spectrum typical of YBa$u The identification of the

-

^194eV

satellite i s positive. The identification of the -9.5 eV satellite i s probable. The features at -3 eV and

-

9.5 eV show large variation with samples and surface compo- sition, and are most likely related

to 0 2p-levels. - - - + - - )

-12.5 -9.5 -4.5 -3 0 eV

As suggested before, the peak at -9.5 eV could correspond to two holes localized on the same ligand (oxygen) atom, L2+0(2p2). This interpretation i s supported by the RESPES results of Thiry et al. 1591, to be further discussed i n Sec. 6.

It seems that i n present applications of molecular cluster models the 0 2p-holes are treated as delocalized over the ligands ( 1 -electron molecular orbitals) and the U Coulomb interaction is small. The -9.5 eV satellite should therefore not be expected to appear i n

h e

calculation of Fujimori et al. [18] which did not include the 0 ( 2 9 ) configuration. However, i n a mrrelated picture, the two-hole L2 state may localize on one of the oxygen ligand

atoms.

From the (probeble) identifi:

cation of the position of the L2+0(2p2) satellite i n Fig. 6, as well as for simple electrostatic reason

,

the oxygan U Coulomb interaction cannot be much less then

4

^{at the}Cu site. Since the 38satellite poslt?!i% Fig. 6 is reproduced with a value of UJd3

-

⁶e f t reasonable ~ 8 1 ~ . f P the oxygen 2p-2p on-site Coubmb interxtian could be

U = $

^eV,of the same order .as the bandwidth WL of the ligand L=p& states. I n fact, for most of!% ligand states the bandwidth IS 2-3 eY, and the mndition for localization, U2p2p > WL. may be fulfilled.

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JOURNAL DE PHYSIQUE

SJ: Cu2pXPS8ndM5

Comparison of the Cu 2p XPS (Fig. 3) and XAS spectra reveals some interesting things, as demonstrated by Bianmni et al. [SO]. Figure 7 gives a rough picture of the superimposed spectra (excitation energy = XPS binding energy or XAS photon energy).

1 2p3d10

-

2p3d1

-

^OL

-

Figure 7. Cu 2p XAS ( f u l l line

2;

XPS (dashed line). The 2p3d1 /

2p3d10L intensity ratio i n XAS Zp3d9

m~ pr&widean e i m a t e of the

-

3 / 3 L mixing i n the ground state ( 3 d 9 ~ represents a ( 3+

cluster state). The identification of the possible structure around 9 2 8 eV is pure speculation.

+-+---+-)

928 931 933 942 eV

Excitation energy

Judging from the 2p-XPS, the most probable screening process, giving the intense 933 eY peak, i s 1i7f'~-c/usf!r L+3d charge transfer leaving a L-hole on the 0-ligands. However, the overlap of the XPS and XAS spectra around 9 3 1 eV shows that a Cu 2p-hole can be "well screened"

while leaving the surrounding essentially unperturbed. This implies ~ f r 8 - c ~ u s f ! charge transfer which neutralizes the cluster and transfers the t i a n d hole (L) to more delocalized levels i n the conduction band: The 9 3 1 eV XPS peak cannot occur i n an isolated cluster.

Similarly the presence of the 9 3 3 eV peak i n XAS indicates a considerable probability to create a 2 p 3 d 1 b ~ configuration, i.e. a Cu 2p+3d excitation together with a charge transfer ( i n the initial and/or final states) from the cluster to delmlized levels i n the uonduction band.

Bianmni et al. SO] suggest that there i s a considerable probability to find the system i n the

-

initial state i n a 3d

b

L configuration. I n addition, there i s the possibility that the 2p-core hole induces a near-degeneracy between the 2p3d1° and 2 p 3 d 1 0 ~ configurations i n the finelstate. In any case, the results indicate the importance of charge "fluctuations" between the cluster and the environment i n the initial and final states, as well as extra-cluster screening i n the final state. An extended configuration-interaction scheme including valence fluctuation on the ligands might then take the form

ground state 1 3 8 + 3 d l 0 ~

-

+ 3d9L

- ( *

3 8 ... )>

VB-hole state 1 3 8 + 3d9L

-

+ 3d1°L2>

-

Cu2p-hole state (2p3d9

-

^{+ 2p3d10}

- +

2p3d10~>

- -

'2p-+3d electron (2p3d10 + 2p3d1°~>

hole pair

- - -

Core-electron ionization spectra from the oxygen-sites are more difficult to analyze: there are inequivalent 0-sites i n the CuO;! planes and Cu-0 chains; the 0 2p occupation ma/ vary between different sites (02-, 0-, intermediate); r e l a t i o n effects can probably be more complicated (Fig.

