Resonance photoemission from single crystalline Bi2Sr2CaCu2O8 at the Cu 3p absorption edge

Physica C 193 (1992) 309-313 North-Holland

Resonance photoemission from single crystalline Bi2Sr2CaCu208 at the Cu 3p absorption edge G E L Flipse, R. Lindsay, G.N. Raikar ~, P.L. Wincott and G. Thornton Interdisciplinary Research Centre in Surface Science and Chemistry Department, Manchester University. Manchester, M I 3 9PL, UK

J.R. Laverty Centrefor High Temperature Superconductivity and Blackett Laboratory. Imperial College of Science. Technologyand Medicine. South Kensington, London. SW7 2AY, UK

W.R. Flavell Chemistry Department, UMIST, PO Box 88, Manchester, 114601QD, UK

Received 7 October 1991 Revised manuscript received24 January 1992

Resonance photoemission measurements of the high-Tosuperconductor Bi2Sr2CaCu2Osat the Cu 3p absorption edge are reported. The resonance behaviour of valence states lying between 10 and 14 eV binding energy is similar to that observed from CuO, confirming their ds character. A resonance of the valenceband states is also observed,although neither an enhancement nor a resonance is observed in Fermi level states. The latter effect is consistent with these final states having little de character.

1. Introduction Resonance photoemission spectroscopy has been widely used to determine the atomic orbital composition of valence electronic structures, particularly those of transition metals and their compounds [ 1 ]. More recently the technique has been employed in the study ~fCu-based high-To oxides and related materials [ 2-13 ]. The aim has been to identify features in the photoemission spectra, especially those states close to the Fermi level in the superconductors [ 25,7,8,11,14,15 ], and hence derive information about the ground state electronic structure. In this work we have re-examined the Cu 3p resonance behaviour o f Bi2Sr2CaCu2Os, employing a single crystal sample and cleaving in-situ to minimise problems associated with sample preparation and contamination. This is of' particular concern i Present address: Chemistry Department, University of Alabama in Huntsville, Huntsville, Alabama 35899, USA.

when dealing with cuprate superconductors [2]. Our studies follow earlier Bi2Sr2CaCuaOs single crystal studies where the resonance behaviour was only partially addressed [ 3,4,7 ] and work in which a ceramic sample was investigated [ 5 ]. In general, valence band photoemission spectra of the heavier first row transition metal compounds cannot be described adequately within a one-particle model because of the large 3d-3d Coulomb interactions compared to the one-particle bandwidth [ 16]. One-particle band structure calculations are therefore unable to properly describe photoemission spectra, a more appropriate approach for many purposes involving impurity a n d / o r cluster configuration in-'.eracfo~s (C~) models [6,12,16]. In this paper we compare Cu 3p resonance photoemission spectra of Bi2Sr2CaCu2Os with the results of recent cluster-CI model calculations and the corresponding experimental results for CuO [ 9,10,16 ]. Those parts of the Bi2Sr2CaCu2Os spectra which are CuO-related, i.e. the valence band and its satellite, display

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310

C.F.J. Flipse et aL/Resonance photoemission

a resonance behaviour consistent with these results. However, the cluster-CI model [ 9,16 ] does not describe the lower binding energy states of Bi2Sr2CaCu2Oa between the ~A~gf'mal state and the Fermi level. Their behaviour is consistent with final states having little d s character.

2. Experimental details Room temperature photoemission measurements employed the toroidal grating monochromator ( ! 5 < h p < 90 eV) and VG ADES 400 instrument on station 6.2 at the SRS, Daresbury Laboratory. The combined (monoehromator+analyser) energy resolution was 0.15 eV, the angular resolution being + 2 ° (ful! width at half maximum (FWHM). In this work ~ e base pressure of the spectrometer was ~ 3 × 1 0 - ' ° mbar. Normalisation of data to the incident photon flux employed the drain current from a W mesh placed between the monochromator exit slit and the sample. Additional normalisation to an absorption spectrum of Pt [ 17 ] was needed to remove Pt N r . 7 structure in tk¢ monochromator output arising from absorption by the beam line optics. Single crystals of Bi2Sr2CaCu208 were supplied by SuperconiX Inc. (St. Paul, Minnesota). The crystals were grown from a molten flux employing a proprietary temperature/gas atmosphere cycle which optimises the superconducting properties. This growth procedure yields crystals which appear as caxis-oriented thin platelets, of face size up to 3 × 3 mm 2, at the surface of the residual melt. Magnetisation measurements showed that the crystals became superconducting at 84 IC Surfaces suitable for photoemission measurements were obtained by peeling the sample at room temperature using a tab technique to expose a clean (001) face. After removal from the spectrometer, the cleaved surface used in the present study was found to be oriented such that photoemission measurements had been carried out at about 12 ° from normal emission.

sorption edge (71
E ::3

L-q~ E: "10 Q)

E 0

Z

k-~

15

3. Results and discussion Valence region spectra of Bi2Sr2CaCuzOs (001) recorded at photon energies around the Cu 3p ab-

10

5

71.5 t

EF

Binding Energy (eV) Fig. 1. Photoemission spectra of Bi2Sr2CaCu:Oa (001) recorded 12 ~ off-normal emission at photon energies around the Cu 3p threshold.