5b); itinermt holes i n the valence band mey localize under the influence of a core-hole. Last, but not least, the chemistry of the sample i n the surface region, including oxygen mordinated to impurities of a l l kinds, may give rise to a spectrum of 0 Is peaks. The Cu-0 chains may be particularly sensitive to surface effects. Different experimental photoemission studies of the 0 I s spectt-um [%7,50,5658,1031 clearly demofistrate the sensitivity of the 0 I s spectrum to the chemical state of the surface of the samples, as well as different opinions of what represents the spectrum of a good sample. For, proper interpretation of 0 Is XPS one probably needs extremely

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well characterized samples to have initial knowledge about oxygen vacancy distributions, conditions i n the surface layers, etc.

From Fig. 5b i t looks l i k e relaxation around 0 1s i n the Cu plane should occur mainly along chains of Cu04 squares, i n the Cu( 1) chains and i n the network o

4

chains i n the Cu(2) planes. (The situation may be different, however, when there are many vacancies (&0.5) i n the Cu( 1 ) chains.) We shall assume that i n the presence of a 0 1s hole, the most localized screening orbital i s an 0 2p atomiclike orbital, giving a local 1 szp6 configuration on the oxygen site. With the 0 Is-hole locally 2p-screened, corresponding to $-( IS), the neighbouring CuO squares should not be much disturbed, and might be represented by the ground state configuration mixture of 1 3 8 + 3 d l 0 b . The

"well screened" Is-hole is then represented by l_s2p6 [ 3 d 4 + 3 d t 0 ~ l .

With the 0 Is-hole nut locally 2p-screened, wrresponding to

( ' d 1s

) there i s a large perturbation on the neighbouring CuO squares. The screening charge around the "poorly screened Is-hole must now reside on the CuO squares, on the Cu atoms [3dl0~+3d1O1 or on the oxygen ligands [3d9~*1. This suggests that the "poorly screened" Is-hole might then be represented by

122p5[ 3d10 + 3 d 1 O ~ + 3 d g ~

.

. .] and should consist of a spectrum of levels.

We then have the fd8f?ve classification of the O 1s X-ray photoelectron spectrum shown i n Fig. 8a,

Figure 8a. Schetch of 0 1 s XPS Figure 8b. Schetch of 0 I s XPS after Sarma after Bianconi et al. [SO] et a1.1561. Full curve, T=300 K; dashed curve ( room temperature). T=80 K. Interpretations from the present paper.

Within this model, the interpretation of the 0 1s XPS becomes straightforward (but s t i l l ten- tative:

a) The 5 2 9 eV and 5 3 1 eV peaks directly reflect the (approximate) mixing coefficients i n the ground state charge distribution

1

^aj3di0$> + b l3dg>I2 of the Cu02 squares i n the infinite system, giving 1529/1 = 82/b2. A likely value of this ratio i n YB Cu307- sam les where the surface has not been r&% (depleted of oxygen) i s !520/1534if?b2 = 8 2 5 ~OI. This means that i n YE Cu3 _S there are approximately 0.75 average d holes on the Cu-atoms or, i.e. the averaae num

%

^er

3

^Cu^3delectrons i s about n =9.25.

b ) ~ h e l eV peak to the condition of the surface [SO] follows immediately from the fact that i t probes the Cu 3d9 configuration i n the immediate neigbourhood: i n a reduced sample with important oxygen loss, the cu 3d9 w i l l certainly have turned i n t o ~ u 3d10 on the Cu( 1) "chain" atoms and w i l l have lost weight on the Cu(2) atoms i n the CuO planes. As a result, the 5 2 9 eV peak should increase and the 531 eV peak decrease during rduction of the surface. This i s precisely what i s observed 1501. The i52g/1531 ratio therefore provides a direct possibly quantitative, probe of oxygen depletion at the surface and of the presence of the Cu 3dSl configuration.

c) The ls2p5 configuration i n Fig. 8 ( i f correcty identified) has very low weight (structure i n this region i s however clearly observed [50,56,58]). Since final state screening effects should dominate, the weak ls2p5 satellite intensity suggests that the probability for leaving the 0 2p screening orbital empty i s very small. This suggests that the 0 2p level i s strongly hybridized and bandlike (metallic) so that screening charge preferentially flows into the 0 2p-screened ground states of the 0 1 s-core hole level.

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Finally, considering 0 1 s excitation to empty p-states the lowest energy state must correspond to a 0 Is-t2p excitation with the configuration ls2p6 3dld, i.e. the same as the lowest level of the

"well screened" 0 1 s-hole. This i s observed es an exciton-like peak around 5 2 9 eV photo energy i n X-ray absorption

[MI.

There i s no clear absorption structure coinciding with the ls2p 3d9 XPS

2

peak at 531 eV, which at f i r s t sight might seem a l i t t l e worrying. However, if the 01s core-hole spectrum i s correctly identified, the absence of an excitonic feature at 5 3 1 eV might be an indication of metallic screening and 02p delocalization. The main 0 1 s absorption structure starts at 5 3 1 eV

[MI

and should correspond to 0 1 % ~ ionization together with charge transfer into an 0 2p-like screening orbital.

Let me emphasize again that there are hardly two experiments which agree on the shape of the 01s XPS for a supposedly

"mu

^{YB Cu}074 surface, which poses problems for a theorist. The above analysis may have to be rndifi% bu? should nevertheless serve as an example of how one may proceal.

EWIFS spectra [66-711 give "direct" information about the local environment (neighbour distances, mrdination, force constants) around the Cu atoms. The structure analysis' agree with the results of crystallography. From the threshold and near-edge region (XANES) one can draw conclusions about empty final levels, relaxation effects, Cu-valency, and the local cluster environment.

A configuration-interaction picture of the Cu 1s XANES initial and final states may take the form

ground state

1 w 3 d electron

I

¹^s3d10+ 1 s 3 d 1 0 ~ >

hole pair

- - -

1 ~ 4 p electron ( l_s4p* (3d1°L + + 3d9 + 3dgL )>

hole pair

- -

From the XANES studies one may conclude the following:

a) The dominating signature of the XANES threshoid i s that of cu2+ l ~ 3 d ~ 4 ~ * , i n agreement

with the results of other spectrosmpies.

-

b) The dominating signature of the XANES above-threshold but near-edge structure i s that of square-planar CuL4 complexes ( L=ligand) [69,1041.

c) There i s a very weak low-ener ls3dlo "dipole forbidden" absorption feature indicating the presence of empty Cu 3d-levels

[la

d) The position of Cu K edge i n XAS of L Sr,CuOq does not move with variation of the doping (variation of the hole concentration, -x) [M?,&gesting that the variation of the id-count must be very small, and that the holes must go into 02p-derived itinerant states.

e) In YBa2Cu30 -6, the density of itinerant Cu p-states at the Fermi level i s very low [70].

f) In Y B ~ C U ~ ~ , - there are weak, low-energy absorption features indicating Cu l n 4 p excitation accompanied&/ L+Cu3d charge transfer (initial andlor final state), ls4p* (3dl0\ + 3d1°L2).

-

5 6: lifaref~#/mns~iii?raf~ikns b d o n va/ence band (YB) mdCu 2p photm/a:I"m spefra Based on the configuration-interaction states i n expressions ( la-c), one can calculate and f i t VB and Cu 2p model spectra to the experimental spectre i n Figs. 3 and 4 i n order to obtain the effective Coulomb interaction U7d3d, the average 3d occupation , etc. In this way, Shen et al.