C.F.J. Flipse et aL ~Relevance photoemission

~d

Z i ,

I

15

I

10

I

I

5

I

I

EF

Binding Energy (eV) Fig. 2. A comparison of CuO and Bi2Sr2CaCu20s (001) photoemission spectra recorded at photon energies of 70 eV and 7I eV, respectively.The CuO spectrum, taken from ref. [9] has been rigidly shifted 1.3 eV to higher BE. A linear background has been subtracted from the B2Sr2CaCu2Os(001 ) spectrum. valence band can be represented by a linear combination of states Id 9) and Id~°_L), where _L represents an O 2p ligand hole. Calculations o f the off-resonance CuO photoemission spectrum [ 16 ] suggest that features lying at binding energies (BE) of up to 6 eV are dominated by [dg_L> states. Final states of IdZ°L2> are situated around 9 eV BE, while the deeper states are mainly of ]d s > type. A comparison of these calculations with experiment [9 ] shows that they can reproduce the dS-satellite position and its resonance behaviour near the Cu 3p threshold rather accurately. Because the IdS> final states are found at comparable binding energies in the cuprate superconductors, the d - d Coulomb interactions will be o f the same magnitude and comparable Cu 3p resonance behaviour is expected. It is generally understood [ 1,18 ] that Cu 3p resonant photoemission spectroscopy on CuO and the cuprate superconductors represents atomic processes of the type: 3p63d 9 + h v-o 3pS3d 1%o 3p63d s ~ e

( 1)

and 3d9.-~3dS+e.

(2)

The first step o f the process (1) is a 3 p - , 3 d photon absorption event, and the second step is a two-elec-

311

tron Coster-Kronig decay. The result is the emission o f a photoelectron e. This process involves the same initial and final states as the direct p h o t o e m J ~ o n process, resulting in interference effects. Resonant photoemission will therefore highlight the Cu 3d s final states. The normalised photoemission spectra shown in fig. 1 evidence resonance enhancement o f the salellite and to a lesser extent the valence band region, with little change to the Fermi level states. As for CuO, satellite enhancement is greatest for the 12.5 eV BE component. This represents a ~G final state [ 16 ], which has a significantly higher Auger-decay transition moment associated with it than the triplet components at ~ 11 eV BE. In fig. 3 we display the resonance behaviour in the form o f constant initial state (CIS) spectra and compare them with CuO experimental data and the re-

Bi2Sr2CaCu208(O01) ....... IJ and CuO I CIS spectra

q

,,

,

~

70

~

-:

v

[

75

80

Photon Energy (eV) Fig. 3. CIS spectra of (a) CuO at 12 eV BE from ref. [9]; (b; calculated CIS spectrum at 12 eV BE for a CuO46- cluster in D4~ symmetry (from fig. 4 in ref. [ 9 ]; (c) Bi2Sr:CaCu2Osfor the BE region 10-14 eV; and (d) Bi2Sr2CaCu:Osfor the BE region I-8 eV. Bi2Sr2CaCu2OsCIS spectra were obtained from photoemission spectra in fig. The resonance enhaacement in (c) is 42% (peak-background at 72.5 eV/background; in (d) it is 26%.