6.5) for Bal $rO 2Cu04 ⁽

7Id

l63l find an average 36 a w p e ion r 1 ~ 6 9 . 4 ( U3d ~ 6 . 5 ) for Y a2Cu30, and ~ ~ 9( U l d g . 5

~ 9 . 5 . ~ ~ ~ ~ ~ = 6 . 5 for Sui), and they also find the cu -02p hybrl zation to be father strong. rma and a, [57] have arrived at similar (somewhat higher) values for the average Cu 3d occupation. This model, then, describes a mixed-valence situation between cu2+(3dg) and~ul+(3dlO), in agreement with the pictureof Fujimori et 61. [IS].

It should be noted that the relaxed value for the free Cu atom i s USdJd

=

15 eY [I061 ; i n Cu metal (filled 3d-band; 4s-4p screening; 3d-screening impossible)

YdJd

^ISdOwn to 8 eV [I061 ; i n Ni metal (partial 3d-screening) it i s down to =4 eV. The value of U

=

6 eV i n CuO compounds therefore indicates very important screening, probably mainly by "7nI"ra-cluster" charge transfer and polarization.

Note however that the results from Cu 2 p XPS and XAS [SO] imply that the model should be

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extended to include "valence fluctuations" on the ligands due to extra- and intra-cluster char

+-i=T

transfers. The 3 d 9 ~ configuration i n expression (2a) has the character of a cluster (CL? 0 ) valency. Inclusion of the 3 d 9 ~ configuration i n the model calculation might possibly lead to a lower value of the averaqe 3d-occupation than the 9.4 found by Shen et al. 1631.

6. RESONANT PHOTOEMISSION 6 /. Cu 3p-t3dresvnanc?; nwy75eY

The 3 w 3 d excitation i s localized on a Cu atom and decays primarily via a ~ u ( 3 ~ 3 d ~ ~ ) - ,

~ u ( 3 d 8 ) + e - super Coster-Kronig process to a uasi-localized Cu two-3d-hole state 12.5 eV below EF. The resonance mainly occurs in the ~ " ( 3 8 ) YB satellite. This establishes the position of the

~ ~ ( 3 8 ) level, and therefore also the size of the scrmned 3d-3d Coulomb interaction UTdq -6 eV (vta model calculations; see e.g. [631). The strong asymmetry of the resonance demons r a es that there i s an important direct excitation mechanism of the satellite.

I n general, Shen et al. [631 find that all VB states show resonant behwiour around the Cu 3 p 3 d resonance, including states very close to the Ferrni level. This demonstrates the presence of 3d-character at the Fermi level due to Cu3d-02p hybridization and should provide an important test of future theoretical calculations. Note, however, that the result does not mean that there are itinerant 3d-holes at the Fermi level.

6 2 U Ps+2p rmnunce; h w 2 U eL

The conclusions i n this subsection are based on the results of Thiry et al. 1591, who found that the -9.5 eV VB satellite shows a strong resonance with strongly asymmetric profile i n the vicinity of the 0 2s-threshold.

The experimental results 1591 can be understood i f the excitation involves a 2 9 2 p transition localized on an oxygen atom. An 0 ( 2s2p6) excitation w i l l primarily decay locally via a 0( 2s2p6) -+ 0( zp4)+e- super Coster-Kronig process to a quasi-localized 0 2p2 two-hole state. The situation

i s analogous to the Cu 3p-t3d resonance and the Cu 3d2 satellite.

The experimentally observed resonance of the -9.5 eV VB satellite probably establishes the position of the localized 0(Zp4) level, and therefore also the size of the screened 2p-2p Coulomb interaction. i n the absence of model calculations, judging from the VB spectrum and comparing with the Cu 3dZ case, a reasonable estimate might be p2 7 4 eV. Again, the strong asymmetry of the resonance indicates that there i s an important d i r a e&tation mechanism of the satellite.

Finally, i n view of the strongly localizedcharacter of the Cu 3d8 and 0

zp4

configurations, it i s not very likely that the - 9 . 5 eV O(2p4) satellite should resonate at the Cu 3 ~ 3 d resonance or the

-

¹^{2.5 eV}^CU(3d8) satellite at the 0 2 w 2 p resonance.

6.3 88 &+4f resvnanup; h&90- 120 eY

The Ba( 4d4f 3 ~ ) resonance occurs around hw=94 eV and the Ba(4dw4f'"

P)

giant dipole resonance i n the 100- 125 eV region. The physics of these resonances and their appearance i n atomic core and valence level photoelectron spectra i s quite well understood [ 107-1101.

The primary decay channel of the Ba(4d4f 3 ~ ) resonance i s via Auger decay to 5p44f reso- nating 5p-satellite configurations around 4 0 eV below EF, showing [I101 a prominent resonance at h w 9 4 eY. In Y-Ba-Cu-0, this has been observed by Onellion et al. [53 The particular asymmetry of resonance profile of the Ba(4d4f D l level i n emission from

J .

EF- 15.7 eV identifies the Sp-core level [63].

The data from EFT2.8 and EF-4.7 eV i n Y-Ba-Cu-0 show a dip at the Ba(4d4f 3 ~ ) resonance and a weak depression i n the region of the Ba( 4dr'4f" ' P I resonance [631. The mrresponding data for La i n La-Sr-Cu-0 [63], however, show resonance profiles characteristic of 4f-emission, suggesting weak but noticeable 4f-hybridzaiion i n the valence band. The results for Ba indicate that any 4f-hybridization i n the VB below EF must be much weaker than in La-Sr-Cu-0. The observed resonace features might also be related to BE( 5d)-O(2p) hybridization.

7 . RESULTS ON OXYQEN DEFICIENT, TETRAQONAL AND SEMlCONDUCTl NO YBa2C~306+E

This section w i l l briefly discuss recent experimental high-energy spectroscopy results 150,55,621 where the oxygen content of superconducting, orthorombic YBa2Cu3O7 has been reduced by e.g. vacuum annealing down to the vicinity of YBa2Cu306, which i s semiconducting and tetragonal.

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C9-1170 JOURNAL DE PHYSIQUE

As discussed i n Secs. 2 and 3, creation of 0-vacancies leads, among other things, to filling of holes and to a rising Fermi level.

There i s at least one published synchrotron radiation PES study [55], comparing the valence band and shallow core levels of Y6a2Cu3O6 with YBa2Cu306 i n the photon energy range of h W 4 0 - 1 0 5 eV. This study indicates a nuni%er of important &anges going from orthorombic, superconducting Y B ~ $ U ~ O ~ . ~ to t e t r w n a l

,

semiconducting YB%Cu 06.2:

a) The general ~mpresslon i s that of a shift of the VB dIstr3butlon and core levels towards higher binding energies.

b) In particular, there i s a 0.9 eV shift of the Ba 4d- and Sp-levels to higher binding energies, consistent with the Fermi level rising with increasing number of oxygen vancies (may he converted to an average constant DOS at the Fermi level of 1.3 e-/eY/cell).

c) The VB satellites at

-

¹2.5 eV (Cu 3 8 ) and -9.5 eV (tentatively 0 2 *) seem to shift much less with the rising Fermi level than the VB and core levels. Indication o$ compensating charge transfers?

d) he YB Cug06 PES shows clear effects of the increased weight of the 3df configuration:

he cu2+ 3 8 B l l i t e Around

-

1 2 5 eV has become quite weak, and the weak cul+ 384ssatellite around

-

^15.5eV has appeared.

A possible interpretation might be the following: The rising Fermi level should partially f i l l the hole band i n the CuO sheets, by f i l l i n g the 0 2p derived hole levels and partially filling Cu 3d levels. This should lead50 the Cu 3dI0 configuration gaining weight (reduced density of Cu 3d- holes), and to reduced 3d8 satellite strength and reduced L 4 Cu 3d charge transfer.

Thus, the large 0-vacancy increase between YB Cu306, and YB Cu 0 seems to have two typical consequences: (i) The Cu-0 chains hecome%oken Boxygen

&14df

leading to spectra characteristic of cul+ (3d1°) and Cu 0, and (ii) the rising Fermi level reduces the cu2+ (3d9)- like character of the C u 4 planes ancfintroduces some CU'+ (3d1°) features. In a f i r s t spproxi- mation, the cu2+ satellite should belong to the Cu02 planes while the cut+ satellite should mainly come from the central Cu( 1 ) layer.