312

C.F.J. Flipse et aL /Resonance photoemission

suits of cluster-Cl calculations [9]. To produce the CIS spectra from the fig. 1 data we have s u m m e d intensity over the satelh'te region (10-14 eV BE) as well as over the valence band (1-8 eV BE). A background was not subtracted from photoemission spectra prior to summation to avoid the introduction o f artifact.~ This has the consequence that the satellite CIS spectra are influenced to an extent by an increase in background due to valence band enhancement. The behaviour o f the satellite is seen to be similar to that observed for CuO [9 ], including the position of the Cu 3p3/2 peak at ~ 74 eV, although our data do not extend to cover the Cu 3pl/2 feature which peaks at 76 eV. Our results are also consistent with the result of cluster-Cl calculations for the CuO satellite [9]. The satellite behaviour is consistent with results obtained form Bi2Sr2CuO6 [7], Nd2_xCexCuO4 [ 11 ], Nd2CuO4 [6 ] and polycrystalline Bi2Sr2CaCu20 s [ 5 ]. However, our results for the valence band ( 1-8 eV) differ from the latter work in evidencing ~sonance enhancement (see fi~ 3 ( d ) ) rather than an anti-resonance (at 3.2 eV BE) [5]. Our valence band results also differ from CuO results where a weak anti-resonance is observed (of the lowest BE feature) [9,10], although they are consistent with recent CuO Cu2p3/2 resonance photoemission results [ 13 ]. Our results are more in line with the behaviour expected on the basis o f the cluster-CI-model calculated 3.5 eV and 1.7 eV BE CIS spectra for CuO [9 ], which show a resonance enhancement at ~ 74 eV. However, such a comparison is not entirely valid since the resonance response will vary across the valence band, due to a variation in d s character. Our results for the fermi level states are shown in fig. 4 in the form o f photoemission spectra recorded on- and off-resonance at the Cu 3p threshold. These data are consistent with earlier results [ 3,5 ], which evidenced neither enhancement or nor anti-resonance behaviour (the apparent enhancement at high BE m fig. 4 is due to the tail of the valence band). The previous interpretation of this behaviour as reflecting the low Cu 3d character of these states [ 3,5 ] contrasts with other work [4] which estimated a ca. 35% Cu 3d character from photeemission cross-section measurements. The intensity in the Bi2Sr2CaCu20~ spectra at

Bi2Sr2CaCu208(O01) Fermi Edge

ne30~l~ormal emission

"i tO

Z I

I

1.0

0.5

I

EF

Binding Energy (eV) Fig. 4. Photoemission Fermi level spectra of Bi2Sr2CaCu2Os ( 001 ) recorded 12 = off-normal emission at: hv= 74 eV; and (b) hu= 73 eV.

BE < 1.0 eV falls outside the area of the CuO clusterCI calculations [9,16]. They predict th ~, lowest BE feature to be the tA]s(D4h) ~tate, which ~: assigned in the CuO experimental spectrum to the 1.2-1.5 eV BE feature [ 9,13 ]. Such singlet states have been considered as the possible origin o f Fermi level states in spectra o f high-To materials [ 6,19,20 ]. Aligning the CuO spectrum with that of Bi2Sr2CaCu2Os by shifting the CuO spectrum ofGhijsen et al. [9] to 1.3 eV higher BE places the tAtg feature at 2.5 eV BE (see fig. 2). The close similarity o f the spectra in fig. 2 at BE>__1.0 eV suggests that the shoulder at ~2.5 eV BE in our Bi2Sr2CaCu2Os spectra also corresponds to a tAtg final state. As for the final states lying to lower BE o f this shoulder (BE < 1.0 eV), the lack o f a Cu 3p resonance effect suggests that d s states do not contribute significantly to this region. We find that this is confirmed by a comparison of the Cu 2p resonance behaviour of Bi2Sr2CaCuzOs and CuO [ ! 3,21 ]. However= this does not exclude the possibility o f a Cu 3d contribution to states at BE_< 1.0 eV, which has been theoretically predicted [ 22 ], and evidenced experimentally [4,15 ]. For instance, ~t may be that final states such as dgL2 contribute to this region, as indicated by X-ray absorption [23] and electron energy, loss experiments [24] which probed the unoccupied states just above EF.

C.F.J. Flipse et al./Reso~,um e pkotoemission

4. Conclusions We have performed a resonance photoemission study of the oxide superconductor Bi2Sr2CaCu2Os. Spectra recorded at the Cu 3p threshold reveal a resonance in the valence band and its satellite. The resonance behaviour of the satellite is found to be similar to that calculated and measured for CuO [9 ] and the behaviour of the valence band is consistent with cluster-CI calculations for CuO [9 ]. This confirms the mainly CuOE-plane origin of the valence band and its satellite in Bi2Sr2CaCu2Os photoemission spectra, and is consistent with the similarity of the CuO and Bi2Sr2CaCu2Os spectra at around the Cu 3p threshold. In contrast to the behaviour claimed for the electron doped superconductor Nd2_xCexCuO,_j, [8 ], no resonance or anti-resonance behaviour of the states close to the Fermi level is observed at the Cu 3p threshold, in agreement with earlier Bi2Sr2CaCu2Os studies [3,5 ]. We take this to indicate that these final states do not have appreciable d s character, but we do not rule out a Cu 3d contribution to Fermi level states, for which there is theoretical [22] and experimental s u p p o r t [ 4,15 ].

Acl~nowledgements

This work was funded by the UK Science and Engineering Research Council, including the award of a studentship to RL, with additional support from Harwell Laboratory.

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