In Cu Is- and 2p-photoabsorption (XAS, XANES) one would then expect (i) a Cu20-like signal proportional to the number of "free" Cu( 1 ) atoms i n the central layer, and ( i i ) a CuO-like signal proportional to the fraction of 3d-holes (weight of 3d9 mnfiguration) on the Cu(2) atoms i n the CuO lanes.

2&cent XANES studies of highly oxygen deficient YB%Cu301_ (6.1 and 0.86) 1621 indicate

CU"/CU~' ratios of 0.3 ( C 0 . 8 6 ) and 0.55 (6=1). assuming t h a the Y-Ba-Cu-0 XANFS can be expressed as a superposition of Cu20 (cul+) and YB cu3+.(cu2+) spectra. EXAFS analysis of the Ysa$u306 1s-absorption spectrum 1621 agreed w i 3 the picture i 7 6 . n ~ of total loss of 0-atoms from the a-b plane of the central layer, leaving only Cu-Cu bonds i n the a-b lane and vertical Cu-0 bonds to th8 0-atoms i n the BaO planes, makes the Cu( 1) atoms adopt a CuP+(3dlo) mnfiguration.

With the YBa2Cu3O6 stochiometry, if the oxygen removal only affects the Cu( 1 ) plane, the formal valence argument requires a l l Cu(1) atoms to go to cul+ corresponding to a cu1+/cu2+ ratio of 0.33. Nevertheless, the XANES analysis gives a CU'+/CU~' ratio of 0.55, suggesting 1621 that there are quite a few 0-vacancies also i n the Cu% planes.

However, according to the previous discuss~on, YB Cu 0, should not be any gmd reference for the Cu02,planes in YB%Cu307-& There should be

less?^^^

and more CU"-like low-energy features, without introducing 0-vacancies i n the Cu02 sheets. The C U ' + / C U ~ + ratio of 0.55 therefore seems compatible with fully oxygen depleted chains and no oxygen vacancies in theCu02 planes.

The study of the Cu 2p-edge i n XAS 1621 has also been done with oxygen deficient semimn- ducting YBa2Cu 06+ indicating the disappearance of the 2e3d1

o~

shoulder at 9 3 3 eV, the intensi- t y reduction d t h e 5p3d1° main absorption line, and the appearance of a Cu20-like absorption spectrum. These chan@s all Imk h i hly consistent and demonstrate the loss of correlation effects.

This study also conludd that the C$+/CU~' ratio was around 0.5 which, again. seems compatible with fully oxygen depleted chains and no oxygen vacancies i n the Cu02 planes.

8. EXPERIMENTALLY OBSERVED TEMPERATURE EFFECTS IN PHOTOELECTRON SPECTROSCOPY.

Temperature dependent effects are likely to be of particular significance since they might give direct clues to the nature of the mechanism causing superconductivity. So far, there are relatively few experiments involving high-energy spectroscopy 155-59,65,72,1031. It seems that nothing dramatic happens around the transition temperature T=Tc Instead there i s a more or less gradual change i n certain spectral features over 8 fairly wide temperature range, some of these changes

(16)

being however rather substantial.

Some of the significant changes going from T ~ 3 0 0 K to T=5OK, well below Tc, may be sum- marized for the Tc=94 K superconductor YBa2Cu307- i n the following manner:

a) The Cu Zp-XPS 156-%&shows ( i l cmsider&ly reduced intensity of the 2 p 3 8 satellite;

(ii) -0.5 eV shift of the 2p3d ,satellite to higher binding energies; (iii) a modification of the lineshape of the main line, shift~ng intensity from the 9 3 3 eV ( 2 p 3 d 9 ~ ) to the 931 eV ( 2 ~ 3 8 ) component; ( i v ) possibly a new small peak at ~ 9 2 8 eV binding energy.

b) In PES 153,591, the VB satellite region from around -7eV to

-

13 eV becomes strongly enhanced.

c) The 0 Is-XPS [56,1031 shows new peak structure around 533-534 eV binding energy, and a corresponding reduction of the intensity of the 5 2 9 eV peak.

d) The Cu 2p-VV Auger spectrum [6Sl shows a new peak around 91 5 eV, below the major Auger peak at =9 1 8 eV.

e) In PES ( h e 1 3 0 eV), the Ba 4d-satellite region becomes strongly enhanced [59]. In eddi- tion, the Ba 4d-main peak shifts by 1.5 eV to lower binding energy 11111. Interestingly enough, the Ba 5p-PE lines do not seem to show shifts or satellites [ ill].

Oenerally speaking, the abovementioned results indicate a change i n electronic structure and dynamics with decreasing temperature. Since holes i n the 0 2p-band probably play a fundamental role for superconductivity,

kev to the oroblem. Unfortunately, so far there are very few published experimental results on the temperature dependence 156-58,1031, and a consistent picture has not yet emerged. Removal of this bottleneck could lead t o important progress i n theoretical descriptions of the 0 2p-conduction band.

The 0 1 s-XPS at T=80 K by Sarma et 81.1561 i s outlined i n Fig. 8b. If the present configuration assignment i s correct, the data indicate that the "well reened" 0 ls2p63dt0 peak loses about 5 0 %

5"

of i t s intensity and that the "poorly" screened 0 1 s2p 3d1° peak gains the same amount of intensity when the temperature i s lowered from 300 K to 80 K.

I would l i k e to suggest that this i s a sign of correlation and localization effects on holes i n a nearly filled, metallic 0 2p-band, analogous to the case of 3d-holes i n Ni metal [17,191. These effects should also influence 0 2p -Cu 3d hybridization and the 0 2p -, Cu 3d charge transfer, and should change the 2p and VB satellite intensi es. I n particular, with the appearance of a pronounced 0 2p VB satellite spectrum. This could also, directly or

Y

0 2p-localization could be associated indirectly, be the reason for the observed VB satellite enhancement 1591.

One possible explanation i s that the Fermi level rises with decreasing temperature. This could account for a number of things, for instance

a) near f i l l i n g of the 0 2p-band (or a high-density part of it) suggested by (i) the increase of the 533-534 eV structure and the decrease of the 5 2 9 eV peak i n the 01s XPS, and (ii) the i n c r e of the VB satellite structure,

b) the increase of the Cu 3d1°/3d9 ratio suggested by (i) the change of the Cu 2p XPS

[=I,

^by

(ii) the decrease of the 531 eV peak i n the 0 1s XPS, and by (iii) the increase of the Cu 2p-VV ( 2p3d1° + 3d8) Auger peak 1651 at 9 15 eV ( 3d1°) relative to the peak at 9 18 eV ( 3d9).

C) the shift of the Ba 46 level to lower binding energy, and the appearance of prominent 4d- satellites below T= 130 K 1591: a rising Fermi level might cross the bottom of a Ba 5d-like band, pulled down bemuse of the deep core hole, and allow metallic screening; since the 4d-potential i s more attractive than the Sp-potential, i t i s not unreasonable that the effect w i l l f i r s t (or only) occur for the 4d-core level.

As a reason for a Fermi level rising with decreasing temperature, one may speculate (wildly) that there may be vacancy ordering (cf. Ref. 18111, clustering of 0-vacancies, creation of more O- vacancies, etc, which might lead to some kind of phase separation between orthorombic and tetra- gnat phases. I n that case, the vacancy regions may donate electrons to the superconducting regions, raising the Fermi level and lowering T,. It has been found that the tetragonal phase lowers the overall Tc i n the mixed, quenched phase i n some proportiorrto i t s relative volume [801.

Alternatively, within a single phase, oxygen vacancy ordering (or other phase transitions) at low temperatures might lead to critically important changes of the electronic structure, e.g. at the Fermi level. For instance, structures may appear with lower concentration of holes i n the 0 2p- derived conduction band, hole mncentrations where strong hole-hole correlation effects become critically important.

I n this connection, i t might also be relevant to mention studies of the temperature and frequency dependence of internal friction, ultrasonic attenuation and Young's modulus [1121, which show a large number of anomalies and lattice contractions and expansions i n the range T=80-300

K.

This might also be relevant for understanding the influence of the temperature-cycling procedures [ 111 used for obtaining reasonably stable samples with Tc=l 6 0 K: I n particular, the results of Ref.